Block sliding down an inclined plane and gravity

[tbcreighton]

Homework Statement

There is a 5.0kg block sliding 1.5m down an inclined plane 12 degrees from the horizontal. the coefficient of kinetic friction is 0.310. Whats is the work done on the block by friction and gravity?

Homework Equations

F=ma, W=fd, fk=uk*N

The Attempt at a Solution

So, i calculated the frictional force to be $$mgcos(12)\mu_{k}$$, but when i found the component force of gravity in the opposite direction ($$mgsin(12)$$), i found that the force of friction was bigger than it. I think I am just overlooking something major, but i can't see what i did wrong.

 

[learningphysics]

You didn't do anything wrong. You're right... the friction is greater than the component of gravity... this means the acceleration is up the plane...

So it is sliding downwards, but slowing down. net work done by friction and gravity will be negative.

Go ahead and calculate the work by friction and gravity.

 

[tbcreighton]

DOH! of course. I feel really dumb now. Thanks for your help.

 

[Zanychap]

If I have understood the above correctly, some initial force must have been provided before it moved. That would have imparted the velocity, and the unbalanced force will decelerate it.