Mass, Speed, and Kinetic Energy

Science background gives teachers more detailed information on the phenomena students explore. Here is an excerpt from the science background section on mass, speed, and kinetic energy.

Mass and Speed

There are rules about how much mass a bobsled can have when empty and when the bobsledders are on it. Remember that mass is the amount of matter in an object. An object’s motion will change according to its mass, as well as the sum of the forces acting on it.

This is because of another law of motion, which states that force equals mass times acceleration. Acceleration is an increase in speed over time. It is measured in meters per second squared (m/s2). Speed is the rate at which an object covers distance in a period of time. It is measured in meters per second (m/s). This law says that the amount of force needed to move an object any distance depends on its mass. An object with greater mass needs more force to accelerate than an object with less mass. In other words, a more massive sled will need more force to cause it to accelerate a certain amount than a less massive sled would need.

Once the sled starts moving, a more massive sled has more energy than a less massive sled when moving at the same speed. This is because there is a proportional relationship between the amount of kinetic energy an object has and its mass. The more mass an object has, the more kinetic energy it has. If the mass of an object doubles, its kinetic energy also doubles. If an object’s mass decreases by half, its kinetic energy also decreases by half.

If you were to graph the relationship between an object’s mass and its kinetic energy, you would see a linear proportional relationship, where two quantities vary directly with one another. If one quantity is doubled, the other, related quantity is also doubled.

Mass and Kinetic Energy

Think of a pebble sitting on top of a hill. To get the pebble moving down the hill, you need to apply a force to it, such as by kicking or throwing the pebble. However, because the pebble doesn’t have much mass, it doesn’t require a lot of force to move it.

Now think of a boulder on top of that same hill. You would have to push it with a lot more force to get it to move because it is so much more massive than the pebble.

However, if both the pebble and the boulder move down the hill at the same speed, the boulder will have more kinetic energy than the pebble does because the boulder is more massive. If both the pebble and the boulder were to collide with a trash can at the bottom of the hill, the boulder would transfer more energy to the trash can, either denting it or moving it to another location. In contrast, the pebble would have much less energy to transfer to the trash can, resulting in a much smaller dent (if any) or a smaller movement.

Because of the relationship between mass and energy, in 1952 the Olympics committee established rules for how much a sled could weigh when it was empty and when the bobsledders were on it. Before 1952, bobsledders tended to be big and heavy. After 1952, the emphasis changed. It became more important for bobsledders to be strong enough to push the sled with a lot of force, but light enough to meet the weight requirements.

There is also a relationship between speed and kinetic energy. The faster an object is moving, the more energy it has. This is why collisions are more dangerous at high speeds compared to low speeds. The speed of an object is also proportional to the amount of kinetic energy it has. However, this isn’t a linear relationship. Instead, it is a nonlinear proportional relationship that changes exponentially where the rate of change increases exponentially each period. When the speed of an object doubles, the kinetic energy quadruples.