Expert: Sherry Wallin - 6/9/2011

Question
Suppose a bike wheel has a diameter of 70 cm and the rider completes one cycle in 0.8 s.  What is the speed of the bike?
Thank you!

Answer
Kerry~

What units are you to report your answer in? cm per sec? And are you allowed to leave pi as pi or do you need to substitute 3.14?

In order to solve this problem you will need to know the circumference of the wheel so that you will know the distance the wheel travels in one revolution. Since the wheel has a diameter of 70 cm you will use the relationship that the circumference is pi*diameter or C = pi*D => C = 70 pi cm
Note that this is done in .8 sec. You want to know the rate from d = rt so solve for r and get that r = d/t. The distance traveled in one revolution is 70 pi cm and the time is .8 sec so r = 70*pi cm /.8 sec => 87.5*pi cm/sec and if you need to get rid of pi just multiply 87.5(3.14) = 274.75 cm/sec
If you need the answer in meters per hour then you will have to convert the units you have to the appropriate units.


Math Prof