Question

A child pulls a block across the floor with force applied by a horizontally held string. A smaller frictional force also acts upon the block yielding a net force on the block that is smaller than the force applied by the string. How does the work done by the string compare with the change in the block’s kinetic energy?

1. the work done is greater than the change in the kinetic energy of the block

2. The work done is equal to the change in kinetic energy of the block

3. The work done is less than the change in kinetic energy of the block

If there is just one force acting on an object, does its work necessarily result in an increase in kinetic energy? (yes or no)

A net force of 65 N accelerates a 4-kg mass over a distance of 16 m. What is the work done by this net force? What is the increase in kinetic energy of the mass?

Answer 1

1)

(a) the work done is greater than the change in the kinetic energy of the block.

As friction is also present, some work (negative) will be done by friction as well. So, the KE will be lesser than the work done by the string.

(b) Yes

2) W = Fd

= 65*16

= 1040 J

Increase in KE = W

= 1040 J

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