Peoples Physics Concepts

8.2. Kinetic Energy http://www.ck12.org

h=

(ωr)^2

2 g

h=

(15 rad/s∗.5 m)^2

2 ∗ 9 .8 m/s^2

h= 2 .9 m

Explore More

1. A bomb with 8 x 104 J of potential energy explodes. Assume 20% of its potential energy is converted to
   kinetic energy of the metal pieces flying outward (shrapnel).
   a. What is the total kinetic energy of the shrapnel?
   b. Assume the average mass of the shrapnel is 0.4 kg and that there are 200 pieces. What is the average
   speed of one piece?


2. A 1500 kg car starts at rest and speeds up to 3.0 m/s.
   a. What is the gain in kinetic energy?
   b. We define efficiency as the ratio of output energy (in this case kinetic energy) to input energy. If this
   car’s efficiency is 0.30, how much input energy was provided by the gasoline?
   c. If 0.15 gallons were used up in the process, what is the energy content of the gasoline in Joules per
   gallon?


3. An airplane with mass 200,000 kg is traveling with a speed of 268 m/s.
   (a) What is the kinetic energy of the plane at this speed?
   A wind picks up, which causes the plane to lose 1. 20 × 108 J per second.
   (b) How fast is the plane going after 25.0 seconds?

Answers to Selected Problems

1. The answers are:
   a. 1. 2 × 106 J
   b. 20m/s


2. The answers are:
   a. 6750J
   b. 2. 25 × 105 J
   c. 1. 5 × 105 J/gallon of gas


3. a. 7. 18 × 109 J