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Q & A: Motorcycle Accident

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Most recent answer: 10/22/2007
I have a question. If a man who is 6 ft. tall and 200 lbs. is riding a motor cycle at 25 mph, and his motor cycle comes to a complete stop, how far will the man be thrown? What if he was going 50 mph?
- Nick Karno
Washington, D.C.
The answer to your question depends on a lot of things. Once the motorcycle comes to a stop the man will keep going because he has inertia (the tendency of an object to stay in motion once it is in motion). How far he goes depends how high he was off the ground when the cycle stopped because that will determine how long it takes him to fall to the ground. It also depends on what the person is wearing and what the road surface is because that will affect how much the force of friction will slow him down. The rougher the surfaces the faster he will be slowed down. However if we compare the case where the cycle is going 25mph to when it was going 50 mph we find that the man travels roughly four times as far when the cycles speed was 50 mph. This is because the amount of distance it takes to stop depends on two things: The force stopping the object(which depends on all that stuff I listed) and how much kinetic energy the object had. The amount of kinetic energy has depends on its mass and its speed squared. So if we double his speed we quadruple his kinetic energy and quadruple the distance it takes for him to be stopped by the friction of the road. The time it takes for the man to stop will be only twice as much for the 50 mph case compared to the 25 mph case. This is because the time it takes to stop depends on the force stopping the object and something called momentum. Momentum is determined by an objects mass and its speed. So if we double the speed we double the momentum and double the amount of time it takes to stop.

(published on 10/22/2007)

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