Laws of Motion: Comprehensive NEET Physics Notes

1. Introduction

In this chapter, we explore the fundamental laws that govern the motion of bodies. Previously, we focused on describing the motion of particles in space, both uniform and non-uniform. Now, we delve into the forces that cause motion.

2. Aristotle’s Fallacy

Aristotle believed that an external force is required to keep a body in motion. This view, although intuitive, is incorrect because it does not account for friction, which opposes motion.

2.1 Common Misconception:

Aristotle's view that a continuous force is needed to keep a body in motion is flawed. In reality, a body in uniform motion does not require an external force if friction is absent.

2.2 NEET Tip:

Remember that friction is the force that opposes motion, and an external force is needed only to counteract friction to maintain uniform motion.

3. The Law of Inertia

Galileo's experiments on inclined planes led to the understanding that a body in motion will continue to move at a constant velocity unless acted upon by an external force. This resistance to change in motion is termed inertia.

3.1 Did You Know?

Galileo's inclined plane experiments paved the way for Newton's first law of motion, also known as the law of inertia.

3.2 NEET Problem-Solving Strategy:

For problems involving inclined planes, always consider the component of forces parallel and perpendicular to the plane.

4. Newton’s First Law of Motion

Every body continues to be in its state of rest or uniform motion in a straight line unless compelled by some external force to act otherwise. This law introduces the concept of inertia.

4.1 Real-life Application:

Seatbelts in cars are designed based on the principle of inertia. In a sudden stop, the seatbelt provides the external force needed to stop the motion of the body.

4.2 NEET Tip:

Always identify all forces acting on a body and determine if they cancel out. If the net external force is zero, the body is either at rest or in uniform motion.

5. Newton’s Second Law of Motion

The rate of change of momentum of a body is directly proportional to the applied force and occurs in the direction in which the force acts. Mathematically, $F = ma$ .

5.1 Momentum

Momentum ( $p$ ) is the product of the mass ( $m$ ) and velocity ( $v$ ) of a body: $p = m v$

5.2 NEET Problem-Solving Strategy:

Break down forces into components and apply $F = ma$ separately in each direction (x and y axes).

5.3 Common Misconception:

A common error is to confuse mass with weight. Remember, mass is a measure of the amount of matter in an object, while weight is the force exerted by gravity on that mass.

6. Newton’s Third Law of Motion

For every action, there is an equal and opposite reaction. This means forces always occur in pairs.

6.1 Did You Know?

The recoil of a gun is an example of Newton's third law. When the bullet is fired forward, the gun is pushed backward with equal force.

6.2 NEET Tip:

Identify action-reaction pairs in problems to correctly apply the third law.

7. Conservation of Momentum

In an isolated system, the total momentum before an interaction is equal to the total momentum after the interaction.

7.1 Real-life Application:

Rockets work on the principle of conservation of momentum. The expulsion of gas at high speed generates a thrust that propels the rocket forward.

7.2 NEET Tip:

Use the conservation of momentum in collision and explosion problems to simplify calculations.

Quick Recap

Aristotle’s Fallacy: Incorrect belief that continuous force is needed for motion.
Law of Inertia: Bodies remain in their state unless acted upon by an external force.
Newton’s First Law: Introduces inertia.
Newton’s Second Law: $F = ma$ .
Newton’s Third Law: Action and reaction are equal and opposite.
Conservation of Momentum: Momentum is conserved in isolated systems.

Practice Questions

Question: A 5 kg mass is subjected to a force of 20 N. What is its acceleration? Solution: Using $F = ma$ , $20 = 5 \cdot a \Rightarrow a = 4 m/s^{2}$
Question: Calculate the momentum of a 10 kg object moving at 15 m/s. Solution: Momentum, $p = m v = 10 \cdot 15 = 150 kg m/s$
Question: A 2 kg mass moving with a velocity of 3 m/s collides with a 3 kg mass at rest. If they stick together, find their common velocity after collision. Solution: Using conservation of momentum, $m_{1} v_{1} + m_{2} v_{2} = (m_{1} + m_{2}) v$ $2 \cdot 3 + 3 \cdot 0 = (2 + 3) v \Rightarrow v = \frac{6}{5} = 1.2 m/s$
Question: A 0.2 kg ball moving at 10 m/s strikes a wall and bounces back at 8 m/s. Find the impulse imparted to the ball. Solution: Impulse, $J = Δ p = m (v_{f} - v_{i}) = 0.2 (8 - (- 10)) = 0.2 \cdot 18 = 3.6 N s$
Question: What is the force required to keep a 10 kg object moving at a constant velocity of 2 m/s on a surface with a friction coefficient of 0.5? Solution: Force of friction, $f = μ N = μ m g = 0.5 \cdot 10 \cdot 9.8 = 49 N$

Glossary

Force (F): An interaction that changes the motion of an object.
Inertia: The resistance of an object to change its state of motion.
Momentum (p): The product of an object's mass and velocity.
Impulse (J): The change in momentum of an object.
Friction: A force that opposes motion between two surfaces in contact.

Quick Reference Guide

Newton's First Law: Inertia
Newton's Second Law: $F = ma$
Newton's Third Law: Action-Reaction
Conservation of Momentum: $m_{1} v_{1} + m_{2} v_{2} = (m_{1} + m_{2}) v$

These comprehensive notes should provide a thorough understanding of the laws of motion, relevant for NEET preparation.