# what is gravity vehicle

A gravity vehicle is a car that is solely powered by the gravitational potential energy it has when it is placed on a ramp. Many science and engineering competitions, such as Science Olympiad, hold gravity vehicle as an event to learn physics concepts.

In this post, we will discuss how you can improve the speed an distance that your gravity vehicle travels.

# improving speed and distance

## Friction

Friction is an important factor to minimize in any vehicle-based competition, especially in gravity vehicle where there is only a limited supply of gravitational potential energy available to power the vehicle.

The best solution to minimize friction between the axles and the vehicle frame is to use ball bearings. However, if you are unable to obtain ball bearings, graphite powder, oil, or other lubricants can be used to reduce the friction between the axle and vehicle frame.

It is important to note, though, that if ball bearings are not used, you should try to minimize the contact between the axles and the vehicle frame by making the vehicle frame thin in areas where the axles are held.

## Weight Placement

It is also important to shift the center of gravity back towards the rear axle of the device. This is based on the equation:

Gravitational Potential Energy = Gravity * Height * Mass

By shifting the center of gravity of the device towards the rear axle, the height of the center of gravity when set on the ramp will increase, which, in turn, will increase the gravitational potential energy.

## Wheel Diameter

If you want to maximize the speed of your device, the wheels of your vehicle should be shorter, which reduces the circumference of the drive wheel, resulting in a shorter time period required to fully rotate each wheel. However, the distance that your vehicle can travel will be reduced.

If you want to maximize the distance your vehicle can travel, the wheels of your vehicle should be larger, which increases the circumference of the wheels, resulting in a longer time period required to fully rotate each wheel. However, the speed of your vehicle will be reduced.

## Ramp Shape

In an idealized model, the shape of the ramp shouldn’t affect the final speed.

Mass * Gravity * Height = Gravitational Potential Energy at the Top of the Ramp = Kinetic Energy at the Bottom of the Ramp = 1/2 * Mass * (Velocity)^2. Mass cancels out, so the final Velocity would be sqrt(Height * Gravity).

That suggests the ramp shape should be designed to both:

(a) Maximize the initial height of the center of mass of the vehicle (maximizing the initial potential energy).

(b) Minimize the energy lost due to friction and bouncing.

It should start off fairly flat at the top (with the rear wheels sitting right at the top edge of the ramp, 1m above the floor, the closer to vertical, the lower the center of mass would be), and it should be reasonably flat at the bottom as well (with a curved transition to flat).