# Homework Help: Inclined Plane Newton's Third Law

1. Oct 28, 2005

### hshphyss

Can anyone help me with this problem? thank-you

A box slides down a 39° ramp with an acceleration of 1.32 m/s2. Determine the coefficient of kinetic friction between the box and the ramp

2. Oct 28, 2005

### Staff: Mentor

Please show what you've done so far.

3. Oct 28, 2005

### hshphyss

I don't know where to start because of the many unknwns. I know that mgcos39=N.

4. Oct 28, 2005

### ritz

I m sorry, if its too late!

Just registered myself...so i dunno if the answer wud still help u!
in ur quest...the acceleration given is the net acceleration actin on the object thats slidin down the ramp. so we know that the difference between the acceleration actin horizontally down the ramp and the acceleration actin opposite to it(acceleration occurin in the direction of friction) has to be 1.32m/s/s.
all we need ta do now is to find the acceleration actin horizontally down the ramp. which has ta be 9.8 times sin39= 6.2m/s/s
as i already mentioned the frictional acceleration has ta be 1.32 less than 6.2(6.2-1.3=4.8m/s/s
so now we know the acceleration in the direction of kinetic friction. acceleration occurin in the direction of normal is balanced by the acceleration towards the gravity. so if we find the y component of gravity...we hav the normal(as it has ta be the same!
thats 9.8 cos39=7.6m/s/s
co-eff of friction= accl towrds friction/acceleration normal
4.8/7.6=.64
thats bout it i guess....generally we use force instead of acceleration...but in this prob we assume mass constant!
if u hav further doubts...u can always post it, it will help me learn more....
take care

5. Oct 28, 2005

### hshphyss

wow thank-you so much