PHYS 2211 Module 7.2

Kinetic Energy

Recommended Reading

7.2 Kinetic Energy

Learning Objectives

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

Kinetic Energy

The kinetic energy of a particle is one-half the product of the particle’s mass m and the square of its speed v:


Practice 7.2.1
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?