The contact force that resists the relative sliding motion between two surfaces in contact is commonly known as which of the following?

Introduction / Context:
Forces in mechanics can be broadly classified as contact forces, which act when bodies are in physical contact, and non contact forces, which act at a distance. When two surfaces touch and there is a tendency for one to slide over the other, a particular contact force arises that opposes this relative motion. This force plays a crucial role in walking, braking, and the functioning of machines. This question asks you to identify that contact force by name.

Given Data / Assumptions:

• Two solid surfaces are in contact with each other.
• There is either actual relative sliding or a tendency to slide between the surfaces.
• The force we are interested in acts along the surfaces and opposes this relative motion.
• Options include friction, magnetic force, electrostatic force, and muscular force.

Concept / Approach:
Friction is defined as the contact force that opposes relative motion or the tendency of relative motion between two surfaces in contact. It acts along the surface of contact and is responsible for resistance to sliding. Magnetic and electrostatic forces are non contact forces that act at a distance through fields and do not require surfaces to touch. Muscular force is the force generated by muscles in the human body or animals and can act through contact or via tools but is not specifically the force between surfaces resisting sliding. Therefore, the correct name for the contact force described is friction.

Step-by-Step Solution:
Step 1: Identify that the force in question arises only when two surfaces are in contact. Step 2: Note that the force acts along the interface between the surfaces and opposes sliding or the tendency to slide. Step 3: Recall that this is exactly how friction is defined in mechanics textbooks. Step 4: Compare with magnetic and electrostatic forces, which can act between objects that are not touching and are therefore not purely contact forces. Step 5: Recognise that muscular force refers to the active force generated by muscles, not the passive resisting force between surfaces.

Verification / Alternative check:
Everyday examples support this identification. When you push a box along the floor, friction between the box and the floor opposes the motion. When a car brakes, friction between the tyres and the road surface is what slows the car down. In both cases, friction is clearly a contact force between surfaces that resists sliding. Magnetic or electrostatic forces might act in some situations but are not responsible for these typical examples of resistance to motion.

Why Other Options Are Wrong:
Magnetic force: A non contact force that can act across air or vacuum and does not require surfaces to touch. It is not specifically the force resisting sliding between surfaces. Electrostatic force: Also a non contact force between charged bodies and does not describe the general resistance to sliding between neutral surfaces. Muscular force: Describes the force produced by muscles, such as when you push or pull an object. It is a cause of motion, not the resisting contact force between surfaces.

Common Pitfalls:
Students may confuse the broad term contact force with friction and think they are always interchangeable. In fact, contact forces include normal reaction, friction, tension, and others. However, when the question specifically mentions resisting sliding motion between surfaces, it is clearly referring to friction. Another misconception is to think of friction as always undesirable, but in many cases such as walking and driving, friction is essential for control and safety.

Final Answer:
The contact force that resists relative sliding between two surfaces in contact is called friction.