Forces & Motion

1. Forces & Motion Work

2. Work • Work – force exerted on an object that causes it to move some distance • Work = Force x Distance • Force is still measured in Newtons • Distance is measured in meters • So…. the units for work are N٠m • Instead of using the N٠m, we have a new unit called the Joule to measure work in.

3. Work • In order for me to do work on an object, the object must move some distance as a result of my force. • Example: • A car is stuck in the snow and you exert a lot of force shoveling, pushing, and trying to get it to move. • Since the car doesn’t end up moving, you haven’t done any work on the car!

4. Work • In order for me to do work on an object, the force I exert must be in the same direction as the object’s motion. • Example: • Lifting a heavy box is work, but carrying the box to another location is not (even though it feels like it!) Force Motion Force Motion

5. Calculating Work • Which do you think involves more work: • Exerting a force of 100 N to lift a potted tree 1 m off the ground or • Exerting a force of 200 N to lift a heavier tree 1 m off the ground? • Again, work = force x distance • 100 N x 1 m = 100 J of work • 200 N x 1 m = 200 J of work

6. Practice Problems • To help rearrange the furniture in your classroom, you exert a force of 20 N to push a desk 10 m. How much work do you do? *work = 20 N x 10 m = 200 J • A hydraulic lift exerts a force of 12,000 N to lift a car 2 m. How much work is done on the car? *work = 12,000 N x 2 m = 24,000 J

7. Practice Problems • You exert a force of 0.2 N to lift a pencil off the floor. How much work do you do if you lift it 1.5 m? *work = 0.2 N x 1.5 m = 0.3 J • Suppose an elevator lifted the previous tree 40 m still with a force of 100 N. How much work is the elevator doing? *work = 100 N x 40 m = 4,000 J