Energy
• Every observable change requires energy.
• Energy comes in several different forms (food,
electrical, solar, chemical), and can be converted
from one form to another.
Kinetic energy
• Energy associated with motion
• KE = ½ mv2
• units are Joules
• mass MUST be in kg
• velocity MUST be in m/s
Example:
What is the kinetic energy of a baseball moving at a
speed of 40 m/s if the baseball has a mass of 0.15
kg?
KE = ½ mv2
KE = ½ (0.15 kg)(40 m/s)2
KE = 120 J
Potential Energy: The energy stored in motionless
objects
Four types:
1. Elastic potential energy
• Stored by something that can stretch or compress
• Energy is released when the object is allowed to
return to its normal state
2. Chemical Potential Energy
• Chemical bonds store energy.
• Breaking the bonds releases the energy
3. Electrical and Nuclear Potential Energy
• Electrical-stored energy due to the position of electrical
charges.
• Nuclear-energy stored in the nucleus of an atom
4. Gravitational Potential Energy
• Anything that can fall has stored GPE
• GPE = mgh
• m = mass in kg
• g = gravitational acceleration (9.8 m/s2)
• h = height above the ground
• The higher above the ground something is, the greater the GPE
EX: Find the height of a baseball with a mass of 0.15 kg that
has a GPE of 73.5 J.
GPE = mgh
73.5 J = (0.15 kg)(9.8 m/s2)h
h = 50 m
Conservation of Energy: Energy is neither created
nor destroyed as is changes from one form to
another
GPE = mgh= (2 kg)(9.8 m/s2)(10 m) = 196 J
2 kg KE = 0 J
While the energy has
changed form, the amount
of energy has not changed.
Energy has been
conserved.
h = 10 m
2 kg
KE = ½ mv2 = 196 J
GPE = 0 J
8m
6m
4m
2m
3 kg
GPE = (3 kg)(9.8 m/s2)(8 m) = 235 J
KE = 0
3 kg
GPE = (3 kg)(9.8 m/s2)(6 m) = 176 J
3 kg
GPE = (3 kg)(9.8 m/s2)(4 m) = 118 J
KE = 235 J - 118 J = 118 J
3 kg
GPE = (3 kg)(9.8 m/s2)(2 m) = 59 J
KE = 235 J - 59 J = 176 J
3 kg
GPE = 0 J
KE = 235 J
KE = 235 J - 176 J = 59 J
8m
6m
4m
2m
3 kg
KE = 0
v=0
3 kg
KE = 59 J = ½(3 kg)v2
v = (2*59/3) = 6.3 m/s
3 kg
KE = 118 J
v = (2*118/3) = 8.9 m/s
3 kg
KE = 176 J
v = (2*176/3) = 10.8 m/s
3 kg
KE = 235 J
v = (2*235/3) = 12.5 m/s
Mechanical Energy = KE + GPE
GPE = max
KE = 0
GPE = max
KE = 0
GPE = 0
KE = max
Q: Why does a real pendulum stop swinging?
A: Because friction converts some of the mechanical energy
into thermal energy.
E = mc2
During nuclear fusion, some of the mass of the
fusing nucleii is transformed into energy.
4
He
2
+
2
H
1
+ Energy
3
H
1
neutron