How Do You Calculate Average Frictional Force?

[losthelp]

Homework Statement

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=?

Homework Equations

vavg= vf-vi, then d/vavg= t, then a=vavg/t, then fnet=m*a, bleh that's all i can really think to do now :S

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

 

[LowlyPion]

Homework Statement

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=?

Homework Equations

vavg= vf-vi, then d/vavg= t, then a=vavg/t, then fnet=m*a, bleh that's all i can really think to do now :S

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

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

 

[losthelp]

but what would that help me get :S

 

[LowlyPion]

but what would that help me get :S

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?

 

[losthelp]

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?

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