Chapter 8

Work, Power and Energy

• Work- applying a force over a distance; must be in the same direction

• Work=Force x Distance W=Fd

• Unit=Joule=Nm=kgm2/s2

• Power-rate of doing work• Power=Work/Time P=W/t• Power=Force x Distance/Time

• Unit=Watt

• Energy- ability to do work

• Mechanical Energy- energy due to motion or position of an object– Kinetic Energy- energy of a moving object– Kinetic Energy=1/2 mass x velocity2 KE=1/5mv2

– Potential Energy- stored energy– Potential Energy=mass x height x acceleration due to

gravity PE=mgh

• Law of Conservation of Energy- energy cannot be created or destroyed, but can be transformed from one form to another

• Machines- devices used to multiply forces or simply to change direction of forces– Simple Machines:

Incline Planes LeversScrew PulleyWedge Wheel and Axle

Review Questions

1. A constant force of 20.0 N applied to an object causes it to move at a constant speed of 4.0 m/s. How much work is done on the object in 6.0 s?

2. A 10.0 kg object and a 5.0 kg object are released simultaneously from a height of 50.0 m above the ground. After falling freely for 2.0 s, the objects have different:

-accelerations-velocities-kinetic energies-displacements

3. If 1375 J of work is done on a mass of 20.0 kg, it can be raised ________ m.

4. Work is not done when you-pick up a box and place it on your shoulder-support the box on your shoulder-carry the box a distance of 15 m-put the box on a shelf

5. A person raises a 2.0 kg mass from the floor to 1.5 m above the floor in 5.0 s. If the person performs the same task again in 10.0 s instead of 5.0 s, what has changed?

-the amount of work done-the total amount of energy involved-the gravitational potential energy involved-the height-the power

6. A 1.0 kg pendulum is released from a height of 3.2 m vertically from a reference level. Its potential energy is _____.

7. A 5 kg cart is traveling at 12 m/s. What is its kinetic energy?

8. Name 6 simple machines.

9. Who is Rube Goldberg?

10. Compare and Contrast Kinetic and Potential Energy.