This guide explains the three fundamental laws that govern why and how objects move. Understanding these is essential for solving problems related to force and motion.

1. Short Notes: Core Concepts

A. Newton's First Law: The Law of Inertia

• The Law: An object at rest will stay at rest, and an object in motion will stay in motion with the same speed and in the same direction, unless acted upon by an unbalanced force.

• In Simple Terms: Things like to keep doing what they're already doing. This tendency to resist a change in motion is called inertia.

• Key Idea: No unbalanced force means no change in velocity (i.e., zero acceleration).

• Classic Example: When a bus suddenly stops, your body continues to move forward because of inertia.

B. Newton's Second Law: The Law of Acceleration (F = ma)

• The Law: The acceleration of an object is directly proportional to the unbalanced force acting on it and inversely proportional to its mass.

• The Formula: Force = mass × acceleration or F = ma

  • Force (F): Measured in Newtons (N)

  • mass (m): Measured in kilograms (kg)

  • acceleration (a): Measured in meters per second squared (m/s²)

• In Simple Terms:

  • To make something accelerate, you need to apply a force.

  • A bigger force produces a bigger acceleration.

  • A bigger mass requires a bigger force to get the same acceleration.

C. Newton's Third Law: The Law of Action and Reaction

• The Law: For every action, there is an equal and opposite reaction.

• In Simple Terms: If you push something, it pushes back on you with the exact same force.

• Key Idea: The action and reaction forces always act on different objects and are equal in size and opposite in direction.

• Classic Example: A rocket pushes hot gas downwards (action). The gas pushes the rocket upwards (reaction), causing it to launch.

D. Related Concepts

• Mass vs. Weight:

  • Mass (m): The amount of matter in an object. It's a scalar quantity (measured in kg) and is the same everywhere.

  • Weight (W): The force of gravity acting on an object's mass. It's a vector quantity (measured in N) and changes depending on gravity.

  • Formula: Weight = mass × gravitational acceleration or W = mg

• Momentum (p):

  • A measure of how difficult it is to stop a moving object.

  • It depends on both mass and velocity.

  • Formula: Momentum = mass × velocity or p = mv

  • Unit: kg m/s. It is a vector quantity.

2. Tips & Tricks for the Exam

• The Magic Triangle for F=ma: Cover the quantity you want to find to see the formula.

  • F = m × a

  • m = F / a

  • a = F / m

• Action-Reaction Pairs: Always remember the forces act on different bodies. When you kick a ball, the action is "foot pushes ball" and the reaction is "ball pushes foot".

• "Unbalanced Force": The key phrase in the first two laws. If forces are balanced (e.g., 10N push right, 10N friction left), the net force is zero and the object's motion doesn't change (it stays still or moves at a constant velocity).

3. Important Points & Common Exam Questions

• Calculations with F=ma: This is the most common type of question. You will be given two of the three variables (F, m, a) and asked to find the third.

  • Example Question: What force is needed to accelerate a 12 kg object at 2 m/s²?

  • Answer: F = ma -> F = 12 kg × 2 m/s² = 24 N.

• Calculating Weight: You will be asked to find the weight of an object given its mass, or vice versa. (Remember to use g ≈ 10 m/s²).

  • Example Question: What is the weight of a 60 kg person on Earth?

  • Answer: W = mg -> W = 60 kg × 10 m/s² = 600 N.

• Explaining Real-World Scenarios: You need to be able to apply the laws to explain everyday phenomena.

  • Why does a gun recoil when a bullet is fired?

  • Answer: Newton's Third Law. The gun exerts a forward force on the bullet (action). The bullet exerts an equal and opposite backward force on the gun (reaction), causing it to recoil.

  • Why is it harder to push a heavy box than a light one?

  • Answer: Newton's Second Law. The heavy box has more mass. According to a = F/m, for the same acceleration, a larger mass requires a larger force.

• Momentum Calculations:

  • Example Question: What is the momentum of a 2000 kg car moving at 10 m/s?

  • Answer: p = mv -> p = 2000 kg × 10 m/s = 20,000 kg m/s.