1. Physical Science Chapter 13
Work and Energy
Physical Science
Chapter 13

2. Work
Examples?
Scientific definition: Work is the transfer of energy through motion.
In order for work to take place, a force must be exerted through a distance.

3. Work
In order for work to be done, there has to be motion, and the motion has to be in the direction of the applied force.

4. Work Equation
Work, like energy, is measured in joules. 1 J = 1 N ∙ m.

5. Example
A student’s backpack weighs 10 N. She lifts it from the floor to a shelf 1.5 m high. How much work is done on the backpack?

6. You try
A dancer lifts a 400-N ballerina overhead a distance of 1.4 m and holds her there for several seconds. How much work is done on the ballerina?

7. You try
A carpenter lifts a 45-kg beam 1.2 m high. How much work is done on the beam?
Remember that weight equals mass times acceleration due to gravity.

8. Power
Power is the rate at which work is done.

9. Watts
Power is measured in watts, named after James Watt, who invented the steam engine.
1 W = 1 J/s
Very small unit, so we often use kW.

10. Discuss Define work and power. How are work and power related?
Determine if work is being done in the following situations:
Lifting a spoonful of soup to your mouth
Holding a large stack of books motionless over your head
Letting a pencil fall to the ground

11. Power
Power is the rate at which work is done.

12. You try
While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts?

13. You try
Anna walks up the stairs on her way to class. She weights 565 N, and the stairs go up 3.25 m vertically.
If Anna climbs the stairs in 2.6 s, what is her power output?
What is her power output if she climbs the stair in 10.5 s?

14. Machine
A device that makes work easier

15. Work and machines
Work is done when a force is exerted through a distance
Machines make work easier by changing the size or direction of the force, or both.
Opening a paint can with a screwdriver
Changes size – you can use less force
Changes direction

16. Mechanical advantage
The number of times a machine multiplies the input force

17. Examples
Find the mechanical advantage of a ramp that is 6.0 m long and 1.5 m tall.
Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a mechanical advantage of 5.2, how much force is exerted on the nail in the claw?

18. Conservation of energy
You can never get more work out of a machine than you put in
If force increases, distance must decrease.
Machines often allow you to use less force, but require you to exert that force over a larger distance.

19. Discuss
Describe how a ramp can make lifting a box easy without changing the amount of work that can be done.