Understanding Work and Power in Physics

1. Chapter 12, Section 1: What is work? • Key concepts: When is work done on an object? How do you determine the work done on an object? What is power? • Key terms: work, joule, power

2. Meaning of work • Work is done on an object when the object moves in the same direction in which the force is exerted. • Examples: pushing a child on a swing, pulling books out of a bookbag • No work is done without motion. • You might exert a force, but no work is being done if the object doesn’t move. • Force in the same direction • To do work, the force must be in the same direction as the object’s motion • Carrying something is vertical, even if you walk with the object.

3. Take note: work is done on the left but not the right. Why??

4. Calculating work • The amount of work done on an object can be determined by multiplying force by distance. • Work = force x distance • What are the units? • What do we measure force in? • Newtons • What do we measure distance in? • Meters • So if work = force x distance, what will our units be?

5. Units for work: • Joule (pronounced JOOL): 1 N x M = amount of work that 1 newton of force does to move an object 1 meter

6. Power • Power is the rate at which the work is done • Power equals the amount of work done on an object at a given time. • Both of those statements are correct; write down whichever one is easier for you to remember. • It takes more power to run up the stairs with your backpack than to walk because it takes you less time.

7. Calculate power • Power = work/time • Power = (force x distance)/time

8. Units of Power • Work is measured in joules and time in seconds, so the SI unit of power is the joule per second j/s • This is also known as the Watt, in honor of James Watt, who made improvements to the steam engine. • 1 j/s = 1 W