Three students are pushing on a box. under what conditions will the motion of the box change?

Explanation:

There are several deductions we could make. If the force applied by the students is less than friction, then the box won't change its state of motion, it will remain under equilibrium, because friction is a force that it's always opposite to any force applied, if the students are pushing forward, the friction will push back.

But, if the force applied by the students is higher than friction, the box will change its state of motion, it won't remain in equilibrium states and it will start to move.

Therefore, the condition is that the force applied by the students must be higher than friction.

a and b sometimes have a positive value

gas and liquid