Study the scenario.

An object is at rest. At five seconds, someone comes over and pushes the object forward with a force of 10 N.
Which of the following choices describes the force needed to keep the object in static equilibrium?

1. A force of 10 N must be exerted in a backward direction on the box at any time after five seconds so that the forces are balanced and the object moves at a constant velocity.

2. A force greater than 10 N must be exerted in a forward direction on the object at exactly five seconds to prevent the object from moving.

3. A backward force of 10 N must be exerted on the box at exactly 10 seconds so that the forces are balanced and the object remains at rest.

4. There is no way to keep the object in static equilibrium once a force has been exerted on it.

Ok, so once you have applied a force at t=5s then there's an acceleration in the object which produces increasing amount of velocity in it as time passes. So only way to keep it in static equilibrium is to apply an equal and opposite force at the same time as the 10 N force was applied to prevent the object from gaining any velocity.

None of the options seem to do that so the last option is what I will select.

Answer Link