14 Energy Associated with Motion – Kinetic Energy

Brokk Toggerson

In the last chapter, we described energy as the capacity to do work: the capacity to push or pull on an object for some distance. We also saw that objects in motion intrinsically have such a capacity. We call this energy due to motion kinetic energy. Kinetic energy depends on two main factors: the of the object, and its speed. Speed is more important than mass. Moreover, kinetic energy is always positive and independent of the direction of motion.

Key Takeaways

The kinetic energy of an object in motion depends on two things:

  1. How much stuff is in the object: its mass m.
  2. How fast it is moving: its speed v.

In addition, there are three other properties of kinetic energy you should know:

  • Speed is more important than mass.
  • Kinetic energy is always positive.
  • Kinetic energy does not depend on the direction of motion.

Example: Car and a Mosquito


A car and a mosquito are both traveling at the same speed when they run into each other. Which has more kinetic energy?


Kinetic energy depends on the speed and the mass. Since the car and the mosquito are traveling at the same speed, whichever has more mass will have more kinetic energy. In this case the car.

Example: Lions and Tortoises


A giant tortoise and a male lion have similar masses with the male lion being slightly lighter: \sim 180 \, \mathrm{kg} for the lion and \sim 200 \, \mathrm{kg} for the tortoise. Which do you think has more kinetic energy at their maximum speed?


A male lion almost certainly has more kinetic energy at his maximum speed. While the lion is a little bit lighter, it travels MUCH faster (74.1 kph for the lion vs. 0.3kph for the tortoise!). Since speed is more important than mass for kinetic energy, the lion almost certainly has more kinetic energy.

Example: Running


Dr. Toggerson is out for a run. He currently runs at 9.5 min/mi (2.8 m/s). He begins his run by traveling due east. At the midway point, he turns around and runs home. During which leg is his kinetic energy the greatest?


The kinetic energy is the same on both legs: the way out and the way back. In both cases, his speed is the same and his mass (presumably!) does not change appreciably over the course of the run.


  • Categories of energy.
  • Kinetic energy definition.
  • How important are the variables that affect kinetic energy?
  • Potential energy examples; kinetic energy examples (second part of the question).



Share This Book