chapter 12, section 1: what is work?

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

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.

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

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?

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

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.

Calculate power

• Power = work/time

• Power = (force x distance)/time

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