Physics Questions-AP physics B?

Question

So my AP physics B teacher is horrible and doesnt teach us anything, he expects us to read the book but im having trouble doing these questions (im preparing for a test on Monday)

1.  A 70kg base runner begins his slide into second base when he is moving at a speed of 4 m/s.  The coefficient of friction between his clothes and Earth is .70  He slides so that his speed is zero just as he reaches the base.
a. How much mechanical energy is lost due to friction actign on the runner
b. How far does he slide

What formulas do I use for this?  Im kinda confused how to relate friction to work to mechanical energy

THanks

jgoulden · Accepted Answer

(1) Your runner has kinetic energy KE = 1/2 m v². He loses this due to the force of friction, which is F = μN = μmg, acting over a distance d.

1/2 m v² = μmg d

Solving for d we find that

d = 1/2 v² / μg = 1/2 ( 4 m/s )² / ( .70 ) ( 9.8 m/s²  ) = 1.2 m.

The mechanical energy lost is all of his original kinetic energy

1/2 m v²  = 1/2 ( 70 kg ) ( 4 m/s )²  = 560 J.

lowly physics grad · Answer

Well you can figure out the force exerted on the runner by friction and from that get the time it takes him to decelerate

F = (mu)mg 

and then plug that into F = ma

from there you can figure out how far he has travelled by

v^2 = vo^2 + 2a(delta s) where v is final velocity, vo is initial velocity, and delta s is the change in distance

unless a is a trick question, it would seem to me that all the mechanical energy lost is due to friction, though it is a non conservative force this is only an approximation

basanti · Answer

sorry i cant help u but i can say search it on google..

? · Answer

idk

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Physics Questions-AP physics B?

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