Energy The capacity to do work Work n

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Energy The capacity to do work

Energy The capacity to do work

Work n Work = Force × displacement n Units: F(N) × d(m) = N·m

Work n Work = Force × displacement n Units: F(N) × d(m) = N·m 1 N·m = 1 Joule = 1 J And remember, 1 N = 1 kg·m/s 2 so 1 J = 1 kg·m 2/s 2

Energy n Energy is the capacity to do work When we do work on

Energy n Energy is the capacity to do work When we do work on an object, we change its energy n Energy is conserved! It may be n transferred, it may be transformed, but it is never created or destroyed

Energy n We will consider two forms of energy n Potential Energy, PE, is

Energy n We will consider two forms of energy n Potential Energy, PE, is stored energy PE = mgh Notice that mgh is weight times height. In other words, a force (weight) times a displacement (the height).

Potential Energy n If we lift a 10 kg box to a height of

Potential Energy n If we lift a 10 kg box to a height of 3 m, by how much does its potential energy change? PE = mgh = (10 kg)(9. 8 m/s 2)(3 m) = 294 J

Energy n Kinetic energy, KE, is the energy of motion KE = ½mv 2

Energy n Kinetic energy, KE, is the energy of motion KE = ½mv 2 Notice the units work! kg × (m/s)2 = J

Energy n The total energy of an object is the sum of its potential

Energy n The total energy of an object is the sum of its potential and kinetic energy Etotal = PE + KE

Etotal = PE + KE

Etotal = PE + KE

Example n A 20 kg object is at rest at the top of a

Example n A 20 kg object is at rest at the top of a 5 m high frictionless hill. n n What is its potential energy? What is its kinetic energy? What is its total energy? The object slides down the hill. As it slides, what happens to Etotal? KE? PE? its speed?

Example n When the object reaches the bottom of the hill n n what

Example n When the object reaches the bottom of the hill n n what is is its its total energy? potential energy? kinetic energy? speed?

Example n When the object was sliding down the hill and was at a

Example n When the object was sliding down the hill and was at a height of 2 m n n What was was its its total energy? potential energy? kinetic energy? speed?

Example n If we drop a 2 kg rock from a height of 30

Example n If we drop a 2 kg rock from a height of 30 m, how fast will it be going when it reaches the ground? Ignore air resistance.

Example n A 5 kg object is sliding 2 m/s at the top of

Example n A 5 kg object is sliding 2 m/s at the top of a frictionless hill that is 3 m high. n n n What is the object’s total energy? What will be the object’s speed at the bottom of the hill? The object then slides up another frictionless hill that is 1. 2 m high. How fast will it be going at the top of this hill?

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