Chapter 5: Work and Energy

1
Chapter 5 Work and Energy

• Last time covered static and kinetic friction
• f lt or ms or k n

This time Work W F d
d
Example F 100 N, d 3 m. Then W 100 N 3
m 300 Nm 300 joules.
Now add a second worker How much work does the
2nd worker do? Zero only component of F along
motion does work.
d
2
Work and Energy (continued)
F
For a general direction of force, decompose
it Work F d cos q
F sin q
q
F cos q
d
3
Work and Energy (ccontinued)
v
v0
Relate work to change in velocity. If F is the
only horizontal force (no friction) the box
accelerates F ma, so a F/m
F
d
From kinematics, 2 a d v2 v02 . 2 F d / m
v2 v02
Work!
2 W / m v2 v02 . W ½ mv2 ½ mv02
D (½ mv2 ) W D (Kinetic Energy)
Call this kinetic energy
Work-energy theorem