Homework Help: Impulse and momentum of a sled

1. Nov 8, 2008

Jennifer001

1. The problem statement, all variables and given/known data
a sled slides along a horizontal surface on which the coeffient of kinetic friction 0.25. its veocity at pt A is 8m/s at pt B its 5m/s. use impulse-momentum theorem to find how long the sled takes to travel from A to B

2. Relevant equations
P=J

3. The attempt at a solution

m(delta)v=Ft
(delta)v=ukgt
vf-vi=ukgt

uk = kinetic friction coeff.

ive gotten this far and im not sure how i'm suppoes to solve for time if theres 2 unknown varibles g and t.. is there a way i can solve for g .

am i doing this question correctly?

2. Nov 8, 2008

PhanthomJay

Re: impulse-momentum

You've got it right....g is assumed to be the acceleration of gravity on earth, and is equal to 9.81m/s^2, a known value.

3. Nov 8, 2008

Jennifer001

Re: impulse-momentum

that g is suppose to be acceleration i used the wrong symbol, it's on the x axis not the y axis
sorry i confused you

4. Nov 9, 2008

PhanthomJay

Re: impulse-momentum

No, you used the correct symbol; it comes out of your correct calculation for the net force acting on the sled, which is u_k(m)(g). You are trying to solve for t, not a. If you want the acceleration in the horizontal direction, which is not called for, it's just F_net = ma, or a = (vf-vi)/t. The impulse-momentun equation is just another form of Newton's 2nd Law.

5. Nov 9, 2008

Jennifer001

Re: impulse-momentum

oo thank you i was confused