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:

Four types:

1. Elastic potential energy

2. Chemical Potential Energy

• Stored by something that can stretch or compress• Energy is released when the object is allowed to

return to its normal state

The energy stored in motionless objects

• Chemical bonds store energy.• Breaking the bonds releases the energy

3. Electrical and Nuclear Potential Energy

4. Gravitational Potential Energy

• Electrical-stored energy due to the position of electrical

charges.• Nuclear-energy stored in the nucleus of an atom

• 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

2 kg

h = 10 m

GPE = mgh= (2 kg)(9.8 m/s2)(10 m) = 196 J

2 kg

KE = 0 J

GPE = 0 J

KE = ½ mv2 =196 J

While the energy has changed form, the amount of energy has not changed. Energy has been conserved.

3 kg

3 kg

3 kg

3 kg

3 kg

2 m

4 m

6 m

8 m GPE = (3 kg)(9.8 m/s2)(8 m) = 235 J

KE = 0

GPE = (3 kg)(9.8 m/s2)(6 m) = 176 J

GPE = (3 kg)(9.8 m/s2)(4 m) = 118 J

GPE = (3 kg)(9.8 m/s2)(2 m) = 59 J

GPE = 0 J

KE = 235 J - 176 J = 59 J

KE = 235 J - 118 J = 118 J

KE = 235 J - 59 J = 176 J

KE = 235 J

3 kg

3 kg

3 kg

3 kg

3 kg

2 m

4 m

6 m

8 mKE = 0

KE = 59 J

KE = 118 J

KE = 176 J

KE = 235 J

v = 0

= ½(3 kg)v2

v = (2*59/3) = 6.3 m/s

v = (2*118/3) = 8.9 m/s

v = (2*176/3) = 10.8 m/s

v = (2*235/3) = 12.5 m/s

Mechanical Energy = KE + GPE

GPE = maxKE = 0

GPE = 0KE = max

GPE = maxKE = 0

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.

H12

H13

+

He24

neutron

+ Energy