7.2 Kinetic Energy
By the end of this section, you will be able to:
- Calculate the kinetic energy of a particle given its mass and its velocity or momentum
- Evaluate the kinetic energy of a body, relative to different frames of reference
The kinetic energy of a particle is one-half the product of the particle’s mass m and the square of its speed v:
|You observe two identical balls heading directly toward each other at equal speeds. What can you say about their total kinetic energy?|
|(a) They have some total amount of kinetic energy, with a magnitude between zero and double the kinetic energy of either ball by itself.|
|(b) They have zero total kinetic energy.|
|(c) They have twice the kinetic energy of either ball by itself.|
|(d) Nothing. The kinetic energy of separate objects cannot be combined.|
Consider how you would answer these questions. Then bring this to class for a group discussion.
One particle has mass m and a second particle has mass 2m. The second particle is moving with speed v and the first with speed 2v. How do their kinetic energies compare?
A particle of m has a velocity of . Is its kinetic energy given by ? If not, what is the correct expression?