How Things Work I
How Things Work I PHYS 6050
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This 9 page Class Notes was uploaded by Ubaldo Jacobson on Monday September 21, 2015. The Class Notes belongs to PHYS 6050 at University of Virginia taught by Richard Lindgren in Fall. Since its upload, it has received 39 views. For similar materials see /class/209738/phys-6050-university-of-virginia in Physics 2 at University of Virginia.
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Date Created: 09/21/15
Bicycles 1 Bicycles Bicycles 2 Question How would raising the height of a small pickup truck affect its turning stability Make it less likely to tip over Make it more likely to tip over Have no overall effect on its stability COM Obsenations About Bicycles Impossible to keep upright while stationary Easy to keep upright while moving forward Require leaning during turns Can be ridden without hands Are easierto ride when they have gears Vehicle s Static Stability Part1 Static stability is determined by base of support polygon formed by ground contact points center of gravity effective point at which gravity acts Static stability occurs when center of gravity is above base of support Vehicle s Static Stability Part 2 Center of gravity above base of support gravitational potential rises when tipped accelerates away from direction of tip vehicle always returns to equilibrium vehicle in stable equilibrium statically stable Vehicle s Static Stability Part 3 Center of gravity not above base of support gravitational potential drops when tipped accelerates in direction of tip vehicle never returns to equilibrium vehicle tips over statically unstable Vehicle s Static Stability Part 4 Center of gravity is above edge of base vehicle in unstable equilibrium accelerates in direction of any tip vehicle never returns to this equilibrium Stationary Vehicles Base of support requires 23 contact points Tricycles have 3 contact points are statically stable and hard to tip over Bicycles have only 2 contact points are statically unstable and tip over easily Vehicle s Dynamic Stability Part1 Dynamic stability is determined by statics base of support center of gravity dynamics inertia accelerations horiz forces Vehicle s Dynamic Stability Part 2 Dynamic effects can fix a vehicle s stability place base of support under center of gravity dynamically stabilize an equilibrium make vehicle dynamically stable Vehicle s Dynamic Stability Part 3 Dynamic effects can ruin a vehicle s stability displace base of support from center of gravity dynamically destabilize an equilibrium make vehicle dynamically unstable Moving Vehicles Tricycles can t lean during turns dynamically unstable and easy to flip Bicycles can lean during turns to maintain stability naturally steer center of gravity under base dynamically stable and hard to flip Bicycle s Automatic Steering A bicycle steers automatically places base of support under center of gravity due to gyroscopic precession of front wheel ground s torque on spinning wheel steers it due to design of its rotating front fork fork steers to reduce gravitational potential Torques and Tipping Over Torques act about bicycle s center of mass Support force acts at wheels causes torque Friction acts at wheels causes torque Weight acts at center of mass no torque lftorques don t cancel net torque on bicycle bicycle undergoes angular acceleration bicycle tips over Leaning During Turns Part 1 When not turning and not leaning zero support torque force points toward pivot zero frictional torque no frictional force bicycle remains upright Leaning During Turns Part 2 When turning and not leaning zero support torque force points toward pivot nonzero frictional torque frictional force bicycle flips over Leaning During Turns Part 3 When turning and leaning correctly nonzero support torque force not at pivot nonzero frictional torque frictional force two torques cancel if you re leaning properly bicycle remains at steady angle Bicycles can lean and thus avoid flipping Tricycles can t lean so flip during turns Bicycles 15 Question How would raising the height of a small pickup truck affect its turning stability 1 Make it less likely to tip over 2 Make it more likely to tip over 3 Have no overall effect on its stability Bicycles 19 Gear Selection From rider s perspective ground is moving With each crank ground moves a distance Ground distance covered increases with gear Work clone per crank increases with gear Pedal forces must increase with gear High gear yields high speed level road Low gear yields easy pedaling steep hills Mechanical Advantage Gears allow you to exchange force for distance or distance for force On hills low gear lets your feet move large distances to exert large force on wheel On descents high gear lets your feet push hard to move rear wheel long distances Rolling and Energy Wheel rim moves and spins A kilogram in the wheel rim has twice the kinetic energy of a kilogram in the frame To start the bicycle moving you must provide its energy Massive bicycles particularly with massive wheels are hard to start or stop Rolling Resistance As a wheel rolls its surface dents inward Denting a surface requires work An underinflated tire has a low coefficient of restitution doesn t return work clone on it well wastes energy as it rolls Bicycles 23 Braking Sliding friction wastes bicycles and riders kinetic energies as thermal energy Braking power is proportional to sliding frictional force between pads and rim support force on brake pads tension of brake cable force on brake levers Braking problems Brake too hard wheels stop rotating and start skidding energy is wasted and steering fails Slowing force exerts a torque on bicycle Rider and bicycle can flip head first Rear wheel loses traction and may fishtail Front wheel has improved traction Wm ammi Water Distribution Wm L7an 2 Question Water enters your home plumbing at ground level Where will you get the strongest spray from a shower In the ground oor shower In the basement shower In the second oor shower Observations About Water Distribution Water is pressurized in the pipes Higher pressure water sprays harder Higher pressure water sprays higher Water is o en stored up high Fluid Motion no gravity Fluids obey Newton s laws Net forces cause accelerations Net forces are due to pressure imbalances Fluids accelerate toward lowest pressures Wm imam 5 Pressurizing Water To pressurize water use of Newton39s third law push inward on the water water pushes outward on you water s outward push is due to its pressure water pressure rises as you squeeze it Pumping Water no gravity Squeeze waterto raise its pressure Water accelerates toward lowest pressure Water begins owing You do work on the water You keep squeezing as water llows Water moves in direction of your force In this case Work Pressure Volume Woodslwes 1 Woodsloves 2 Question Woodstoves Which is more effective at heating a room a black woodstove a white woodstove Observations Thermal Energy About Wood Stoves They burn wood inside closed fireboxes is disordered energy They often have long chimney pipes is kinetic and potential energies of atoms They are usually black gives rise to temperature You get burned if you touch them does not include order energies 0 Heat rises off their surfaces kinetic energy of an object moving or rotating lt feels hot to Stand near them potential energy of outside interactions Heat Burning Wood is energy that flows between objects Fire releases chemical potential energy because of their difference in temperature wood and air consist of molecules is thermal energy on the move Molecules are bound by chemical bonds Technically objects don t contain heat When bonds rearrange they release energy Burning involves bond rearrangement Chemical Forces Part 1 Atoms interact via electromagnetic forces Large separations atoms attract Attraction is weak at great distances Attraction gets stronger as atoms get closer Attraction reaches a maximum strength Attraction weakens as they approach further Chemical Forces Part 2 Medium separations equilibrium Attraction vanishes altogether at equilibrium Small separations atoms repel Repulsion gets stronger as atoms get closer Chemical Bonds Part 1 When atoms are brought together they do work release chemical potential energy By the time they reach equilibrium they have released a specific amount of energy have become bound together chemically Chemical Bonds Part 2 To separate the atoms you must do work on them return the specific amount of energy to them Chemical Concepts Molecule atomsjoined by chemical bonds Chemical bond chemicalforce linkages Bond strength work needed to break bond Reactants starting molecules Reaction products ending molecules Chemical Reactions Breaking old bonds takes work Forming new bonds does work If new bonds are strongerthan old chemical potential energy a thermal energy Breaking old bonds requires energy reaction requires activation energy to start Burning Wood Reactants carbohydrates and oxygen Products water and carbon dioxide Activation energy a burning match Thermal Energy and Bonds Thermal energy causes atoms to vibrate Atoms vibrate about equilibrium Experience restoring forces about equilibrium Energy goes potentialekineticepotential Total energy is constant unless transferred Temperature set by thermal kinetic energy Heat and Temperature Objects exchange thermal energy Microscopic energy flows both ways Average energy flows from hotter to colder Temperature predicts energy flow direction No flow a thermal equilibrium a same temp Temperature is Average thermal kinetic energy per particle Woodsloves 16 Open Fire Burns wood to release thermal energy Good features Heat flows from hot fire to cold room Bad features Smoke enters room Fire uses up room s oxygen Can set fire to room Woodslwes 17 Fireplace Burns wood to release thermal energy Good features Heat flows from hot fire to cold room Smoke goes mostly up chimney New oxygen enters room through cracks Less likely to set fire on room Bad features Inefficient at transferring heat to room Woodsloves 15 Woodstove Burns wood to release thermal energy Good features Heat flows from hot fire to cold room All the smoke goes up chimney pipe New oxygen enters room through cracks Relatively little fire hazard Transfers heat efficiently to room Woodstoves 19 Heat Exchanger Woodstove is a heat exchanger Separates air used by the fire from room air Transfers heat witho ut transferring smoke Woodstoves 20 Heat Transfer Mechanisms Conduction heat flow through materials Convection heat flow via moving fluids Radiation heat flow via light waves All three transfer heat from hot to cold Woodstoves 21 Conduction Heat flows but atoms don t In an insulator adjacent atoms jiggle one another atoms do work and exchange energies on average heat flows from hot to cold atoms In a conductor mobile electrons carry heat long distances heat flows quickly from hot to cold spots Woodstoves 22 Woodstoves Conduction moves heat through the stove s metal walls Woodstoves 23 Convection Fluid transports heat stored in its atoms Fluid warms up near a not object Flowing fluid carries thermal energy with it Fluid cools down near a cold object Overall heat flows from hot to cold Natural buoyancy drives convection Warmed fluid rises away from not object Cooled fluid descends away from cold object Woodstoves 24 Woodstoves Conduction moves heat through the stove s metal walls Convection circulates hot air around the room
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