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I. Forms of Energy

Energy is one of the most important concepts in science. Simply defined, energy is “the ability to do work” and there exist several forms of energy. Potential energy is the energy associated with an object’s position or configuration and its surroundings. One common example of such energy is gravitational potential energy (PE = mass * gravitational acceleration constant * height), which stems from an object’s relative position to the earth. Thermal energy, also called heat, is the energy from the random vibrations of an object’s molecules. Other forms include electric energy, chemical energy, and nuclear energy.

However, we are primarily concerned with kinetic energy. From the Greek word kinetikos meaning “motion,” kinetic energy is the energy in the motion of an object. Thus, all objects or particles that are moving have kinetic energy. There are actually two forms of kinetic energy: rotational and translational or linear kinetic energy. Rotational kinetic energy is the kinetic energy of a body rotating about an axis, while translational kinetic energy is the energy of a body moving in a straight line. Various forms of energy can coexist in one body. For example, a thrown football has translational kinetic energy due to its forward displacement, potential energy because of its height off the ground, and rotational kinetic energy because of it is spinning.

Translational kinetic energy is calculated by the following formula:

Where m = the body’s mass and v = the body’s velocity. The SI units for this calculation are kilogram (kg) for mass, meters per second (m/s) for velocity, and Joules (J) for energy. 1 Joule = 1 kg * m²/s²

Work through the following problems to gain a better understanding of translational energy.

1. Calculate the kinetic energy in Joules of the following:

a. Shaquille O’Neil, weighing 315 lbs (143 kg), running down a basketball court at a speed of 18 mph (8.1 m/s)

b. A marathon runner 150 lbs (68 kg) running down a street at 18 mph (8.1 m/s)

c. A 5 ounce (0.14 kg) baseball tossed into a glove at a speed of 35 mph (15 m/s)

2. If a 0.05 kg bullet has a kinetic energy of 500 J, how fast is it going in m/s?

3. Calculate the speed in m/s a grain of sand (0.0001kg) must travel in order for it to have the same amount of kinetic energy as the baseball in problem 1.c above.