Work and Energy

1. 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.