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Gravitational potential energy is gained when an object is moved against gravity. The amount of energy depends on the object’s mass, height, and the strength of the gravitational field. The energy can be calculated using a formula and is used in hydroelectricity and counterweights.
Gravitational potential energy is gained by an object when it has been moved against a gravitational field. For example, an object lifted above the surface of the Earth will gain energy, which is released if the object is allowed to fall back to the ground. In order for an object to be lifted vertically upwards, it must work against the downward force of gravity. This work is then stored as gravitational potential energy. When the object is released and falls towards the Earth, the potential is converted into kinetic energy, or motion.
A pendulum is a good example of the relationship between gravitational potential and kinetic energy. At its highest point, the pendulum has only potential energy. As it descends, this is converted into kinetic energy, reaching a maximum at its lowest point, where it has no potential energy. As it swings again, the kinetics are converted into potential energy.
The amount of potential energy an object possesses depends on the mass or weight of the object, its height above the surface, and the strength of the gravitational field. If the other factors are the same, a heavy object will have more gravitational potential energy than a lighter object. An object 1 mile (1.6 km) in the sky will have more energy than the same object 1 foot (30.48 cm) from the surface. On the Moon, which has a weaker gravity field than Earth, an object will have less potential energy than the same object at a similar height above the Earth.
Calculation of gravitational potential energy
The potential energy of an object can be calculated as the mass of the object, times the gravitational force, times the height of the object above a given point. That point could be the surface of the Earth or it could be the floor of a room. In fact, the potential can be calculated for any point underneath the object.
Gravitational force is usually expressed as the acceleration experienced by an object dropped freely, ignoring the effects of air resistance or friction. Although the strength of the gravitational field on the earth’s surface varies from place to place, the variations are so small as to be almost negligible. In physics, therefore, the acceleration due to gravity near the surface of the Earth is generally considered to be a constant, with a value of about 32 feet (9.8 meters) per second per second (feet/s2 or om/s2). A simple potential energy formula for an object lifted off the surface of the earth could then be formulated as follows:
potential energy = mass of object in pounds × 32 × height of object in feet or potential energy = mass of object in kilograms × 9.8 × height of object in meters
This formula works well for objects close to the Earth’s surface. It can be easily adapted to cope with similar scenarios in other gravitational fields, for example on the Moon or Mars, by changing the value of the gravitational force accordingly. Since the strength of any gravitational field decreases with distance from its source, however, this formula will only work for objects relatively close to the surface of a gravity source, where the reduction in gravitational force is too small to matter. For objects relatively distant from a surface, the mass of the source of gravity and the distance from its center to the object must be taken into account.
it is used
Probably the most important use of gravitational potential energy is in hydroelectricity. Here, the potential energy of the water in a lake or reservoir located high above a power plant is harnessed to generate electricity. The water can descend from the tank, converting the potential into kinetic energy, and the movement of the water drives a turbine, which generates an electric current. During times of low demand, the power plant may have an excess of electricity, which can be used to pump water into the reservoir, storing more potential energy.
Another application is in counterweights, which are used in a number of mechanical devices. For example, some types of elevators employ a counterweight so that it descends when the elevator car ascends and vice versa. As the car climbs, the potential energy of the counterweight is converted into kinetic energy as it descends, so it helps lift the car, reducing the amount of work that has to be done by the motor driving the device. As the elevator car descends, aided by gravity, the counterweight is lifted, gaining potential energy.
