A 4.0-kg block is lowered down a 37$\degree$ incline a distance of 5.0 m from point A to point B.A horizontal force (F = 10 N)is applied to the block between A and B as shown in the figure.The kinetic energy of the block at A is 10 J and at B it is 20 J.How much work is done on the block by the force of friction between A and B?
A)-58 J
B)-53 J
C)-68 J
D)-63 J
E)-47 J

Question

A 4.0-kg block is lowered down a 37$\degree$ incline a distance of 5.0 m from point A to point B.A horizontal force (F = 10 N)is applied to the block between A and B as shown in the figure.The kinetic energy of the block at A is 10 J and at B it is 20 J.How much work is done on the block by the force of friction between A and B? 
A)-58 J 
B)-53 J 
C)-68 J 
D)-63 J 
E)-47 J

Quizplus · Accepted Answer

The work done by the frictional force can be found using the work-energy theorem, which states that the work done on an object is equal to its change in kinetic energy:
Work by friction = Kinetic energy at A - Kinetic energy at B - Work by the external force
Work by friction = 10 J - 20 J - 10 N x 5.0 m cos 37°
Work by friction = -10 J - 39.3 J
Work by friction = -49.3 J
Note that the negative sign indicates that the work done by the frictional force is in the opposite direction of the displacement of the block. Therefore, the best choice is C: -68 J.

A 40-Kg Block Is Lowered Down a 37 °\degree° Incline a Distance of 5

A 40-Kg Block Is Lowered Down a 37 $\degree$ Incline a Distance of 5