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CONCEPTUAL PHYSICS. ENERGY. Work Work= force x distance W= Fd Unit: Joule (J) We do work when: We exert a force We move something with that force ( distance ) Work generally falls into 2 categories: Work done to change the speed of something Ex. Speeding up a car or slowing it down.
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CONCEPTUAL PHYSICS ENERGY
Work • Work= force x distance • W= Fd • Unit: Joule (J) • We do work when: • We exert a force • We move something with that force (distance) • Work generally falls into 2 categories: • Work done to change the speed of something Ex. Speeding up a car or slowing it down. • Work done against another force. Ex. Doing pushups, archer stretches a bow.
Power: indicates how fast work is done. • Power= work done P= W time T • Unit: watt (W) • Mechanical energy : energy due to position or the movement of something • Energy: the ability to do work
Potential energy (PE): energy stored and held in readiness • Ex. A compressed spring, stretched rubber band, chemical energy in food • Gravitational potential energy: potential energy due to elevated positions. • Gravitational potential energy= weight x height weight=mg PE= mgh Fig 8-3 p105
Kinetic energy (KE):energy of motion • Ex. A moving car, a moving baseball etc. • K.E. = ½ mv2 • The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest: • Fd=1/2mv2 • Conservation of Energy • Law of conservation of energy: energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.
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Machines: devices for multiplying forces or simply changing the direction of forces. • Work input=work output • Mechanical advantage: the ratio of output force to input force for a machine • Types of simple machines: • Levers fulcrum: pivot point • Type 1: fulcrum is between the input and output ends of the lever. Ex. seesaw
Type 2: load is between the fulcrum and the input end. • Type 3: fulcrum is at one end and the load is at the other.
Pulley: changes direction of force. Can multiply forces. • Efficiency: ratio of useful work output to total work input . • Efficiency= useful work output x 100 total work input
Classwork • Answer questions p 116 2-20
Examples 1)How much work is done in lifting a 500N barbell 2.2m above the ground? What is the potential energy of the barbell once it is lifted to the that height? Calculate the power expended if the barbell is lifted 2.2m in 2 s.
2) Calculate the change in potential energy of 8 million kg of water dropping 50m over Niagara Falls. If 8 million kg of water flows over Niagara Falls each second, calculate the power available at the bottom of the falls.
3)A. Calculate the kinetic energy of a 3kg toy cart that moves at 4 m/s. B. Calculate the kinetic energy of the same cart at twice the speed.
4) A lever is used to lift a heavy load. When a 50 N force pushes one end of the lever down 1.2 m, the load rises 0.2m. Calculate the weight of the load.
5) a) In raising a 5000N piano with a pulley system, the workers note that for every 2m of rope pulled down, the piano rises 0.4m . How much force is required to lift the piano? B) If the workers actually pull with 2500N force to lift the piano, what is the efficiency of the pulley system?
CONCEPTUAL PHYSICS ENERGY
