University of Wisconsin Green Bay

You throw a 0.14 kg baseball 2.7 m into the air. How much does its gravitational potential energy change as it goes up?

  • In this problem, you are given the change in height of a baseball and asked for the change in its gravitational potential energy. Potential energy is defined as the energy of position as related to a conservative force (such as gravity), 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 height information asked for the closely-related gravitational potential energy. A picture will not provide any additional insight or organization beyond what is already present in the problem.

  • In equation form, gravitational potential energy is defined as

    PEg = mgh

    This is the only relation you need for this problem.

  • There is no further calculation required in this problem.

  • This problem is merely a definition problem. You will use the definition of potential energy in Conservation of Energy problems, much as you use Fg = mg when you solve force problems. Whenever you have information about the height of an object, you can restate that information as its gravitational potential 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. In this case, the weight (mg) of the baseball is (0.14 kg)(9.8 m/s2) or 1.4 N and so your answer makes sense.