Potential and Kinetic Energy

An object does not have to be moving to have energy. Some objects have energy because of their shape or position. An archer uses energy to pull back on the string of the bow, and changes its shape. At this point, the bow and string have potential energy. When you lift a box, you use energy that works against the force of gravity, and changes the position of the box. The held box has potential energy due to its position.

The potential energy of an object due to gravity depends on the object’s height and weight. An increase in either of these will increase the object’s potential energy. A skier at the top of a small slope has less potential energy than a skier of the same weight who is at the top of a higher slope. Similarly, if two skiers are at the top of the same slope, the skier who weighs more has more potential energy. When these skiers begin skiing down the hill, their potential energy changes to kinetic energy.

To calculate gravitational potential energy, multiply height times weight. The box is at a height of 1.5 meters and weighs 40 Newtons. The box’s potential energy equals 60 joules. The skier is at a height of 50 meters and weighs 500 Newtons. The skier’s potential energy equals 25,000 joules.

Kinetic energy is the energy of motion. Objects in motion have the ability to do work. Wind in motion can move rocks across the surface of the Earth. A hammer in motion can drive a nail into a board. The amount of kinetic energy an object has depends on its mass and velocity. An increase in either of these will increase the object’s kinetic energy.

The equation for calculating kinetic energy is one-half times mass times velocity squared. A ball that weighs .3 kg and is moving at 10 m/s has the kinetic energy of 15 joules. Velocity has more of an impact on an object’s kinetic energy than mass. If the ball’s mass is doubled to .6 kg, then the kinetic energy is doubled. However, if you double the velocity of the ball to 20 m/s, then the ball’s kinetic energy quadruples.