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BYU - PHY S 100 - Chapter Five Notes - Class Notes

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BYU - PHY S 100 - Chapter Five Notes - Class Notes

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background image Chapter 5 Notes  How do the laws of motion and the interactions of gravity and magnetism work together? 
Finding Forces: Ask Yourself 
•  Is gravity present? What's its role?  •  Are the objects involved charged? Are there magnets? Do those forces attract or repel?  •  Is there direct contact on the object?  •  Is there friction force?  •  Paired forces (equal and opposite) will result in a net force of ZERO, and there will be no 
acceleration on that object 
Unbalanced forces cause acceleration in the direction of the net force  Remember: if there's acceleration, there's a net force/unbalance force     
Standing on a scale 
•  When you stand on the scale, your acceleration due to gravity pushes down on the springs within 
the scale 
•  Those springs will, in return, push up with a larger force, causing you to bounce up and down 
•  As the springs find balance and push up with the same force as you push down, your weight can 
be measured 
The scale actually measures the contact force pushing you up!    
Jumping into the Air 
•  Initially, the only forces acting upon you are the force of gravity and the equal and opposite 
contact force from the floor pushing up on you (net force = zero, so there is no acceleration) 
•  Crouching down:  Experience acceleration downward (contact force less than gravitational force)  Once in crouched position, contact force is now greater than gravitational force (you slightly 
bounce back up after downward acceleration) 
Now have a net force of zero  •  Jump:  Contact force increases as you push off the floor (which pushes off you too!)  ▪  Floor only pushes on you when you're touching the floor, so once you're in the 
air…contact force equals ZERO 
With only gravity acting upon you in the air, you being to accelerate downwards back to the 
An arched jump and a vertical jump hit the ground at the same time    
Moving Furniture: 
•  Your feet push against the ground as you pull a couch, which means the ground pushes against 
you AND the couch pulls back on you (inertia) 
•  Friction between you and the ground, friction between the couch and the ground  The friction between the couch and the ground is LESS than the force you exert on the 
couch (pull) = couch accelerates towards you 
•  Constant speed:  Force of friction on couch equal to forward force of your legs; if you stopped pulling friction 
would decelerate the couch to a stop 
background image   
Air Friction 
•  When objects move, they come in contact with air molecules that accelerate out of the object's 
path (F = m*a), while the object experiences an incredibly small acceleration backward 
But think of how many air molecules are colliding with moving objects! Ultimately, the 
object's motion is affected significantly 
That is air friction  •  Air friction increases with object's surface area and shape     
Sky Diving 
•  At zero seconds (right as you jump out), only acceleration of gravity is acting upon you  •  As you fall faster, air friction increases and pushes up on you  •  Eventually, friction force equals gravity and acceleration equals ZERO   Velocity is now constant  •  Parachute increases surface area and friction force on you; net force is now upward, creating an 
upward acceleration that slows you down 
•  As you slow, friction force decreases until it balances again with gravity  •  You land safely    
Falling Elevator 
•  Friction force is pushing against car as it falls  •  Constant acceleration of car (and you) means you will fall faster and faster  Result: "weightless floating"    
Going into Orbit 
•  Burning fuel of engines push against spacecraft, which pushes back on fuel  •  As gases escape from nozzle, spacecraft pushed upward with equal and opposite force  •  In space, spacecraft experiences freefall (always falling under gravity only)    
Centripetal Acceleration 
•  Net sideways force exists on object  •  Centripetal force: a force that is sideways to the motion of the object, turning them towards the 
center of the circle 
•  Strength of net force to create circular 
Motion depends on object's speed and mass 
•  F = (m * speed^2)/radius  •  Weaker force = looser circle  •  Stronger force = tighter circle    

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School: Brigham Young University
Department: OTHER
Course: Physical Science
Professor: Patricia Ackroyd
Term: Spring 2017
Tags: force gravity and newton's laws
Name: Chapter Five Notes
Description: How do the laws of motion and gravitational interaction work together?
Uploaded: 09/26/2017
4 Pages 22 Views 17 Unlocks
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