Gravity Vehicle Fundamentals
what is gravity vehicle
A gravity vehicle is model car that is powered solely by gravitational potential energy. Many science and engineering competitions, such as Science Olympiad, host gravity vehicle competitions to help teach basic physic concepts to students.
In this post, we will outline the basic concepts needed to consider when creating a mousetrap vehicle and how to optimize those aspects of your device for the best results.
what to consider
weight
Contrary to other vehicle-based events, you should maximize the weight of your gravity vehicle. This is based on the equation:
Gravitational Potential Energy = Gravity * Height * Mass
Because gravitational energy is what propels the device forward it is extremely important to maximize the vehicle’s weight, which, in turn, will increase your gravitational potential energy.
Center of gravity
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.
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.
vehicle width
Vehicle width refers to the distance between the wheels attached to a vehicle’s axles.
To maximize your vehicle’s stability, you must optimize the vehicle width of your gravity vehicle. This increases the base-area of your device, resulting in a more stable vehicle. It is important to balance vehicle width with your devices weight for the best results.
vehicle length
Vehicle length refers to the distance between the front and back axles.
To maximize your vehicle’s stability, you must optimize the vehicle length of your gravity vehicle. This increases the base-area of your device, resulting in a more stable vehicle. It is important to balance vehicle length with your devices weight for the best results.
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).