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Name: Luka
Status: student
Grade: K-3
Country: Australia
Date: Summer 2012


Question:
I have a ramp that is hot wheels and it is equal and all the same I weigh my cars and then race them down the ramp and the heavy car is winning every time., I have been doing it alot and this always happens and the cars are usually the same size. I have tried this on another ramp too and it is the same. What are the rules behind this? Someone told me a heavy ball and a lighter ball of the same size will fall or roll at the same speed but my tests with my cars is not the same as that. Does weight difference make something roll down a ramp faster or slower? If my cars were on a flat surface would the heavy car win or lose, I cannot test it fairly because I might accidentally push one harder. Please can you explain some of this stuff to me, I love learning about this right now. My kindergarten teacher or my mum or dad or my sisters all do not know the answers. From Luka age 4. (dad)



Replies:
Cars have to push air out of their way to move. (It might be easier to see this if you put your ramp in the bathtub, because water is much harder to push out of the way than air is. The heavy cars will have an even bigger advantage under water.) The faster a car goes, the harder it is to push air out of its way.

When a car is pushing on air, air is pushing back on the car, and with exactly the same force.

It takes more force to change the speed of a heavy thing than a light thing. The force that makes the cars go down the ramp (gravity, of course) is larger for the heavier cars - that is what it means to be heavier, after all. The force is just enough larger that all cars would move at the same speed if not for air resistance (and other kinds of friction). The thing that makes them heavier is /exactly/ the same thing that makes them harder to push around. (This is amazing if you stop and think about it for a minute. You do not have to be a four-year-old to be baffled by this; few understand it.)

But air resistance is the same for all cars. The heavy cars are less affected, because air resistance is a smaller fraction of the total force on them.

Tim Mooney


In an ideal world with no air resistance, both cars should fall at the same rate – but that is in an ideal world. The “real cars” can have different friction in the wheel and its axle and the wheels and the ramp. You might try adding weights to the lighter car and observe how that modified car behaves.

Vince Calder



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