Question

For an object sliding on the ground, the friction force a) always acts in the same direction as the displacement. b) always acts in a direction perpendicular to the displacement. c) always acts in a direction opposite to the displacement. d) acts either in the same direction as the displacement or in the direction opposite to the displacement depending on the value of the coefficient of kinetic friction.

Short answer

Answer: The friction force always acts in a direction opposite to the displacement.

Step-by-step solution

Identify the correct statement

First, let us recall that the friction force opposes the motion or the tendency of motion of an object in contact with a surface. Keeping this in mind, let's evaluate each option: a) Friction force always acts in the same direction as the displacement: This statement implies that the friction force is helping the object move, which contradicts the fundamental nature of friction force as an opposing force. b) Friction force always acts in a direction perpendicular to the displacement: This statement implies that the friction force would not have any effect on the object's motion because the force is perpendicular to the direction of motion. In reality, friction force directly opposes the motion, so it cannot be perpendicular to the displacement. c) Friction force always acts in a direction opposite to the displacement: As previously mentioned, friction force opposes the motion or the tendency of motion. This statement correctly demonstrates the nature of friction force as it works against the object's displacement. d) Friction force acts either in the same direction as the displacement or in the direction opposite to the displacement depending on the value of the coefficient of kinetic friction: This statement is incorrect because the direction of the friction force doesn't depend on the coefficient of kinetic friction, its magnitude does. The friction force always acts in the direction opposite to the displacement. The correct answer is option c) Friction force always acts in a direction opposite to the displacement.

Key concepts

Kinetic Friction

When two surfaces in contact move relative to each other, the force that resists the motion is known as kinetic friction. This force arises due to the interactions between the surface molecules of the two objects in contact. The magnitude of kinetic friction depends on two key factors: the normal force and the coefficient of kinetic friction, which is a measure of how rough or smooth the surfaces are relative to each other.

It's vital to realize that kinetic friction does not rely on the speed of the moving object or its surface area in contact. However, it is always directed opposite to the direction of the object's motion. To remember this, one might think of kinetic friction as the force that 'rubs' against the direction of travel, attempting to bring objects to a halt. This plays a crucial role in everything from braking a car to the simple act of walking, as it is what allows us to apply the necessary force against the ground.

Displacement in Physics

Displacement is a vector quantity that represents the change in position of an object. It's defined by both magnitude and direction. For instance, if you walk 3 meters east, your displacement is 3 meters to the east, regardless of your path to get there, as it is the shortest distance from the starting point to the ending point.

In problems involving motion on a surface, like the exercise at hand, displacement plays a key role in understanding how forces such as friction act on moving objects. While the path traveled can be winding and complex, it's the straight-line displacement that matters when evaluating forces like friction that oppose motion. Therefore, for an object sliding on a surface, displacement will help in determining the work done against the force of friction over that direct line from start to finish.

Opposing Forces

In physics, opposing forces are forces that act in direct opposition to each other. They can be involved in cases of balancing forces leading to static equilibrium, or they can be part of dynamic scenarios where they impact an object's motion. Kinetic friction is a classic example of an opposing force, as its entire role is to counteract the motion or the tendency of motion of an object.

Opposing forces are not only limited to friction. For example, air resistance is another opposing force that acts against the direction of an object moving through the air. In all scenarios that involve opposing forces, Newton's third law is at play: for every action, there is an equal and opposite reaction. This principle guides us in understanding the push and pull that occurs in countless physical situations, from the everyday to the extraordinary.