Definition of work
leads to units of energy
Energy
Work, Potential Energy and Kinetic
Energy
Work and Energy
|
“Useful work”
What is “work”?
| Work = Force × distance
| Work = (kg m/s2) × m
| 1 J = 1 N × 1 m = 1 kg (m/s)2
|
Work and Energy
|
“Useful work”
|
Potential energy
|
Energy is not
always in the
form of “useful
work”!
W=F×d
|
Potential energy
PE = F × d
z Gravity:
PE = mgh
W=F×d
PE = F × d
1
Ramps and Work
|
Work = ΔPE = mgh
Ramps and Work
|
|
Work = ΔPE = mgh
BUT Work = F × d
z Increasing
the distance decreases
the force required to move the object
W = ΔKE
Kinetic Energy
|
|
Kinetic energy vs
Potential Energy
Kinetic energy in
terms of mass and
velocity:
|
Suppose the cannon has a barrel length of 2 m and
the muzzle velocity of the 5-kg cannonball is 50 m/s.
z
z
What is the work done on the cannonball?
What is the force supplied by the gunpowder charge?
Ball-bounce
paradox
KE = ½ mv2
|
The Work-Energy
Theorem:
W = ΔKE
KE = 1/2 mv^2 (one-half m-v-squared)
W=Fd
2
W = ΔKE
|
|
Suppose that the car (500 kg) is
accelerated from zero to 5 m/s in 10 s.
What is the work done on the car?
W = delta-KE
KE = 1/2 mv^2
What is the work done
on the green railroad car?
1000 kg
1000 kg
W = delta-KE
KE = 1/2 mv^2
3