Chapter 6 Energy and Oscillation
I.
Energy, Work, and Power
A.
Energy = ability to change the condition of matter
1) When you add energy to a system, something must change
2) Heat is a form of Energy
a) Heat a beaker of water
b) Motion of the water as it boils
c) Vaporization of the water into steam
3) Push down a spring—the spring changes shape then snaps back
4) Move a suspended pendulum, it starts swinging back and forth
B.
Simple Machines = multiply the effect of an applied force
1) Make it easier to change the condition of a system
2) Lever
a) F1d1 = F2d2
b) Same Energy
c) Smaller force over larger distance
F2  100N, d 2  0.2m, d1  1m, F1  ?
F1d 2  F2 d 2  F1 
F2 d 2 (100N)(0.2m)

 20N
d1
1m
3)
4)
Pulley
a) Use half the force to move the object
b) Pull twice the length of rope
c) Same energy, less force
output force 2
 2
Mechanical Advantage =
input force
1
C) Work
1)
2)
3)
4)
Force applied and the distance moved tell us about what happens when we add
energy to a system
Work = W = Fd
Units = N x m = Joule = J = units of Energy
Work is a form of energy and has units of Joules (J)
Only the component of the Force in the
direction of the movement counts
a) Since no movement vertically work
W?
is done only horizontally
b) If you push on a wall that doesn’t move
have you done any work?
2m
D) Power = measure of work done per unit time
1)
W
P
2)
3)
4)
t
 rate of work
W = 200 J
t = 10 s
P = 200 J/10 s = 20 J/s = 20 Watts
1 J/s = 1 Watt = 1 W
1000 W = 1 kW
1 hp = 746 watts = 0.746 kW
W = weight = work = Watt
II. Kinetic Energy = Energy associated with motion
A) Work involves transfer of energy to a moving object (from you to a box)
B) W = Energy transferred = F x d implies motion
1) F causes acceleration
2) As the velocity increases, distance is
covered at a faster rate
3) Apply same F, but distance increases
4) Energy increases with velocity
1
KE  mv 2
2
Find W and KE for m = 100kg v = 2 m/s F = 50 N d = 4 m
If velocity doubles, KE quadruples
C)
Negative Work
1) A car stopping is losing Kinetic Energy = negative Work
W  KE  ( F )d
2)
3)
4)
KE = negative = W
Friction opposing motion is the force slowing down the car (-f)
Stopping distance
a) Remember W = KE directly proportional to v2
b) Double velocity, we quadruple work required to stop
c) Braking power (friction) is constant
d) A car going twice as fast takes 4 times farther to stop
W60  4W30
Fd60  4Fd30
d 60  4d30
III. Potential Energy
A) Lifting a weight up to a higher position
1) We have performed work on the box
2) KE at the end is still = 0 if its not moving
3) What happened to the Energy we transferred?
Friction and Heat
B) Gravitational Potential Energy = stored energy
depending on how far from the Earth’s surface an object is
1) W = F x d = PE = (weight)(height) = mgh
2) PE = mgh
3) Work is performed by pulling against the force of gravity
D) What happens when we drop the box?
1) Potential Energy is turned into Kinetic Energy
2) Call ground h = 0
PE = mgh = 0
3) Box 6.3 PE? m = 100 kg, h = 2 m
KE?