Circular Motion – The Wall Of Death

For shear terror, you have to watch this clip from the Diamond Maruti Car Circus in Delhi

So how does a car drive around in a horizontal circle?

Well for a start, friction is needed to keep the car form sliding down the wall. The friction force must be equal to the weight. Friction on the ground is normally a fraction of the normal reaction – here that is still the case, but turned sideways. A friction co-efficent of around 0.8 is good, meaning that the friction force is 80% of the normal reaction. So to find the normal reaction needed to produce 15,000N of friction (equal to the weight of a 1500 kg car), we need to divide by 0.8 to get 18750N.

This normal reaction provides the centripetal force required to turn the car through the circle. We we can work backwards from 18750N to find the speed needed – around 40km/h. (I’ve estimated the radius of the circle at the top of the bowl to be 10m.)

(You might notice in the diagram that the while the forces are in balance, the forces are not all acting through the same point, causing a torque. The lean of the car helps to balance this.)

Wall of death

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