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Jinx00007 · Oct 14, 2009, 11:39 PM

A multiple choice question

An Olympic athlete of mass 80 kg competes in a 100 m race. What is the best estimate of his mean kinetic energy during the race?

A 4x10^2 J

B 4x10^3 J

C 4x10^4 J

D 4x10^5 J

The answer to this question according to the booklet is B..!

I am trying to relate an equation with mass and distance in order to get the answer...

I was trying to use K.E= 1/2mv^2 but the velocity is unknown... I just don't know where to start.. I am stuck please help me..! Thanks in advance..! 11

Unknown008 · Oct 15, 2009, 01:52 AM

Well, you should know that the world record for the 100 m race is about 9 seconds. That leaves us with a speed of 10m/s if we take it as 10s roughly.

Speed = Distance/Time = 100/10 = 10m/s

With that, you get your answer as B.

Now, let's check the other ones. To have 10^2, the man will be going at

$E_k = \frac12 mv^2$

$v = \sqrt{\frac{400 \times 2}{80}} = 3.16$

People can go faster than that!

Now, the others, you'll end up with speeds of 31.6 m/s and 100m/s which are too fast! We won't be using cars if people could run that fast :p