# Homework Help: I dont understand how to find the work done

1. Oct 13, 2011

### snash1057

i dont understand how to find the "work" done

A driver of a 7475 N car passes a sign stating "Bridge Out 25 Meters Ahead." She slams on the brakes, coming to a stop in 10 s. How much work must be done by the brakes on the car if it is to stop just in time? Neglect the weight of the driver, and assume that the negative acceleration of the car caused by the braking is constant.

W=Fdcos

i tried finding acceleration by doing 25=1/2a(10^2) = .125
then finding mass by doing 7475=m(9.8)= 762.76
and then Fnet = 762.76 x .125 = 95.35
w= 95.35(25) = 2383.63
___________________________________________________

i also tried finding Vf by using 25=1/2(0+Vf)10 = 1.25

then i tried finding kinetic energy by 1/2(7475)(1.25^2)= 5839.84

but neither of them are right so i have to be doing something wrong

Last edited: Oct 13, 2011
2. Oct 13, 2011

### SammyS

Staff Emeritus
Re: i dont understand how to find the "work" done

You had 25=1/2a(10^2) = .125 . ???

I think you meant that a = .125, but that's wrong. Check your algebra.

3. Oct 13, 2011

### snash1057

Re: i dont understand how to find the "work" done

is it .5 rather?

4. Oct 13, 2011

### snash1057

Re: i dont understand how to find the "work" done

OH GOSH! i cant believe i made that dumb of a mistake! haha i got the right answer! thank you for correcting my algebra!