A 25-kg block on a horizontal surface is attached to a light spring (force constant = 8.0 kN/m).The block is pulled 10 cm to the right from its equilibrium position and released from rest.When the block has moved 2.0 cm toward its equilibrium position,its kinetic energy is 12 J.What is the change in mechanical energy caused by the frictional force on the block as it moves the 2.0 cm?
A)-4.0 J
B)-3.5 J
C)-2.4 J
D)-2.9 J
E)-15 J

Question

Quizplus · Accepted Answer

The change in mechanical energy is equal to the work done by all forces except the spring force. We can find the work done by the frictional force by using the work-energy theorem: 
Work done by frictional force = kinetic energy at 2.0 cm - kinetic energy at 10 cm 
= 12 J - (1/2)(25 kg)((0.10 m)^2)((8.0 kN/m)/(1000 N/kN)) 
= 12 J - 1 J 
= 11 J

Therefore, the change in mechanical energy caused by the frictional force on the block is 11 J. Choice C is the correct answer.

A 25-Kg Block on a Horizontal Surface Is Attached to a Light