In basic mechanics, which of the following best describes a contact force, and which option gives a correct example?

Introduction / Context:
Forces in physics can be broadly classified into contact forces and non-contact (or action-at-a-distance) forces. Understanding this classification helps in analysing how objects interact. This question asks you to identify the correct definition of a contact force along with a suitable everyday example of such a force.

Given Data / Assumptions:

• We are distinguishing between forces that require physical contact and those that do not.
• Examples such as friction, normal reaction and tension are familiar contact forces.
• Examples such as gravity, electrostatic and magnetic forces are familiar non-contact forces.
• The question combines both definition and example in each option.

Concept / Approach:
A contact force is a force that acts only when two interacting bodies are in physical contact with each other. If the bodies are separated and there is no contact, the force disappears. Examples include friction between surfaces, the normal reaction of a table on a book, tension in a rope when pulling an object and air resistance on a moving body. In contrast, non-contact forces such as gravity, electrostatic and magnetic forces act even when the bodies are separated by some distance.

Step-by-Step Solution:
Step 1: Recall the definition of a contact force: it requires actual touching or physical interaction between bodies. Step 2: Check option A: it states that a contact force acts only when two bodies are physically touching and gives friction between a book and a table as an example. This matches the definition and provides a correct example. Step 3: Check option B: it describes a force that acts at a distance without physical contact, such as gravity between Earth and the Moon. This is clearly a non-contact force, so B describes the opposite of a contact force. Step 4: Check option C: it claims the force can act with or without contact, which does not fit the strict definition of a contact force, and it confuses weight (a gravitational force) with contact forces. Step 5: Check option D: it focuses on whether the force changes motion, which is not how contact force is defined; a contact force can change motion or be part of a balanced system.

Verification / Alternative check:
Textbooks typically list contact forces such as friction, normal reaction, tension, applied push or pull and air resistance, all of which require contact between interacting bodies (or contact with a fluid in the case of drag). Non-contact forces listed include gravitational, electrostatic and magnetic forces. When a book rests on a table, the table exerts an upward normal force and friction can act if we try to slide the book. Both are present only because the book is touching the table. If you lift the book off the table, these contact forces vanish, confirming the definition.

Why Other Options Are Wrong:
A force that acts at a distance without physical contact, such as gravitational attraction between Earth and the Moon: This clearly defines a non-contact force, not a contact force.

A force that can act with or without contact, such as weight acting on a block on a table: Weight is due to gravity and remains even if the block is in free fall, so it is not a contact force.
A force that never changes the motion of an object, such as a balanced normal reaction: Whether a force changes motion depends on net force, not on whether it is a contact or non-contact force.

Common Pitfalls:
Some learners mix up contact and non-contact forces because both can coexist in a scenario (for example, a block on a table experiences both weight and normal reaction). Another confusion is to think that only forces that change motion are "real." In fact, a contact force can be part of a balanced force system and still be genuine. The defining feature of contact forces is the need for physical contact between interacting bodies.

Final Answer:
A contact force is a force that acts only when two bodies are physically touching, such as friction between a book and a table.