# Homework Help: Average frictional force.

1. Sep 30, 2008

### losthelp

1. The problem statement, all variables and given/known data
A 0.5 kg block is sliding across a table top with an inital velocity of 0.2 m/s. It slides to rest in a distance of 0.7m. Find the average friction force that retracted its motion.

m=0.5kg
vi=0.2m/s
vf=0
d=0.7m
ff=?

2. Relevant equations
vavg= vf-vi, then d/vavg= t, then a=vavg/t, then fnet=m*a, bleh thats all i can really think to do now :S

3. The attempt at a solution
vavg= 0-0.2= -0.2
t=d/vavg= .7/-.2= -3.5sec
a= -0.2/-3.5=0.057
fnet= 0.5*0.057=0.0287

:S i have no ide ahow to find average frictional force from this :S

2. Sep 30, 2008

### LowlyPion

To figure the force you need to figure the deceleration. But you won't find it by using the Average Velocity.

One relationship you can exploit is the Vf2 =Vi2 +2a*x

3. Sep 30, 2008

### losthelp

but what would that help me get :S

4. Sep 30, 2008

### LowlyPion

You'd think if you had the negative acceleration you might be able to figure the Force that was retarding the block wouldn't you?

5. Sep 30, 2008

### losthelp

i probalay would if i had any idea what i was doing. :(
thanks anyways.