Kinetic energy of a soapbox derby car
A soapbox derby racecar has a light wooden body and steel axles, which altogether have a mass of 35 kg. Each wheel is a bicycle wheel that can be approximated by a 36 cm diameter hoop with a mass of 3.1 kg. The rider has a mass of 36 kg. What is the ratio of his rotational potential energy to his translational kinetic energy?
How can we know kinetic energy without knowing his velocity?
Answer
You have to make an assumption about how the racecar is rolling. Normally, our cars roll without slipping, meaning that the wheels do not slip against the ground. In other words, one revolution of the wheels corresponds to 2πR distance covered on the ground.
Another way of writing the no-slip condition is v_t = Rω, where v_t is the speed of the car. Try using this relationship to find the energies.
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You have to make an assumption about how the racecar is rolling. Normally, our cars roll without slipping, meaning that the wheels do not slip against the ground. In other words, one revolution of the wheels corresponds to 2πR distance covered on the ground.
Another way of writing the no-slip condition is v_t = Rω, where v_t is the speed of the car. Try using this relationship to find the energies.