The correct answer is: D. All the above
The laws of dynamic friction are:
- The force of friction always acts in a direction opposite to that in which a body is moving.
- The magnitude of the kinetic friction bears a constant ratio to the normal reaction between two surfaces. The ratio being slightly less than that in the case of limiting friction.
- For moderate speeds the force of friction remains constant but decreases slightly with the increase of speed.
The first law of dynamic friction states that the force of friction always acts in a direction opposite to that in which a body is moving. This means that the force of friction always opposes the motion of a body. For example, if you are pushing a box across the floor, the force of friction will be acting in the opposite direction to the motion of the box.
The second law of dynamic friction states that the magnitude of the kinetic friction bears a constant ratio to the normal reaction between two surfaces. The ratio being slightly less than that in the case of limiting friction. This means that the force of friction is proportional to the normal reaction between two surfaces. The normal reaction is the force that is perpendicular to the surface of contact. The coefficient of kinetic friction is a constant that is determined by the materials of the two surfaces in contact.
The third law of dynamic friction states that for moderate speeds the force of friction remains constant but decreases slightly with the increase of speed. This means that the force of friction is constant for moderate speeds, but it decreases slightly with the increase of speed. This is because the surfaces of the two objects in contact become smoother as the speed increases.
I hope this helps!