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, [tex]f_k[/tex] = 3.9 N
For an object to move at a constant speed, the applied force must be equal to the frictional force.
[tex]f_k = N(F_a)\\\\f_k = N(\frac{mv}{t})\\\\f_k = \frac{N}{t} (mv)\\\\\frac{N}{t} = \frac{f_k}{mv}[/tex]
where;
[tex]F_a[/tex] is the applied force
N/t is the number of rocks per minutes thrown
Substitute the given parameters;
[tex]\frac{N}{t} = \frac{f_k}{mv}\\\\ \frac{N}{t} = \frac{3.9}{0.64*7.5}\\\\ \frac{N}{t} = 0.8125 \ \frac{rocks}{second} * \frac{60 \ seconds}{1 \ min} = 48.75 \ \frac{rocks}{min} \[/tex]
Approximately 49 rocks/min
