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PHYS107 Test 2 Study Guide

by: Mallory McClurg

PHYS107 Test 2 Study Guide Phys 107

Marketplace > University of Mississippi > Physical Science > Phys 107 > PHYS107 Test 2 Study Guide
Mallory McClurg
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These notes cover all of chapters 6 through 8 - Momentum, Energy, and Rotational Motion. I outlined everything in the chapters, included in-class examples/explanations, and even worked the quiz pr...
Physical Science I
Study Guide
momentum, rotational, motion, inertia, Energy
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This 20 page Study Guide was uploaded by Mallory McClurg on Monday October 3, 2016. The Study Guide belongs to Phys 107 at University of Mississippi taught by Quinn in Fall 2016. Since its upload, it has received 27 views. For similar materials see Physical Science I in Physical Science at University of Mississippi.

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Date Created: 10/03/16
PHYS107 Test 2 Study Guuide Chapters 6- 8; Quinn Chapter 6: Momentum v Momentum is the measure of inertia (mass) in motion (velocity) MOMENTUM = MASS x VELOCITY = mv o Only moving objects can have momentum o Momentum is large when either the mass or the velocity is large, so it’s directly proportional to one of the mass/velocity variables. MOMENTUM =MASS xVELOCITY MOMENTUM = MASS x VELOCITY MOMENTUM = MASS x VELOCITY § Important concepts to understand about momentum: • A truck with a large mass and a car with a smaller mass are travelling at the same speed = truck has more momentum • Two trucks with the same mass, but one has a bigger velocity than the other = faster truck has more momentum • Huge barge moving at a low velocity = can have a large momentum • Small bullet moving at a high velocity = can have a large momentum • A truck parked in a parking lot = no momentum • An object is harder to stop when it is moving fast v Impulse is when the momentum of an object changes; this means either the mass or the velocity (or both) is changing; remember, velocity is speed AND direction, so if an object changes direction, its momentum is changing and it’s experiencing impulse o Most of the time, mass will stay the same, so if velocity changes, then acceleration occurs, and an object will experience impulse o The greater the impulse exerted on something, the greater it’s change in momentum will be. The two concepts are always linked. o How to calculate impulse: § When recording a change in momentum (impulse), TIME is an important factor. § For example, if you hit your brakes for just a half second, the force will change the car’s momentum, but if you apply that same force to your brake pedal over a longer period of time, you produce a greater change in the automobile’s momentum. IMPULSE = FORCE x TIME v The Impulse-Momentum Relationship: FORCE x TIME = Δ MASS x VELOCTY (impulse = a change in momentum) § Increasing momentum: PHYS107 Test 2 Stuudy Guuidee Chapters 6- 8; Quinn • Apply the greatest force possible for as long as possible o For example, hitting a golf ball: zero force is exerted on the ball until the club makes contact, then the force increases rapidly while the golfer follows through with his swing, then diminishes as the ball comes up to speed § Decreasing momentum over a long time period: • If the change in momentum is over a long period of time, the hitting force is small; if the change in momentum is over a short period of time, the hitting force is large o For example, if your car is out of control and you had the option to drive it into a brick wall or a haystack, which would you choose? Probably the haystack, but why? Either way, momentum would be brought to zero by the same impulse (or the same product of force and time). But by hitting the haystack, you extend the time during which your momentum is brought to zero. o For example, when you jump off a table, you don’t want to land with your legs straight and stiff… instead, you bend your knees when your feet make contact with the ground. This extends the time during which your momentum decreases by 10 to 20 times compared to landing with stiff, straight legs. § Decreasing momentum over a short time period: • The smaller the amount of time the momentum is decreased over, the bigger the hitting/impact force will be o For example, you’re in a boxing ring and you’re trying to dodge your opponent’s next hit. If you move into (towards) his quick swinging hand, the time is reduced, and you must withstand a greater force than if you moved away from his hand, which would make the impact force smaller. v Bouncing is what happens when an object reacts to an impact force. o Impulses are greater when an object bounces o The impulse required to bring an object to a stop and to “throw it back again” is greater than the impulse experienced when just bringing the ball to rest. § For example, a flowerpot just fell from a balcony above you. If you catch the flowerpot above your head, you provide an impulse to reduce its momentum to zero. If you throw it upward again, you must provide additional impulse, although this additional impulse is the same as the impulse your head would supply if the flowerpot where to bounce off of it. PHYS107 Testt2 Sttudy Guuide Chapters 6- 8; Quinn v The Conservation of Momentum – states that if you wish to change the momentum of an object, exert an impulse on it; similar to Newton’sLaw o Only an impulse external to the system can change the momentum of the system; internal systems and impulses wont work o If there are no external forces on a system, the momentum of the system doesn’t change o Momentum is a vector quantity, so it can cancel itself out with direction and magnitude. § For example, the moment as a cannon is fired, the cannonball inside the barrel has a force that is equal and opposite of the force that causes the cannon to recoil. Since force from the exploding gunpowder acts on the cannon and cannonball for the same amount of time, the impulses are equal and opposite. Since the impulses are internal to the cannon/cannonball system, they don’t change the momentum of the system. Momentum is neither gained nor lost, because, before the cannon is fired, momentum is zero. After the cannon is fired, the total momentum is still zero. v Collisions occur when two objects hit each other, whether they’re both moving, or if one object hits another at rest. o The Conservation of Momentum is present in all collisions; the net momentum of a system if colliding objects is unchanged before, during, and after the collision, because the forces that act during the collision are internal forces. Momentum is simply shared or redistributed. NET MOMENTUM BEFORE COLLISION = NET MOMENTUM AFTER COLLISION o There are two different types of collisions: § Elastic collisions – when two objects bounce off one another with no deformation in the objects nor heat generated § Inelastic collisions – when two objects stick together when they collide, and deformation and heat generation will always occur; NOTE: only in a perfect elastic collision will objects actually stick together PHYS107 Test 2 Studdy Guuide Chapters 6- 8; Quinn Review questions for Chapter 6 : 1. Which has more momentum: a 1-ton car moving at 100 km/h or a 2- ton truck moving at 50 km/h? (Multiply 1 ton and 100 km/h, then multiply 2 ton and 50 km/h. They’re equal!) 2. Does a moving object have an impulse? (No, an object never has impulse, it just provides or experiences impulse. An object cannot possess impulse just as it cannot possess force.) 3. Does a moving object have momentum? (Yes, but only with respect to a frame of reference, which is usually Earth’s surface. The momentum possessed by a moving object with respect to a stationary point may be quite different from the momentum is possesses with respect to another moving object.) 4. For the same force, which cannon imparts a greater impulse to a cannonball: a long cannon, or a short one? (The longer cannon imparts a greater impulse because the force acts over a longer time.) 5. If a boxer is able to increase the duration of impact to three times as long by riding with the punch, by how much will the force of the impact be reduced? (The force of the impact will be only 1/3 of what it would have been if he hadn’t pulled back.) 6. If a boxer instead moves INTO the punch so to decrease the duration of impact by half, by how much will the net force be increased? (The force of impact will be two times greater.) 7. A boxer being hit by a punch wants to extend time for best results, while a karate expert delivers a force in a short time for best results. Isn’t this contradictory? (No, because he best results for each sport are very different. The best result for the boxer is reduced force by maximizing time. The best results for the karate expert are increased force in a minimal amount of time.) PHYS107 Testt2 Sttudy Guuide Chapters 6- 8; Quinn 8. When does impulse equal momentum? (If the initial momentum of an object is zero when the impulse is applied, final momentum = impulse applied. And, if an object is brought to rest, initial momentum = impulse delivered.) 9. How does the force that a karate expert exerts on bricks compare with the force exerted on the karate expert’s hand? (The forces are equal – Newton’s Third Law. Only the resilience of the human hand and the training he/she has undergone allows for this to be performed without broken bones.) 10. How will the impulse resulting from the impact of a karate expert’s hand on a pile of bricks differ if her hand bounces back upon striking the bricks? (The impulse will be greater if the karate expert’s hand bounces from the bricks upon impact. If the time of the impact is not correspondingly increased, a greater force will then be exerted on the bricks – and the hand!) 11.Are impulses greater or lesser if an object bounces off of something else? (Greater, because the momentum change when stopping an object and throwing it back is greater than the momentum change when simply stopping an object.) 12.If you’re in a car and you push forward on the dashboard, why won’t the momentum of the car increase even if there’s forward force acting on it? (A passenger inside of a car is considered an internal impulse. Only an impulse external to a system can change the momentum of the system.) 13.If a traincar with a velocity of +10 m/s has an inelastic collision with another traincar of equal mass that is at rest, what is the velocity of the 2 traincars after the collision? What happened to momentum after the collision? (After the collision, the two traincars have a velocity of +5 m/s and the momentum is shared equally between them because they stuck together in an elastic collision.) PHYS107 Tesst2 Studdy Guidee Chapters 6- 8; Quinn 14.If a fish with a mass of 5-kg swims 1 m/s toward a 1-kg fish, then eats it, what is the velocity of the 5-kg fish after he eats the little fish? Momentum after, we can calculate it like this.) (5 kg) (1 m/s) + (1 kg) (0 m/s) = (5 kg + 1 kg)(v) 5 kg * m/s = (6 kg)(v) v = 5/6 m/s PHYS107 Test 2 Studdy Guidee Chapters 6- 8; Quinn Chapter 7: Energy v Work is the effort exerted on something that will change its energy. WORK = APPLIED FORCE x DISTANCE o Work is measured in a unit called a joule (J), which is equal to 1 Newton-meter (N*m); one joule of work is completed when a force of 1 newton is exerted over a distance of one meter o If a force is applied but distance does not change, for example, a little kid trying to push a car, work is not being done. o There are two basic categories of work: § Work done against another force § Work done to change the speed of an object o In-class example: Which requires more work to be done: raising a 50-kg block 2 meters upward, or raising a 25-kg block 4 meters upward? 50 kg = 500 N 25 kg = 250 N (500 N)(2 m) = 1000J (250 N)(4 m) = 1000 J v Power is the amount of work done per time it takes to do it. POWER = WORK/TIME o Power is measured in a unit called a Watt (W), which is equal to 1 joule of work done in 1 second o In-class example: cont.’d from example above. If the 50-kg block is raised 2 meters upward in 1 second, and the 25-kg block is raised 4 meters upward in 2 seconds, which experienced more power? 50 kg = 500 N 25 kg = 250 N (500 N)(2 m) = 1000J (250 N)(4 m) = 1000 J 1000J/1 s = 1000 W 1000J/2 s = 500 W v Energy is what enables an object to do work; it is what is given to an object when a force is exerted on it o Mechanical energy – comprised of energy due to the position of the object (potential energy), energy due to an object’s motion (kinetic energy), or a sum of the two § Potential energy – the energy that is stored and held in readiness • Work must be done on an object to give it potential energy, which then allows that object do work itself. If an object has a constant velocity, there is no work being done. • Potential energy due to height is called Gravitational Potential Energy (PE). Remember, height is relative to whatever arbitrary point is used. Also, horizontal distance/movement is GRAVITATIONAL POTENTIAL ENERGY = MASS x GRAVITY x HEIGHT PHYS107 Test 2 Stuudyy Guidee Chapters 6- 8; Quinn o For example, a box sitting on the floor has no potential energy. But if you lift a box up off the floor, it has gravitational potential energy. § Kinetic Energy – the energy of motion KINETIC ENERGY = (1/2) MASS x VELOCITY 2 • An object can only have kinetic energy if it is in motion. • Kinetic energy can NOT be negative • Velocity affects kinetic energy more than mass does • In-class example: Which has more kinetic energy: a 1- kg block thown 1 m/s, a 2-kg block thrown 2 m/s, or a 1-kg block thrown 2 m/s? (1/2)(1 kg)(1 m/s) = (1/2)J 2 (1/2)(2 kg)(1 m/s2 = 1J (1/2)(1 kg)(2 m/s) = 2J o The Work-Energy Theorem states that work is equal to a change in kinetic energy NET WORK = Δ KINETIC ENERGY o Conservation of Energy – energy is neither created nor destroyed; it transforms from one form into another and from one place to another; total amount of energy never changes § In-Class example: When a pendulum is dropped from a certain height, it has the maximum gravitational potential energy before it is let go, and the maximum kinetic energy when it is at the lowest point in its swing. o Machines are devices that multiply the force or the distance of work, or simply change the direction of the force; machines aren’t used to do more work – they’re used to do work more quickly/efficiently § They cannot multiply the energy or the work, because of the conservation of energy! § An ideal machine loses no work (energy) to friction, heating, etc., but this doesn’t really exist because energy is often wasted through heat and friction FORCE xDISTANCE )input FORCE x DISTANCE )output § An example of a simple machine is a lever. You do work on one end, while the other end does work on the load. A lever changes the direction of the force. • An example of a lever is a pair of scissors. Scissors are made up of two levers with a fulcrum near the applied force (which is typically the wrong way for a lever to work efficiently). However, the fulcrum is “wrongly placed” because it increases cutting distance. § Efficiency – expressed as a ratio to convey how much energy a machine is wasting/conserving EFFICIENCY = USEFUL WORK input USEFUL WORK output PHYS107 Testt2 Sttudyy Guidee Chapters 6- 8; Quinn Review Questions for Chapter 7: 1. If a new-model forklift has twice the power of an older one, how much more load can it lift in the same amount of time? If it lifts the same load as the old one, how much faster can it operate? (The new forklift delivers twice the power, so it can lift twice the load in the same time, or the same load in half the time.) 2. How much work is done when lifting a 100-N block of ice a vertical distance of 2 meters? FORCE x DISTANCE = WORK 100 NEWTONS x 2 METERS = 200 JOULES a. How much work is done when pushing the same block of ice up a 4-m long ramp, whose maximum vertical height is 2-m, when the pushing force needed is only 50-N? FORCE x DISTANCE = WORK 50 NEWTONS x 4 METERS = 200 JOULES b. Comparing lifting a 100-N block of ice vertically 2-m, and pushing it up a ramp to a distance of 2-m above ground zero, what is the increase in the blocks’ gravitational potential energy in each case? (In both cases, the block’s potential energy increases by 200J. The ramp simply makes the work easier to perform!) 3. Can an object have energy? (Yes, but in a relative sense. For example, an elevated object may possess potential energy relative to the ground below but no energy relative to a point 0at the same elevation. Similarly, the kinetic energy that an object has is relative to a frame of reference – usually Earth’s surface.) 4. Can an object have work? (No. Unlike momentum or energy, work is not something that an object has. Work is energy in transit! It’s the energy transferred when a force acts on an object as it moves over a distance.) 5. When you’re driving 90 km/h, how much more distance do you need to stop compared with driving 30 km/h? (1/2) * MASS * ( 3 * VELOCITY ) = (1/2) * MASS * ( 9 ) * VELOCITY = ( 9 )(1/2) * MASS * VELOCITY2 (9 times as much work requires 9 times as much distance.) PHYS107 Test 2 Studdy Guuide Chapters 6- 8; Quinn 6. Does a car consume more fuel when the a/c is on? When its lights are on? While the radio is on with the motor off? (All in all, yes. All the energy consumes in these activities ultimately comes from the fuel. Even the energy taken from the battery must be given back to the battery by the alternator, which is turned by the engine, which runs from the energy of the fuel!) 7. Say a 100% energy-efficient car burns fuel that has an energy content of 40MJ per liter. If the total air drag and overall frictional forces on the car traveling at highway speed is 500N, how far can the car travel per liter of fuel at this speed? WORK = FORCE x DISTANCE DISTANCE = WORK/FORCE 1 MJ = 1,000,000 J 40,000,000-J per liter/500 N = 80,000-m per liter = 80-km per liter a. If that same car is powered by s 25%-efficient engine, how far can it travel at highway speed on 1 liter of fuel? (25% fuel efficiency means that this car will travel one-fourth (1/4) as far as the one above, which is 20-km per liter!) PHYS107 Teest 2 Studdy Guiide Chapters 6- 8; Quinn Chapter 8: Rotational Motion v Circular motion – as we have learned there are different types of motion; here is a breakdown of the relationship between linear and circular speed o Linear speed (v, velocity) – how far an object moved (in meters) per unit of time § Points on the outside of a rotating object have greater linear speed than those inside closer to the axis § The speed of something moving along a circular path is referred to as tangential speed, since the direction is tangent to the circle o Rotational speed (ω, angular speed) – the number of rotations per unit of time § All points on a rigid rotating object have the same rate of rotation § Typically measured in revolutions per minute (RPM), degrees per second, or radians per second, although the latter is the most common; Note: there are 360 degrees or 2π radians in one revolution. o Tangential speed is directly proportional to rotational speed, and directly proportional to radial distance from the axis VELOCITY = RADIUS * ω § This formula only works for a rigid rotating system (which means the system does not deform under the action of applied forces); this does NOT work for a non-rigid rotating system like that of the orbiting planets or horses running around a track v Rotational Inertia – the property of an object to resist changes in rotational motion o An object rotating around an axis will remain rotating around that axis at the same speed unless interfered with by an external influence; this external influence on an object rotating around an axis is called torque o Like regular inertia, rotational inertia depends on the mass of the object; however, it also depends on the distribution of the mass relative to the axis of rotation § The more mass that’s concentrated away from the axis, the greater the rotational inertia § To illustrate, think of a solid cylinder and a ring (cylinder with no center) rolling down a hill. A disk rolls faster than a ring because more mass is concentrated near the axis of rotation PHYS107 Test 2 Sttudyy Guiide Chapters 6- 8; Quinn o To calculate simple rotational inertia problems: Simple pendulum: INERTIA = MASS x RADIUS 2 Hoop rotating around a normal axis: INERTIA = MASS x RADIUS 2 Hoop rotating around its diameter: INERTIA = (1/2) MASS x RADIUS 2 Rod rotating around its end: INERTIA = (1/3) MASS x LENGTH 2 Rod rotating around its center: INERTIA = (1/2) MASS x LENGTH 2 Solid cylinder rotating around normal axis: INERTIA = (1/2) MASS x RADIUS 2 Solid sphere rotating around its center: INERTIA = (2/5) MASS x 2 RADIUS v Torque – a force that causes a change in rotational motion o A lever arm is the shortest distance between the line of applied force and the rotational axis (perpendicular from the line of applied force to the axis) TORQUE = FORCE x LEVER ARM § For example, a wrench turning a bolt. • In order to turn it the easiest, your arm would be perpendicular to the wrench • If it’s still too hard to turn, you can increase the length of wrench o Mechanical equilibrium – an object rotating around an axis can be in mechanical equilibrium § ΣF must equal 0 § ΣT must also equal zero (clockwise rotation is usually noted to be positive, while counter-clockwise rotation is usually considered negative.) v Center of Mass/Center of Gravity o An object’s center of mass is the average position of all the mass in the object o An object’s center of gravity is the average position of all the weight distribution of an object § The center of mass and the center of gravity is the same, unless gravity varies from one part of the object to another • For example, a super tall skyscraper has a center of gravity a few feet below it’s center of mass because the higher the skyscraper is above the earth, the less gravity pulls on it. § Center of gravity can exist even where there is no mass • For example, an L-shaped piece of wood has a center of gravity that is not actually located on the object. o How do you find an object center of gravity? § Attach a string at a single point on the object and suspend it in the air. Let another string hang straight down from that point. Trace the vertical line made by the string. It’s center of gravity lies along that line. PHYS107 Test 2 Stuudy Guuidee Chapters 6- 8; Quinn § Now, detach the strings and suspend the object from another point. Let the other string hang straight down. The object’s center of gravity lies where that line and the prior line intersect. o Center of Gravity in Men vs. Women: § Men have a higher center of gravity than women - about 1 or 2% higher – because men typically have more upper body muscle mass § In both men and women, center of gravity is near the belly button • Why? So the umbilical cord won’t get tangled up when a baby moves around in the womb v Stability – the stability of an object has to do with the size of its support base o Smaller support bases have less stability; larger support bases have more stability o If we draw a line straight down from the center of gravity of an object of any shape and it falls inside the base of the object, then the object is in stable equilibrium – it will balance. If the straight line falls out of the center of gravity, the object will be unstable § Example: The Leaning Tower of Pisa has a center of gravity at about mid-height in the center of the building. The line that’s drawn straight down from there, falls within the support base boundaries, which means it will not fall (unless acted on by something else…) v Centripetal force – any force directed toward a fixed center; means “center- seeking” or “toward the center” o An example of centripetal force: whirling a can around at the end of a string, we find that we must keep pulling on the string – exerting a centripetal force. The string transmits the centripetal force which pulls the can into a circular path o The Moon is also held in its orbit by centripetal force. CENTRIPETAL FORCE = 2 2 MASS * TANGENTIAL SPEED (aka VELOCITY ) / RADIUS OF CURVATURE o Notice that speed is squared; that means, twice the speed needs four times the force! The inverse relationship with the radius of curvature tells us that half the radial distance requires twice the force. o Centripetal force is NOT a basic force of nature – just the name given to any force, whether string tension, gravitational, electrical, or whatever, that is directed toward a fixed center. v Centrifugal Force – although centripetal force is directed toward the center, an occupant inside a revolving/rotating system experiences an outward force called centrifugal force o What causes centrifugal force? – Inertia and “inertial force” or “apparent force”! Like whirling a bucket of water on the end of a rope…if the rope were to break the bucket of water would fly of in a straight path tangential to the point it broke loose. That’s because of inertia – the tendency of an object to continue moving in a certain PHYS107 Teest 2Sttudy Guuide Chapters 6- 8; Quinn direction. The water remains at the bottom of the bucket despite being whirled upside down because the water wants to continue going in the direction it is swung. o Simulated gravity is created by centrifugal force v Angular momentum – ANGULAR MOMENTUM = ROTATIONAL INERTIA x ROTATIONAL VELOCITY o The momentum we learned about in Chapter 6 is linear momentum. o Angular momentum is “the inertia of rotation” of rotating objects § A planet orbiting the sun, a rock whirling at the end of a string, tiny electrons whirling around atomic nuclei – all have angular momentum o It is a vector quantity, and has direction and magnitude o An object or system of objects will maintain its angular momentum unless acted upon by an external net torque o If a whirling object is small compared to the radial distance to its axis of rotation, like a planet orbiting the Sun, we can calculate angular momentum as follows: ANGULAR MOMENTUM = (MASS * VELOCITY) (aka the magnitude of its line* RADIUSn) v Conservation of Angular Momentum – if no external net torque acts on a rotating system, the angular momentum of that system remains constant o With no external torque, the product of rotational inertia and rotational velocity at one time will be the same as at any other time. o For example, an ice skater begins a spin on the ice with her arms stretched out, which allows her rotational inertia to be relatively large. Her angular momentum is the product of her rotational inertia and rotational velocity, ω. When she pulls her arms inward, her rotational speed increases because her rotational inertia is reduced. PHYS107 Teest 2 Studdy Guiide Chapters 6- 8; Quinn Review Questions for Chapter 8: 1. Imagine a ladybug sitting halfway between the rotational axis and the outer edge of a turntable (record player). When the turntable has a speed of 20 RPM and the bug has a tangential speed of 2 cm/s, what will be the rotational and tangential speeds of another ladybug who sits at the outer edge? (Since all parts of the turntable have the same rotational speed, the ladybug at the edge of the turntable also rotates at 20 RPM. Tangential speed is a different story… Since that ladybug is twice as far from the axis of rotation, she moves twice as fast – 4 cm/s.) 2. Is it easier to balance a hammer with the head balanced on your finger, or the end balanced on your finger? Why? (It’s easier to balance it with the most mass at the top – this way, the rotational inertia will be greater and therefore the rotational movement of the hammer will be more resistant to change.) 3. Why are manhole covers circular instead of any other shape? (Circular covers are the only shaped covers that will not fall into the hole and injure someone underneath! A square cover, for example, could be tilted vertically and turned diagonally so it could drop into the manhole. Every other shape is the same way. Pretty cool trivia fact, there.) 4. If a pipe effectively extends a wrench handle to three times its length, by how much will the torque increase for the same applied force? (Three times more leverage for the same force produces three times more torque.) 5. Where is the center of gravity (CG) of a donut? (In the center of the whole. Yep, even where there’s no mass.) 6. Can an object have more than one center of gravity? (No, a rigid object has only one center of gravity. If an item is non-rigid, like a piece of clay, which can be distorted into different shapes, then its center of gravity may change as its shape changes. Even then, it has one center of gravity for any given shape.) PHYS107 Test 2 Stuudy Guuidee Chapters 6- 8; Quinn 7. Where is the center of mass of Earth’s crust? (Like a giant basketball, Earth’s crust is a spherical shell with its center of mass at Earth’s center.) 8. A meter stick supported at the 25-cm mark balances on a fulcrum when a 1-kg rock is suspended at the 0-cm end. What is the mass of the meter stick? (The mass of the meter stick is 1-kg. This system is in equilibrium, so all torques must be balanced. The torque produced by the weight of the rock is balanced by the equal but oppositely directed torque produced by the weight of the stick applied at its center of gravity, the 50-cm mark. The fulcrum’s support force at the 25-cm mark is applied midway between the rock and the stick’s center of gravity, so the lever arms around the fulcrum are equal (25-cm). 9. When a car drives off a cliff, why does it rotate forward as it falls? (When all the wheels are on the ground, the car’s center of gravity is above a support base and no tipping occurs. When the car drives off a cliff, the front wheels are the first to leave the ground, and its back wheels only support the car. The center of gravity then extends past its support base, and rotation occurs.) 10. If Earth were to spin faster about its axis, your weight would be less. If you were in a rotating space habitat that increased its spin rate, you’d “weigh” more. Explain why greater spin rates produce opposite effects in these cases. (You’re on the outside of the spinning Earth. In a rotating space habitat, you’d be on the inside of the system. A greater spin rate on the outside of Earth shows a decrease in weight on the weighing scale. Inside a rotating space habitat, you’d see an increase in weight on the weighing scale.) PHYS107 Testt2 Sttudyy Guidee Chapters 6- 8; Quinn Practice test questions: (from quizzes) 1. A heavy truck and a light truck roll down a hill. Neglecting friction, at the bottom of the hill, the heavy truck will have greater a. Acceleration b. Speed c. Momentum d. All of these e. None of these 2. Two billiard balls of the same mass roll toward each other, each moving with speed 10 m/s. What is the combined momentum of the two balls? a. 20 kg*m/s b. 0 kg*m/s c. 10 kg*m/s 3. A man tries to catch a home run baseball while sitting in the outfield bleachers. Unfortunately, the ball misses his glove and hits the top of his head. If the ball bounces off his head, he will be hit ________ he would if the ball sticks to his head. a. The same as b. Less hard than c. Harder than 4. When you jump from an elevated position, you usually bend your knees upon reaching the ground. By doing this, you make the time of impact about 10 times as great as for a stiff-legged landing. In this way, the average force your body is experiencing is a. More than 1/10 as great b. Less than 1/10 as great c. About 10 times as great d. About 1/10 as great 5. A 1 kg glider and a 3 kg glider both slide toward each other at 2 m/s on an air track. They collide and stick. The combined mass moves a. 1 m/s M 1 1 kg M 2 3 kg b. 1/3 m/s V1= 2 m/s V2= -2 m/s c. 1/6 m/s M Final 4kg VFinal ? d. 0 m/s (M 1 V 1 + (M 2 V )2= M Final VFinal e. 1/2 m/s (1 * 2) + (3 * -2) = 4 * x 2 + -6 = 4x -4 = 4x x = -1 (Remember, the negative here is just indicating a direction!) PHYS107 Teest 2Sttudy Guuide Chapters 6- 8; Quinn 6. Two identical arrows, one with twice the kinetic energy of the other, are fixed into a bale of hay. Compared with penetration of the slow arrow, the faster arrow penetrates a. Twice as far b. More than 4 times as far c. Four times as far d. The same distance e. None of the above 7. 50W of power can do how much work in 2 seconds? a. 25 J POWER = WORK/TIME b. 200 J 50 W = x /2 s c. 100 J 100 W = x d. 12.5 J 8. If a 4-kg object gained 40 J of potential energy when it was lifted, about how high was it lifted? a. 1600 m GRAVITATIONAL POTENTIAL ENERGY b. 10 m = MASS * GRAVITY * HEIGHT c. 1 m 40 = 4 * 10 * x d. 160 m 40 = 40x x = 1 9. Which task requires more work? a. Lifting a 50-kg sack 2 meters WORK = FORCE * DISTANCE b. Lifting a 25-kg sack 4 meters 50 * 2 = 100 J c. Both require the same 25 * 4 = 100 J d. Need more information 10.What is the mass of a Porsche 911 Turbo sports car if it has 2,000,000 J of kinetic energy while driving 50 m/s (i.e. 180 km/h) a. 800 kg KINETIC ENERGY = (1/2) MASS * VELOCITY 2 b. 80,000 kg 2,000,000 = (1/2) * x * (50) 2 c. 40,000 kg 2,000,000 = (1/2) * x * 2,500 d. 2,500,000,000 kg 800 = (1/2)x e. 1600 kg 1,600 = x 11. Strictly speaking, your car consumes more fuel if the a/c, headlights, or the radio is on. This statement is a. True b. False c. True, only if the car’s engine is running PHYS107 Test 2 Studdy Guuide Chapters 6- 8; Quinn 12. A jack system will increase the potential energy of a heavy load by 400 J and generate 600 J of heat with a work input of 1000 J. The efficiency of the jack system is a. 100% EFFICIENCY = WORK InputWORK Output 100 b. 60% x = 400/1000 * 100 c. 25% x = 0.4 * 100 d. 40% x = 40 e. Not enough information 13. Phil applies 100 N to a pulley system and raises a load one-tenth of his downward pull. Ideally, the weight of the load is a. More than 10,000 N b. 1,000 N c. 100 N d. 10,000 N 14. The bob of a simple pendulum has its maximum kinetic energy at the a. Midpoint between top and bottom b. At all points along its path of swing c. Top of its swing d. Bottom of its swing 15. When a cannonball is fired, it recoils as the cannonball is set in motion. The cannon and the cannonball ideally acquire equal a. But opposite amounts of momentum b. Amounts of kinetic energy c. Both of these d. Neither of these 16. To tighten a bolt, you push with a force of 40 N at the end of a wrench handle that is 1/8 meter from the axis of the bolt. Assuming that you push perpendicular to the handle, the torque you are exerting is a. 5 Nm TORQUE = FORCE * LEVER ARM b. 20 Nm x = 40 * (1/8) c. 1,600 Nm x = 5 d. 320 Nm 17. Compared with a force, a torque involves a. Distance from an axis of rotation b. Leverage c. Rotation d. All the above 18. For a very tall skyscraper, the center of mass is a. In the same position as the center of gravity b. Is slightly higher than the center of gravity c. Is slightly lower than the center of gravity d. Is about 10 times higher than the center of gravity e. Is about 10 times lower than the center of gravity PHYS107 Testt2 Sttudy Guuide Chapters 6- 8; Quinn 19. The rotational inertia of a pencil is greatest about an axis a. About its end, like a pendulum b. About its midpoint, like a propeller c. About its length, where the lead is 20.Your pet hamster sits on a record player whose angular speed is constant. If he moves to a point twice as far from the center, then his linear speed a. Doubles b. Remains the same c. Halves 21. Centripetal force does no work on a circularly moving object because a. Centripetal force has no component in the direction of motion b. No change in energy occurs c. Rotational energy transfers to kinetic energy d. None of the above 22.When a twirling ice skater brings her arms inward, her rotational inertia a. Increases b. Decreases c. Remains the same 23.A difference between linear momentum and angular momentum involves a. Two different types of speed b. A radial distance c. Both of these d. Neither of these 24.An SUV will tip over more easily than a sedan built on the same chassis because a. It has a smaller base b. It is more massive c. Its center of mass is higher d. It wont tip over more easily because SUVs are always safer than a sedan 25.Consider a rotating donut-shaped space habitat where living quarters are on the inside surface farthest from the axis. If the rotational speed of the habitat decreases, the apparent weight of the people inside a. Stays the same b. Increases c. Decreases d. Is always zero


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