Natural Forces and Physics
The Physics of Braking

Potential & Kinetic Energy: The Physics of Braking & Traffic Collisions

Updated Nov. 2, 2019

In simple terms, energy is the capacity to do work. The more energy something has, the longer or harder it can work.

There are many different forms of energy, including:

  • Thermal (heat) energy, such as that produced in your engine when your vehicle burns fuel.
  • Radiant (light) energy, such as that emitted by your car’s headlamps.
  • Kinetic (movement) energy, such as that produced when your vehicle is in motion.

In this module, we will discuss kinetic energy and the effect it has on your vehicle.

Potential energy

Before energy is expressed in any form (kinetic, thermal or otherwise), it exists as potential energy. All objects possess kinetic energy while they are in motion. When an object is stationary, that kinetic energy is stored as potential energy. For instance:

  • Draw an arrow back against the string of a bow and the tension it creates is potential energy. When you release the arrow, it becomes kinetic.
  • Park a car on an incline and the gravity working to pull it downward creates potential energy. This becomes kinetic energy when the car begins to roll.

Some objects have more potential energy than others. A car parked on a hill has more potential energy than a car parked on a flat surface, just as an arrow pulled against a very tense bow string has more potential energy than an arrow against a slack bow string. The more potential energy an object has, the harder it is to stop it from moving.

Kinetic energy

Kinetic energy is produced by motion. In the driving example above, the only thing keeping the vehicle’s potential energy from becoming kinetic energy is the car’s brakes. Should the brakes fail, the car will begin to roll and gather kinetic energy. The amount of kinetic energy an object has effects how easily it can be stopped. As speed increases, so does kinetic energy. The car in our example would be harder to stop, the further it rolls down the hill.

Your vehicle’s total kinetic energy will be influenced by outside forces. For example, a car traveling uphill will have less kinetic energy than a car traveling downhill, as gravity will be working against or with the vehicle, respectively.

Newton’s second law determines how much kinetic energy an object has before it is affected by outside forces, based on its weight and speed. However, these two factors do not increase kinetic energy equally; speed has a far greater influence. Kinetic energy increases proportionally to weight, so a car three times as heavy as another car would have three times the kinetic energy. Where speed is concerned, the rate at which kinetic energy increases is proportional to the object’s speed squared. Let’s illustrate this with a couple of examples:

  1. Car A and Car B are traveling at the same speed. However, Car A is twice the weight of Car B and therefore has twice the kinetic energy.
  2. Car C and Car D are the same weight. However, Car C is traveling twice as fast as Car D and therefore has four times as much kinetic energy.

Energy can never be destroyed; it can only be transferred from one object to another, transformed into a different kind of energy, or both. When your vehicle stops, the kinetic energy it has gained while in motion must go somewhere. If you are controlling the stop with your brakes, they will absorb the kinetic energy. When a vehicle stops abruptly, there is too much energy for the brakes to handle in the time allowed. With nowhere else to go, the kinetic energy will be absorbed by your vehicle’s body, the bodies of any people occupying the vehicle and any object you collide with. This is how physical injuries are sustained during car crashes.

The physics of braking

When slowing down or stopping, your vehicle’s brakes must overcome its kinetic energy. It is important to realize that you are not simply working against speed, as the car’s kinetic energy will be four times as great as the speed at which it is traveling. This means that your stopping distance will be squared too. For example:

  • A vehicle traveling at 30mph will take 45 feet to stop from the moment you begin braking.
  • A vehicle traveling at 60mph (twice as fast) will take 180 feet (four times the distance) to stop from the moment you begin braking.

Keep in mind that these distances do not take into account the time it takes you to perceive a threat and choose to apply the brakes. The faster you are traveling, the harder it is to perceive threats. This means that at high speeds, your total stopping distance may be significantly greater than the distance provided in this example. Remember that your braking distance will be extended further, if the road surface offers poor traction.

Kinetic energy in a collision

If your vehicle collides with an object, the force of the impact will be equal to its kinetic energy, divided by your stopping distance. The greater stopping distance you have, the more time your brakes will have to absorb kinetic energy from the vehicle and lessen the severity of an impact. This is why collisions are often more destructive when the driver has little time to apply the brakes.

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