Chapter 6 Energy and Oscillation

I. Energy, Work, and PowerA. Energy = ability to change the condition of matter

1) When you add energy to a system, something must change2) Heat is a form of Energy

a) Heat a beaker of waterb) Motion of the water as it boilsc) Vaporization of the water into steam

3) Push down a spring—the spring changes shape then snaps back4) Move a suspended pendulum, it starts swinging back and forth

B. Simple Machines = multiply the effect of an applied force1) Make it easier to change the condition of a system2) Lever

a) F1d1 = F2d2

b) Same Energyc) Smaller force over larger distance

20N1m

m)(100N)(0.2

d

dF FdF dF

? F 1m, d 0.2m, d 100N, F

1

2212221

1122

3) Pulley

a) Use half the force to move the object

b) Pull twice the length of rope

c) Same energy, less force

4) Mechanical Advantage =

C) Work1) Force applied and the distance moved tell us about what happens when we add

energy to a system

2) Work = W = Fd Units = N x m = Joule = J = units of Energy

3) Work is a form of energy and has units of Joules (J)

4) Only the component of the Force in the

direction of the movement counts

a) Since no movement vertically work

is done only horizontally

b) If you push on a wall that doesn’t move

have you done any work?

21

2

forceinput

forceoutput

W?

2 m

D) Power = measure of work done per unit time

1)

2) W = 200 J t = 10 s P = 200 J/10 s = 20 J/s = 20 Watts

3) 1 J/s = 1 Watt = 1 W 1000 W = 1 kW

4) 1 hp = 746 watts = 0.746 kW

II. Kinetic Energy = Energy associated with motionA) 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

workof ratet

WP

2

2

1mvKE

If velocity doubles, KE quadruples

Find W and KE for m = 100kg v = 2 m/s F = 50 N d = 4 m

W = weight = work = Watt

C) Negative Work

1) A car stopping is losing Kinetic Energy = negative Work

2) KE = negative = W

3) Friction opposing motion is the force slowing down the car (-f)

4) 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

dFKEW )(

3060 4WW

3060 4FdFd

3060 4dd

III. Potential EnergyA) 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?

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?

Friction and Heat