CBSE Class 9 Science Notes: Force and Laws of Motion

Effects of Force on Motion

Force is a fundamental concept in physics that can significantly alter the motion of an object. Here’s a breakdown:

• Change in Speed: A force can cause an object to speed up (accelerate), slow down (decelerate), or even maintain a constant speed.
• Change in Direction: A force can change the direction of an object’s motion. Think of a soccer ball being kicked, changing its path.
• Change in Shape and Size: A force can deform an object, altering its shape or size. Consider squishing a ball of clay.
• State of Rest to Motion: A force can initiate motion. A stationary object begins to move when a force is applied.

In essence, force is the ’cause’ and motion is the ‘effect’.

Newton’s Three Laws of Motion

Newton’s laws of motion are the cornerstone of classical mechanics.

1. First Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force.
   Core Principle: Objects resist changes in their state of motion.


2. Second Law: The acceleration of an object is directly proportional to the net force acting on it, is in the direction of the net force, and is inversely proportional to its mass.
   Formula: $F = ma$ (where F = Force, m = mass, a = acceleration)
   Core Principle: Force causes acceleration.


3. Third Law: For every action, there is an equal and opposite reaction.
   Core Principle: Forces always come in pairs.

Action–Reaction Forces

This directly relates to Newton’s Third Law.

When one object exerts a force (action) on another object, the second object simultaneously exerts a force back on the first object (reaction).

• Equal in Magnitude: The action and reaction forces have the same strength.
• Opposite in Direction: The forces act in opposite directions.
• Act on Different Objects: The forces act on different objects and don’t cancel each other out.
• Example: When you walk, your foot pushes the ground backward (action), and the ground pushes your foot forward (reaction).

Inertia and its Types

Inertia is the tendency of an object to resist changes in its state of motion. It’s directly related to an object’s mass.

• Inertia of Rest: The resistance of a stationary object to start moving.
  Example: A passenger in a bus leans backward when the bus suddenly starts moving.
• Inertia of Motion: The resistance of a moving object to stop moving or change its velocity.
  Example: A passenger in a moving bus leans forward when the bus suddenly stops.
• Inertia of Direction: The resistance of an object to change its direction of motion.
  Example: When a car takes a sharp turn, passengers tend to be thrown outwards.

Mass and Momentum

Mass is a measure of the amount of matter in an object. Momentum is a measure of the “motion” of an object.

• Mass: A fundamental property of an object, measured in kilograms (kg). It’s a scalar quantity.
• Momentum: The product of an object’s mass and velocity.
  Formula: $p = mv$ (where p = momentum, m = mass, v = velocity)
  Core Principle: Momentum is a vector quantity (it has both magnitude and direction). A change in momentum is called impulse.
• Conservation of Momentum: In a closed system, the total momentum remains constant if no external forces are acting.

Relation between Force and Acceleration

Newton’s Second Law provides the critical link between force and acceleration.

As stated before, $F = ma$

• Direct Proportionality: The acceleration of an object is directly proportional to the net force acting on it. Doubling the force doubles the acceleration (assuming mass remains constant).
• Inverse Proportionality with Mass: The acceleration of an object is inversely proportional to its mass. Doubling the mass halves the acceleration (assuming the force remains constant).
• Direction of Acceleration: The acceleration always occurs in the same direction as the net force.