B$.$ A $50kg$ box is pushed to the right with a force of $300N,$ along a horizontal rough surface. It accelerates at $4\frac{m}{s^2}.$ You can neglect any drag force due to the air.

On the diagram at right, draw and label a free$-$body diagram for the box.

Argue about which Newton law or $($:$2^{nd}\}$ should be used to use to obtain information about the magnitude of the frictional force. Use that law to calculate the magnitude of the frictional force.