What is the Difference Between Potential and Kinetic Energy?

Victoria Blackburn

The difference between potential and kinetic energy all comes down to a very simple property of the object. If an object is moving, then it has kinetic energy, or kinetic energy is the energy of movement. Potential energy is energy that is stored in an object and can be released under the right conditions. Therefore, the difference between these two types of energy comes down to whether an object is moving or not.

Kinetic energy is released as roller coaster riders descend over a hill.
Kinetic energy is released as roller coaster riders descend over a hill.

Many objects have potential energy that can be released in many different ways. Fuels and food contain potential energy, which is released as heat or chemical energy, respectively. Springs also contain potential energy. When a spring is released, the potential energy is released as kinetic energy. Another example of potential energy is gravitational potential energy.

Swinging on a swing is a good example of both potential and kinetic energy.
Swinging on a swing is a good example of both potential and kinetic energy.

Gravitational potential energy is a type of energy an object has due to it being raised above the ground. The amount of gravitational potential energy the object has as it is lifted can be determined by measuring the amount of work it took to get the object there using the height it was raised (h), its mass (m) and the gravitational pull of the earth (g), which is 10 N/kg. The formula for gravitational potential energy is GPE = mgh. For example, a 50 kg weight that is lifted 5 m would have a GPE of 50 x 5 x 10, or 2500 joules (J), the unit for energy.

If the weight used in the previous example were released, its gravitational potential energy would change into kinetic energy. The weight starts out with only gravitational potential energy while it is held above the ground. When it is released, it begins to accelerate towards the ground due to the gravitational pull. As it does, it will have both potential and kinetic energy. The closer it gets to the ground, the more of its potential energy is transferred to kinetic energy until it is about to hit the ground and all of the energy has turned into kinetic energy.

This is the principle behind conservation of energy in physics. Conservation of energy states that energy can neither be destroyed nor created and it is a fundamental law of physics. From above, initially, the gravitational potential energy was transferred from the work to lift the weight. Second, the kinetic energy was transferred from the stored gravitational potential energy of the lifted weight. In other words, the potential and kinetic energy are the same.

Kinetic energy is calculated using the mass (m) of an object and its velocity (v) in the following equation – KE = ½ mv2 (mass x velocity squared divided by 2). Using our above example, we already know the kinetic energy of the weight because the potential and kinetic energy are the same. The weight has a kinetic energy of 2500 J just before it hits the ground. We can use this to determine its velocity as it hits the ground, or its impact velocity. By rearranging the formula for kinetic energy, we get - v2 = 2KE/m or v2 = 2 x 2500 / 50 = 100, so the impact velocity of the weight is 10 m/s (the square root of 100).

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