Momentum Momentum and Energy Momentum is inertia in motion Mass x velocity Has both magnitude and direction Large mass or high speed can give object great amount of momentum Chapter 3 Momentum and Impulse Apply force over time to change velocity and momentum Greater time of application greater change in momentum Force x time interval is IMPULSE Impulse F t Force x time interval Impulse changes momentum Technically Ft mv Realistically Ft m v Momentum m v Change momentum by changing mass or velocity usually velocity What causes changes in velocity Force Time is also important Impulse Increasing Momentum Ft m v Pushing a child on a swing the force Increases momentum Longer push increases momentum more than a short one Ft mv Impulse Decreasing Momentum Stopping the out of control car Change momentum to zero Less force if time is greater Ft m v Impulse Changes Momentum Can change force by changing time http www bcm edu oto research cochlea Volta 16 html Bounce increases impulse There is a change in velocity direction making a greater v So greater force is required Water changes direction has greater impulse than a flat paddle Ft m v Move away increases time reduces force Toward decreases time increases force 1 Conservation of Momentum Conservation of Momentum No net force No net momentum The system the cannon and the ball When it fires momentum is conserved they both have momentum in opposite directions Conservation of Momentum Conservation of Momentum Nothing about time in definition Slow or fast Same force same distance same work Work Transference of Energy Work Force x distance a Yellow ball starts stationary b Both balls moving opposite directions c Green ball moving faster W Fd In the absence of external force the momentum of the system remains unchanged Consider individual balls as individual systems momentum of each does change Net momentum is the same before the collision As after the collision Work Conservation of Momentum Work Lifting load against the force of the weight of the object Twice the distance results in twice the work Twice the weight is twice the work W Fd Work W Fd Twice the weight Twice the distance 2 Work W Fd Units of work are Joules Work is energy Work W Fd Units of force Newtons kg m s2 Force x distance Newton meters kg m kg m 2 m s2 s2 Joules Work vs Energy Same units Work occurs with transfer of energy Work occurs when you store potential energy Work vs Energy Mechanical energy Moving things has two forms 1 Potential mechanical energy Waiting to work Work W Fd Weight lifter does work to lift barbell expends energy to keep the potential energy in the barbell But he does no work on the barbell after it is lifted Work vs Energy Energy stored in bow Work is done to create the potential energy 2 Kinetic mechanical energy Work being done Work vs Energy Potential Energy Lift heavy ram of pile driver Work transfers energy to lift into potential gravitational energy Potential Energy Fuel is chemical potential energy http www alternativefuels com au Biodiesel dragster htm http www howstuffworks com inside clock htm http www lilligren com Redneck redneck lawnmower htm http www himalayan pdx edu virtualjourney slideshow se photos web pages Boy 20with 20Slingshot 2C 20J htm 3 Gravitational potential energy Gravitational potential energy Due to object s position Relative to a surface weight x height Gravitational potential energy mgh Work done for object to gain potential energy Gravitational potential energy Gravitational potential energy EP mass x acceleration of gravity x height EP mgh mg weight h height Height is above some reference level Potential energy is always referenced to a zero level defined in the system Gravitational potential energy EP mgh Horizontal distance is not factor in EP Kinetic Energy of Motion 1 EK mv2 2 Work is a change in kinetic energy W EK Delta change Gravitational potential energy EP mgh Path to the height is not factor in EP Kinetic Energy of Motion How much further will a car skid if you lock up the wheels at 90 km h vs 30 km h EK mv2 mass x 90 km h 2 mass x 8100 km2 h2 mass x 30 km h 2 mass x 900 km2 h2 mass x 8100 mass x 900 9 Nine times further 4 Kinetic Energy of Motion Heat Sound Electricity and light Work Energy Theorem Work is change in kinetic energy Work KE Kinetic Energy of Motion W EK Work energy theorem Net work force x distance Fd Due to net force mv 2 EK 2 Kinetic energy and momentum Properties of moving things Momentum is a vector quantity Conservation of Energy Cannot be created or destroyed Can be converted from one form to another can be cancelled with opposite momentum Kinetic Energy is a scalar quantity Cannot ever be cancelled mv 2 2 Conservation of Energy Transformation from one form to another Potential energy of stretched rubber of slingshot Transformed to kinetic energy of rock flying through air Kinetic energy of rock flying through air Potential energy of stretched rubber of slingshot Transformed from potential to kinetic Rock transfers its kinetic energy to the object it hits May be transformed to heat upon impact Energy cannot be created or destroyed It may be transformed from one form into another but the total amount never changes http www littletheatre net Firecracker Miss miss firecracker contest htm Conservation of Energy Sun s energy from fusion of hydrogen to helium 4 H He energy 1 1 Fd 4 2 Sun s energy converted to chemical energy by plants Sun s heat converted to potential energy when it evaporates water Conservation of Energy Does a car use more fuel when its lights are on What about when the air conditioner is on Power Work done over time Power Work done time interval How about using the radio when the engine is off 5
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