Flea Jumping Problem

[sniderwes]

Hey, here's the problem-A 1 mg flea can jump to a height of .2m. The length of its leg is .5mm.
What is the approximate force F a flea exerts on the ground during a jump?
a) 2*10^07
b)1*10^-5
c)4*10^-3
d) Can't be determined

I said d because you don't know how long the leg is exerting the force for, so you don't know what velocity it is launched with (since you don't know impulse).
However this seems to be going in the completely wrong direction.

Their answer was just mgh=Fd, where d is the length of the leg. How does this make sense physically? I don't get how you know the leg will be exerting the force for its full length...like if a person is standing upright and they jump, their legs only exert force for a little bit until they leave the ground. Are you supposed to assuming their legs are crumpled to the ground before jumping?

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[tdod]

you are right, they make an assumption that the flea's leg will exert a force for the distance of its leg. i am not an entomologist, so i can't tell you whether or not that is a good assumption. it's a flawed question in my opinion.

 

[DrknoSDN]

Only suggestion I might have is look for trigger words in the question. "What is the approximate" would make you think you are intended to make an approximation based on an equation if at all possible. Thus you would need to know what equation they want you to plug and chug. Check if all the variable required are given and approximate it.

You should only choose "can't be determined" if you know what equation is required to make the approximation, and also that they did not give you all the variables required to perform the calculation.
If it truly can't be determined it should be somewhat obvious because they are asking you specifically about an equation you are supposed to memorize. And the MCAT is usually more focused on your ability to reason.

This question was probably just related to the current scientific news of 2011, so it may not be the most intuitive because prior to that, "how a flea jumps" was debated.
http://www.npr.org/2011/02/10/133602679/itching-to-know-how-fleas-flee-mystery-solved