A box is being pushed across the floor at a constant velocity with an applied force of 30N, what is the magnitude of the frictional force acting
on this box?

Magnitude of frictional force = 30 N

Explanation:

According to the second Newton's law, the net force exerted by an external agent on an object of mass m is:

Fn=m.a

The net force is the vector addition of each individual force. If the sum of all the forces acting on an object is zero, then the acceleration is zero. That means the object moves at a constant speed or is at rest.

When an object is pushed across a horizontal rough surface, there are two forces acting in the direction of the motion: The applied force and the frictional force.

If the applied force is greater than the frictional force, then the object moves at a constant positive acceleration. If the frictional force is greater than the applied force, then the object won't move at all (if it was at rest) or will start a deaccelerated motion (braking).

Finally, if both forces are equal, the object will move at a constant speed or remains at rest. Since the box is moving at a constant speed, we can conclude the frictional force equals the applied force:

Magnitude of frictional force = 30 N

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at right angles to each other