Science Starter!

How much money could a 55 kg boy earn generating energy for the electric company if

he can climb a 6.0 m tall flight of stairs in 4.0 s, and spends 10 hours climbing stairs at this

rate? (The electric company charges $0.11 per kWh)

Work – Energy Theorem

Kinetic Energy

A 5 kg dog is surfing, traveling at a speed of 2 m/s. How much kinetic energy does the dog possess?

K = ½ mv2

m = 5 kg

v = 2 m/s

K = ?

K = ½ mv2

K = ½ (5)(2)2

K = 10 J

Gravitational Potential Energy

A 70 kg bungee jumper is about to take the plunge from a 100 m

tall crane. How much gravitational potential

energy does the jumper possess?

Ug = mghm: 70 kgg: 9.8 m/s2

h: 100 mUg = ?

Ug = mgh

Ug = (70)(9.8)(100)

Ug = 68600 J

A boy is shooting a rock using a sling shot thathas a spring constant of 5 N/m. He pulls the string back 10 cm. How much elastic potential energy is stored in the slingshot?

Ue = ½ kx2

k = 5 N/m

x = 0.1 m

Ue = ?

Ue = ½ kx2

Ue = ½ (5)(.10)2

Ue = 0.025 J

Total Energy

Work – Energy TheoremExample:

A 20 kg bike traveling at 4 m/s coasts through a patch of dirt, slowing down to 1 m/s.

(a) How much work was done on the bike by the dirt in slowing it down? (b) How much force was exerted on the bike if the dirt stretched for 3 m?