What Is the Energy Being Used When a Bike Is Coasting Down a Hill?

Energy is the ability to do work. In the case of a bicycle coasting down a hill, the force that is doing work on your bike is gravity. Gravity itself is not a form of energy; however, gravity combined with the height of the hill and the total mass of you and your bike creates potential energy. Potential energy, about to be converted into kinetic energy, is the first form of energy you and your bike experience when you're standing at the top of a hill preparing to coast to the bottom.

Kinetic Energy

“Energy” is the capacity of a system to do work, according to “Taber’s Cyclopedic Medical Dictionary, 19th Ed.” All forms of energy are classified as either kinetic or potential energy or some combination of the two. Kinetic energy is the energy of movement; moving objects perform work by imparting movement to other matter. A coasting bicycle has kinetic energy since it is moving down the hill.

Potential Energy

Potential energy is energy that matter possesses because of its location or structure. A motionless object, no less than a moving one, can have energy so long as it has the capacity to impart motion to other matter. Before it begins coasting downhill, a bicycle at rest on the hilltop has energy — potential energy. In descending, the bicycle converts its potential energy into kinetic energy.

Gravity

Pondering the movement of a bicycle coasting downhill, you might at first think that gravity is doing the work and is therefore the energy involved. But gravity is not energy; it is a force. Specifically, it is the force exerted by all objects having mass on all other objects having mass. Strictly speaking then, it is not just gravity but also the bicycle’s mass and its location — at the hilltop — which gives the bicycle potential energy that the bicycle’s descent converts into kinetic energy.

Momentum

A bicycle coasting downhill develops momentum. “Momentum” is the mass of an object times its velocity, according to Zona Land Education, an online source of scientific and mathematical knowledge. As the bicycle rolls downhill, how much its velocity increases will depend not on its energy but on two other factors: how steep the hill is and how much friction the hill’s surface presents to the bike. The bicycle’s momentum will increase faster down a steep slope than down a shallow one and faster down a smooth slope than down one marked by ruts and bumps.