EnergyBy. Jonathan Lee and Harry

Chun

What is “energy”?

Energy is the ability to do work

Potential Energy (PE) is the “possible” ability to do work

Kinetic Energy (KE) is how much energy a moving object has

Gravitational Potential Energy

The potential energy gained or lost as a result of changing it’s vertical position.

PE= W= Fd

→ F= W= mg

→ d= height

→ PE= mgh

Elastic Potential Energy

The amount of energy stored in a spring when it is compressed or elongated.

PEs = (kx²) / 2

Kinetic Energy

KE = (mv²) / 2

Practice Question # 1

A 50kg tire is rolling at a speed of 20 m/s.Assuming that there is no friction, what is thekinetic energy of the tire?

Solution: KE= (mv²) / 2 KE= ((50kg)(20m/s) ²)/2KE= (20000kgm²/s²)/2KE= 10000J

Practice Question #2

Compared to the potential energy of a 25kg box on top of a

50cm shelf, the potential energy of a 50kg box on top of a

25cm shelf is…

Solution:

Box 1 Box 2

PE= mgh PE= mgh

PE= (25kg)(g)(50cm) PE= (50kg)(g)(25cm)

PE= 12262.5J PE= 12262.5J

The potential energy is the same

Practice Question #3

A spring, with a spring constant of 10Nm, is elongated to a

length of 0.5m. If the spring is elongated 1.0m more, the

potential energy of the spring will increase by…

Solution:

PEs= (kx²) /2

PEs= (k(2x)²) /2

PEs= (4kx²)/2

PEs= 4 * (kx²) /2

Answer: 4 times

Homework

Work, power, energy sheet

# 138, 145, 149

BONUS QUESTION

A 0.5kg bullet is traveling at 20m/s. It then hits a1kg wooden block that is hanging on a stringattached to the ceiling. As a result, the bullet getsembedded in the block, and the wooden blockswings upward. Assuming that the string has nomass and there is no friction between the stringand the ceiling, what is the maximum heightreached by the wooden block?

GOOD LUCK ON MIDYEAR!