Calculate Acceleration, Friction & Speed on 30° Incline | Block Homework

[Ab17]

Homework Statement

3.00-kg block starts from rest at the top of a 30.0° incline and slides a distance of 2.00 m down the incline in 1.50 s. Find (a) the magnitude of the acceler- ation of the block, (b) the coefficient of kinetic friction between block and plane, (c) the friction force acting on the block, and (d) the speed of the block after it has slid 2.00 m.

Homework Equations

The Attempt at a Solution

a = 1.78 m.s2

(B) fk - mgsin@ = ma
Ukmgcos@ - mgsin@ = ma
Ukgcos@ -gsin@ = a
Uk= (a+ gsin@)/gcos@
Uk = 0.787

 

[Ab17]

Not getting the correct answer where am I going wrong?

 

[gneill]

You have a sign issue with the directions of the vector components. Draw an FBD that shows the directions of the vectors involved. Include the acceleration vector so that your choice of coordinate system is clear (so that the signs you assign to the force vectors when you write your equations match your choice of + direction for the acceleration).

 

[Ab17]

Oh yes its suppost to be mgsin@ -fk

 

[Ab17]

But i still get the same eqn

 

[cnh1995]

But i still get the same eqn

Signs should change in your equation.

 

[Ab17]

Ya I see it now. I made such a stupid mistake