Question

A boy exerts a force of 9.0 N horizontally to push his sister on a sled. He pushes her through a distance of 15 m. How much work did he do?
Was this positive work or negative work? If he stops pushing, his sister and the sled will eventually slow to a stop. Is this negative work or positive
work? What force is doing the work?
Please help with the last 2 questions above^^ (negative or positive,, and what force is doing the work)

Answer 1

Answer:

Work done by the boy is 135 J  and is positive work.

Friction is the force acting that stops the sled and her sister.

Work done by friction is negative work.

Explanation:

Given:

Force acting on the sled by the boy is, $F=9.0\ N$

Displacement of the sled in the direction of force is, $S=15\ m$

We know that, positive work is said to be done by a force, if the force causes displacement in its own direction. Also, negative work is said to be done by a force, if the force causes displacement in the direction exactly opposite to the direction of the applied force.

Here, the force applied by the boy causes a displacement in the direction of the applied force. So, work is positive and is given as:

Work = Force × Displacement

$Work=F\times S\\Work=9\times 15=135\ J$

Therefore, the work done by the boy is positive and equal to 135 J.

Now, when the force of push is removed, then the body will experience only the frictional force in the opposite direction which eventually causes the sled to stop.

Here, the direction of force is backward while the displacement is in the forward direction. So, both of them are in opposite direction.

So, work done by frictional force is negative.