University of Wisconsin Green Bay

A woman with a mass of 63 kg is running at a speed of 4.8 m/s. What is her kinetic energy?

  • In this problem, you are given the speed of a person and asked for her kinetic energy. Kinetic energy is defined as the energy of motion, so you aren’t asked for new information in this problem but rather to restate what you know about motion in a slightly different way.

  • There is no need for a picture in most definition problems, and this is one of them. You are given velocity information at one point and asked for the closely-related kinetic energy information at that same point. A picture will not provide any additional insight or organization beyond what is already present in the problem.

  • In equation form, kinetic energy is defined as

    KE = ½ mv2

    This is the only relation you need for this problem.

  • KE = ½ mv2
    KE = ½ (63 kg)(4.8 m/s)2
    KE = 730 J

    There is no further calculation required in this problem.

  • KE = ½ mv2
    KE = ½ (63 kg)(4.8 m/s)2
    KE = 730 N

    This problem is merely a definition problem. You will use the definition of kinetic energy in Conservation of Energy problems, much as you use Fg = mg as you solve force problems. Whenever you have information about the speed of an object, you can restate that information as its kinetic energy (or vice versa.)

    The MKS unit of energy is the Joule. For context, one Joule is the amount of energy required to lift a one N (0.22 lb) object one meter (3.3 ft) in the air.