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Physics · Class 9 · Chapter 4: Motion and Force

8 min

Key points

Motion is the change in position of a body with respect to time and place
Force is a push or pull that changes or tends to change the state of rest or uniform motion of a body
Newton's First Law: A body at rest remains at rest and a body in motion remains in uniform motion unless acted upon by an external force
Newton's Second Law: The rate of change of momentum is directly proportional to the applied force and takes place in the direction of force
Newton's Third Law: To every action there is always an equal and opposite reaction
Momentum is the product of mass and velocity of a moving body (p = mv)
Inertia is the property of matter by virtue of which it resists any change in its state of rest or uniform motion
Weight is the force with which earth attracts a body towards its center (W = mg)
Friction always opposes the relative motion between two surfaces in contact

Definitions

Force: Force is a push or pull that changes or tends to change the state of rest or uniform motion of a body.
Momentum: Momentum is the product of mass and velocity of a moving body.
Inertia: Inertia is the property of matter by virtue of which it resists any change in its state of rest or uniform motion.
Weight: Weight is the force with which earth attracts a body towards its center.
Friction: Friction is the force that opposes the relative motion between two surfaces in contact.

Worked example

Define motion and rest. Explain that motion and rest are relative terms with suitable examples. (5 marks)

Motion: A body is said to be in motion if it changes its position with respect to time and its surroundings. Rest: A body is said to be at rest if it does not change its position with respect to time and its surroundings. Motion and rest are relative terms because they depend upon the frame of reference or observer. Example: A passenger sitting in a moving bus is at rest with respect to the bus but in motion with respect to a person standing on the road. Similarly, trees appear to be moving backward to the passenger but are at rest for the roadside observer.

Common mistakes

Students confuse mass and weight - remember weight = mg, not just mass
Writing momentum formula as p = m/v instead of p = mv
Forgetting to state Newton's laws word-for-word as given in textbook
Not drawing proper free body diagrams when solving force problems
Mixing up action-reaction pairs in Newton's Third Law examples

Quick recap

Motion occurs when position changes with time relative to surroundings, while rest means no position change. Motion and rest depend on frame of reference (observer). Force is a push or pull that can change motion, shape, or direction of objects. Newton's three laws govern force and motion relationships, with applications in daily life phenomena.

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