Forces and Motion

Science background gives teachers more detailed information on the phenomena students explore. Here is an excerpt from the science background section on forces and motion.

Pushing a Bobsled

The initial push is a key part of a successful bobsled run because it is the only chance the bobsledders have to apply force to the sled, transferring energy into the bobsled energy system. An energy system is a set of connected parts that change an input of energy to a different output of energy.

At the top of the hill, the bobsledders have gravitational potential energy, which is the energy stored in an object as a result of its vertical position or height. The height of the hill causes the amount of energy stored in the bobsledding energy system to change. The higher above the ground they are, the more gravitational energy they have.

But the sled needs a force to get it to move in the beginning. This is because of the law of inertia, which states that an object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force. When the bobsledders push the sled, they provide the outside force that changes their motion. The force of their pushing transfers kinetic energy to the sled, which begins to move because it has gained an input of kinetic energy.

The more force the bobsledders apply to the sled in the beginning, the more speed they will have on the rest of the track. Because of this, bobsledders need to have tremendous speed and power for pushing the bobsled. Vonetta Flowers first heard about bobsledding because of a flier urging track and field athletes to try out for the U.S. bobsled team. The Olympic bobsled team recognized that having strong, fast runners was essential to a strong bobsled team

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.

Mechanical Engineers Design Vehicles

Before the 2014 Olympics, the U.S. bobsled team hadn’t won in the men’s two-person bobsled event in more than 60 years.

So the U.S. turned to Michael Scully, a creative designer for BMW, to apply his background knowledge to make the bobsleds more like a race car. This field of engineering concerned with applying forces and energy transfer to design and build machines is called mechanical engineering. Machines are technologies that use kinetic energy to do work. The best engineered machines balance the forces the machine uses to do work.

Scully was able to work with a team to design a faster bobsled because race cars and bobsleds have similar goals: keep the people inside them safe while moving with as much velocity as possible. Velocity is the rate at which an object changes its position in a specific direction. Bobsleds and race cars are both a kind of machine called a vehicle, which moves passengers or goods from one place to another. Race cars are built with powerful engines to increase force and acceleration. Engines are important because they convert an input of chemical potential energy stored in gasoline into outputs of kinetic energy that moves the car and heat. The average racing engine can push a car with the force of 750 horses. In contrast, bobsleds depend on the force of pushing applied by the team members and the force of gravity as the sled moves down the hill.