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Chapter 5 Work and Energy

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Chapter 5Work and Energy

- Work is done on an object only when a net force acts on the object to displace it in the direction of a component of the net force.
- Work = Force x displacement x cosΘ
- W=fd(cosΘ)
- Work is measured in Nm or Joules

When Force and Distance are in the same direction, cosΘ is 1 and W = Fd

A box is dragged across a floor by a 100N force directed 60o above the horizontal. How much work does the force do in pulling the object 8m?

Decide if work is done and if so, the sign of the work for each case:

a) A crane lifting a bucket of concrete

b) The force of gravity on the bucket being lifted

c) An athlete holds a weight up in a fixed position

d) An athlete lowers a weight slowly

e) A person pushes a book across the table.

- Power is the rate at which work is done.
- Power = work/elapsed time
- P = W/Δt
- The SI Unit for power is the watt (W) which equals one Joule per second (J/s)

- A 50 kg girl climbs a flight of stairs that is 5.0 m high. Calculate the power output if she takes 10.0 s to do this.
- Find the work done. (Recall that her force is equal to mg)
- 2. Calculate the power.

- Energy: The ability to do Work
- Potential Energy: Energy of position or stored energy
- ΔPE = mghwhere “h” refers to height.

- Compressed spring
- Bow pulled back in archery
- Stretched rubber band

- Kinetic Energy: the mechanical energy of motion. It is how much work an object is currently doing.
- KE = ½ mv2

- The SI unit for energy is the Joule. This is the same unit for work.
- When work is done on an object, energy is transformed from one form to another.
- The sum of the changes in PE, KE and heat energy are equal to the work done on the object.
- Mechanical energy is transformed into heat energy when work is done to overcome friction.

- When a string is stretched or compressed, it gains elastic potential energy.

- The force that pulls it back and attempts to restore the spring to equilbriumis the restoring force.
- PE = ½ kx2
- Elastic PE = ½ (spring constant)(distance compressed or stretched)2

- A spring with a force constant of 5.2 N/m has a relaxed length of 2.45 m. When a mass is attached to the end of a string, and allowed to come to rest, the vertical length of the spring is 3.57 m. Calculate the elastic potential energy of the spring (page 180 problem #1, Answer: 3.3 J)

- Law of Conservation of Energy: Energy cannot be created or destroyed.
- Total amount of ME in a system remains constant if no work is done by any other force besides gravity.
- Δ KE = ΔPE
- KE and PE before an interaction equals all the KE and PE after the interactionl
- KE0 + PEo = KEf + PEf

- Bo flings a 0.20 kg pool ball off a 0.68 m high pool table and the ball hits the floor with a speed of 6.0 m/s. How fast was the ball moving when it left the pool table?