1. Energy

2. Work – The Physics definition:
The act of adding energy to a system
Can only occur when you are applying a force to an object AND causing it to move.
Work is only done when the force is in the same direction as the direction of travel for the object.

3. You are holding a 50-lb box for your cousin
You are holding a 50-lb box for your cousin. He leaves you standing there for 5 minutes. Are you doing any work?
No! Not according to our definition

4. You carry a 50-lb. box from your locker in the senior bank to the world language office. Are you doing any work?
No! The force you are applying is upward! The displacement is sideways. Work only happens when the force and the displacement are in the same direction.

5. Your teacher pushes as hard as she can against the wall for an hour
Your teacher pushes as hard as she can against the wall for an hour. Is she doing any work?
No. The wall is not moving.

6. Joules (J) = (Newtons)(meters)
To calculate work:
W = F(d)
F = force
d = displacement
Units for work:
Joules (J) = (Newtons)(meters)

7. Example Problem:
How much work is done on a vacuum cleaner pulled 3.0 m by a horizontal force of 50.0 N?

8. Power Definition: Mathematically:
Power is the rate at which work gets done
Mathematically:
P = W/t
The units are N·m/sec or J/s or Watts (W)
Examples:
Horsepower Lab
Since , Power also =

9. Example Problem:
An electric motor lifts an elevator 9.00 m in 15.0 s by exerting an upward force of N. What power does the motor produce?

10. Energy Kinetic Energy Definition: Mathematically:
The ability of an object to produce a change in itself or the world around it.
Or: The ability to do work.
Kinetic Energy
The energy resulting from motion (KE)
Mathematically:
KE = ½ mv2

11. Example Problem:
A 7.00-kg bowling ball moves at 3.00 m/s. How fast must a 2.45-g ping pong ball move in order to have the same kinetic energy as the bowling ball? Is this speed reasonable for a ping pong ball?

12. Example problem:
A car has a kinetic energy of 4.32 x 105 J when traveling at a speed of 23 m/s. What is the mass of the car?

13. Gravitational Potential Energy
Definition:
The energy an object has just because of its position above the Earth
Mathematically:
PEg = mgh
m = mass (kg)
g = gravitational constant 9.8 N/kg
h = height above zero level (m)

14. Example problem:
We’re storing a 7-kg bowling ball on a shelf that is 2 m above the ground. How much GPE does it have?