1. Kinetic Energy Energy due to motion K = ½ m v2 Unit: Joules
K: Kinetic Energy
m: mass in kg
v: speed in m/s
Unit: Joules
Can an object have KE if it’s stationary?

2. Sample problem A 10.0 g bullet has a speed of 1.2 km/s.
What is the kinetic energy of the bullet?
What is the bullet’s kinetic energy if the speed is halved?
What is the bullet’s kinetic energy if the speed is doubled?
UNITS!
KE = .5(.01)(1200m/s)(1200m/s) = 7,200J
KE would be quartered; 1,800J
KE would be quadrupled; 28,800J

3. The Work-Energy Theorem
The net work due to all forces equals the change in the kinetic energy of a system.
Wnet = DK
Wnet: work due to all forces acting on an object
DK: change in kinetic energy (Kf – Ki)
W = Fd or W = mad

4. Sample problem
An 8.0-g acorn falls from a tree and lands on the ground 10.0 m below with a speed of 11.0 m/s.
What would the speed of the acorn have been if there had been no air resistance?
Did air resistance do positive, negative or zero work on the acorn? Why?
UNITS
14.01m/s
Air resistance did negative work – it lessened the total mechanical energy. Prove this with a W=Fdcos.

5. Sample problem
An 8.0-g acorn falls from a tree and lands on the ground 10.0 m below with a speed of 11.0 m/s.
How much work was done by air resistance?
What was the average force of air resistance?
c) .3J
d) .03N

6. Sample Problem
.3J
Area under the curve
How much work is done by the force shown when it acts on an object and pushes it from x = 0.25 m to x = 0.75 m?