Ramp & Friction Lab

When the car leaves the ramp, it is moving fast. Nothing pushes it forward after that. On the flat surface, friction is the force that slows it down until it stops. Different surfaces make different amounts of friction.

Friction

A force between two surfaces that touch and rub. It always pushes against the motion.

Inertia

An object keeps doing what it is doing unless a force changes it. A rolling car would keep rolling if nothing slowed it down.

Contact points

The places where the wheels touch the surface. Rough or soft surfaces grab the wheels in more places. Smooth surfaces grab in fewer places.

Mass

How much matter is in an object, measured in grams (g). The car is 60 g, and you can add hooked masses to it. Keep the car's mass the same for every surface.

Coefficient of friction (μ)

A number that tells how much two surfaces grab each other. A bigger number means more friction. The Greek letter μ is said "mew."

Trial and average

A trial is one run of the test. To find the average, add the three times, then divide by 3.

Why do three trials? Real measurements change a little every time. The car may be let go a tiny bit differently. The surface may have a small bump. One odd result could fool you. Doing three trials and using the average gives you a result you can trust more.

Why keep the ramp height, the release point and the mass the same? A taller ramp, or a release point higher up the ramp, gives the car more speed at the bottom. More mass makes the car a little faster too. If you change one of them and the surface at the same time, you can't tell which one changed the result. It doesn't matter which height, release point or mass you pick, as long as you keep them the same for every surface.

Does adding mass change the result? Only a little. More mass presses the wheels down harder, so friction pushes back harder. But a heavier car also has more inertia, so it is harder to slow down. Those two cancel out. The wheels also have to spin, and spinning takes some energy. A heavier car loses a smaller share of its energy to the spinning wheels, so it is a little faster.

Speed at the bottom of the ramp. The higher the release point, the faster the car. Here h is how high the release point is above the counter (the box "Height of the release point" shows it). It depends on both the ramp height and how far up the ramp the car starts. The 0.96 means the ramp's own friction takes away a little of the car's energy on the way down.

Slowing down on the surface. Friction slows the car at a steady rate, a. A grabbier surface (bigger μ) slows it faster.

Time to reach a mark. The time t to roll a distance d after leaving the ramp:

Adding mass. The wheels spin, and spinning takes energy. In this model the wheels act like 8 g of mass that only soaks up energy. A car with total mass m keeps the share m ÷ (m + 8) of its energy for moving forward: 0.88 for the 60 g car, 0.97 for a 260 g car. So a heavier car's v is a little bigger. Its slowing, a, grows by the same share, so it stops in the same place, just a little sooner.

The μ for each surface (typical values for a rolling toy car):

Trial-to-trial wobble. Each trial changes the launch speed by about 1% and the friction by up to about 3%. That is why the three trials are close but not the same. Every computer also gets its own car and surfaces: the launch speed is up to 3% different, and each surface's friction is up to 3% different. They stay the same on that computer. That is why your times are a little different from a classmate's. Here g = 980 cm/s² and h is the height of the release point in cm.