# What factors affect the velocity of an object moving down an inclined plane?

If you have a small toy car rolling down an wooden plank, what factors would increase or decrease the velocity of said car. In the experiment my team originally stated that adding mass to the car would increase it's velocity as it is creating more downward momentum. However we had to change our experiment to the different surfaces that apply more friction and would therefore change the velocity because other teams told us that mass doesn't affect velocity. Were they correct in telling us that mass doesn't affect velocity or vice versa. Also what other factors can influence the velocity of the car rolling freely down the inclined plane, other than the surface, incline, air resistance, and mass. What formulas or theories are related to this topic that i could include in my discussion to back up my results and finding?

• I think it's just (gravity = 9.8 m/s^2) *sin(theta) theta for the incline angle, and the friction along the x axis where -(coefficient of friction)*x*cos (theta). You just need to add the component's up as you find different forces along different angles. – pandoragami Apr 19 '15 at 1:09

As it turns out, adding mass (while keeping the dimensions of the car fixed) will make the car go faster. This sounds contrary to what all physics students are taught, but the reason is while friction scales with mass, air resistance doesn't. That's why a thirty-foot rock will fall faster than a thirty-foot parachute in an atmosphere. It's also why the winning car in pine-wood or soap-box derby contests usually seem to be right at the weight limit.

Other factors that will affect the car's speed include:

1. The mass of the wheels. It takes a little bit of energy to get the wheels spinning (rotational kinetic energy), and the less mass they have the less energy they will absorb.
2. You want an axle with a very small diameter. The upward force from the wheel to the axle isn't affected by the axle diameter, but the drag torque (essentially friction) exerted by the wheel onto the axle will be less the smaller the diameter of the axle. If the car is large enough, you could go with a ball-bearing wheel/axle bearing, like real cars or roller blades, since rolling friction is less lossy than sliding friction.
3. If you can get a spring-like suspension on the wheels, or use air-inflated tires, that might help. Rigid wheels on a rigid suspension will cause the car to bounce up and down whenever it hits a tiny bump, wasting energy. A good suspension will compress when it rises over an obstacle (slowing the vehicle down slightly), but that energy will be recovered when the wheel rolls down the other side of the bump (allowing the vehicle to speed back up).

You've already mentioned making the car as aerodynamic as possible. If it's running down a track where it might bump up against a side guardrail or something, using a low-friction material will help (Teflon, or the kind of plastic used in gallon milk jugs, or even tiny wheels mounted on the sides). That's all I can think of at the moment. Good luck.

• Spoken like a former pinewood derby winner. Made pretty much all the points I was going to make... – Floris May 23 '15 at 20:26

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