Be a Scientist/Engineer

Challenge
How far can your car roll from the end of the ramp?

Supplies

 * ~4"x5" Polystyrene tray or corrugated cardboard
 * 4 Kelvin wheels or similar
 * 2 Plastic drinking straws
 * 2 Bamboo kitchen skewers
 * Tape
 * Scissors
 * Weights (optional)
 * Ramp

What to do

 * Tape your straws (bearing) on the bottom of the polystyrene or cardboard chassis
 * One towards the front
 * One towards the back
 * Make sure the straws are parallel
 * Remove the pointy ends of the skewers
 * Put a wheel on the end of a skewer
 * Run the skewer (axle) through a straw
 * Place a wheel on the other end of the skewer
 * Place your racer at the top of the ramp and let go
 * How far did it go? Can you make it go farther?

Inquiry questions

 * What happens if the car is heavier?
 * Can you reduce the friction or rubbing between the wheels and the chassis?
 * What happens if you release the racer lower on the ramp?
 * What happens if you change the angle of the ramp?
 * Does the angle of the ramp or the height of release make more of a difference?

What’s happening?
Lifting the racer to the top of the ramp gives it potential energy which is converted to kinetic energy as it goes down the ramp. The higher up the ramp you go, the faster and farther the racer will travel. Friction is what brings the racer to a stop. You can find sources of friction by listening and watching how it rolls. Any place where you see or hear rubbing is a sight of lost energy. Reducing the amount of friction should allow your car to go farther.

When you add weight to the car, something interesting happens. If we start by looking at the physics and math, we can see that adding weight shouldn't change how far the racer goes. When you lift the racer, you are providing gravitational potential energy (U) which is equal to the mass (m) of the car times the acceleration of gravity (g) time the height you raised it (h):

U = mgh

When you release it, the gravitational potential energy is converted to kinetic energy (E) which is equal to 1/2 times the mass (m) times the velocity squared:

E = 1/2 mv^2

Since all the kinetic energy comes from the potential energy, we can set it up so these equations are equal to each other:

mgh = 1/2 mv^2

Now we can see that we have mass (m) on both sides and since the mass of the racers doesn't change from the top of the ramp to the bottom, it completely drops out of the equation. This means that (in an ideal situation) adding weight to the racer won't make it go any faster or farther. Is that what happened when you tried it? Normally you can add some weight without increasing the friction very much and the increased mass will make your racer go faster. At some point, you get close to the ideal and more weight doesn't change the outcome. If you continue adding more weight, it starts bending the axle and drastically increasing the friction at which point, your racer will go slower.