Which of these is exhibiting kinetic energy?
The object that is moving is exhibiting kinetic energy, the energy of motion, such as a rolling ball, a running person, or falling water. Stationary or stored-energy examples show potential energy, not kinetic energy.
The answer
Kinetic energy is the energy an object has because it is moving, so the correct choice is always the example that is in motion. A rolling ball, a moving car, a running person, flowing water, or a swinging bat all exhibit kinetic energy. If an object is sitting still, it has zero kinetic energy at that instant, no matter how much stored (potential) energy it holds.
The defining test is simple: Is it moving? If yes, it has kinetic energy. If it is only positioned to move later, or storing energy in some other form, that is potential energy instead.
The formula and what it tells you
Kinetic energy is calculated with:
KE = 1/2 x m x v squared
where m is mass (kg) and v is speed (m/s), giving energy in joules. Two things follow from this equation:
- If v = 0 (the object is stationary), then KE = 0. This is why a book resting on a shelf has no kinetic energy.
- KE depends on the square of speed. Doubling the speed quadruples the kinetic energy. A car at 40 mph carries four times the kinetic energy it had at 20 mph, which is why stopping distances grow so quickly.
Worked example: a 2 kg ball rolling at 3 m/s has KE = 1/2 x 2 x 3^2 = 9 joules. Speed it up to 6 m/s and KE = 1/2 x 2 x 36 = 36 joules, four times as much.
Why the other options are wrong
Typical distractors on this question are things that are not moving:
- A book on a shelf or a rock at the top of a hill has gravitational potential energy (stored because of height), but no kinetic energy until it starts to fall.
- A stretched rubber band or compressed spring holds elastic potential energy, again stored, not motion.
- A battery or a piece of food holds chemical potential energy.
- A parked car has zero kinetic energy even though its engine could produce motion later.
All of these become kinetic only once movement actually begins. The moment the rock falls or the spring releases, potential energy converts into kinetic energy.
The bigger picture
Energy is constantly converting between potential and kinetic forms while the total stays conserved. A roller coaster is the classic illustration: at the top of the first hill it has maximum potential energy and (briefly) minimum kinetic energy; as it plunges, potential converts to kinetic and it moves fastest at the bottom. Recognizing which example is moving, versus merely storing energy, is the whole skill this question tests.
Which of these is exhibiting kinetic energy?
Frequently asked
What is kinetic energy?
Kinetic energy is the energy an object possesses because of its motion. Any moving object, from a rolling ball to a flying plane, has kinetic energy, and it is calculated as one half times mass times velocity squared.
What is the difference between kinetic and potential energy?
Kinetic energy is energy of motion, present only when an object is moving. Potential energy is stored energy due to position, shape, or chemical composition, present even when an object is still. Potential energy can convert into kinetic energy when the object starts to move.
What is the formula for kinetic energy?
The formula is KE = 1/2 x m x v squared, where m is mass in kilograms and v is speed in meters per second, giving energy in joules. Because speed is squared, doubling the speed multiplies kinetic energy by four.
Does a stationary object have kinetic energy?
No. A stationary object has zero kinetic energy because its velocity is zero, and plugging v = 0 into the formula gives KE = 0. It may still hold potential energy, but not kinetic energy until it moves.