If the input machine produces an
output of 80 Newtons, then the mechanical advantage is that it produces work. If
the required output must be 80 Newtons, then the input force is desirable
having 100 percent production of force even though it requires a large amount
of input to produce 80 Newtons of force.
Complete question:
A child sits in a wagon with a pile of 0.64-kg rocks. If she can throw each rock with a speed of 7.5 m/s relative to the ground, causing the wagon to move, how many rocks must she throw per minute to maintain a constant average speed against a 3.9-N force of friction
Answer:
The number of rocks per minutes thrown is 49 rocks/min
Explanation:
Given;
mass of the rock, m = 0.64 kg
speed of the rock, v = 7.5 m/s
frictional force,
= 3.9 N
For an object to move at a constant speed, the applied force must be equal to the frictional force.

where;
is the applied force
N/t is the number of rocks per minutes thrown
Substitute the given parameters;

Approximately 49 rocks/min