1. Statement: “An object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.”
2. Explanation: The law means that objects do not change their state of rest or uniform motion on their own — they need a net force to do so. If the net force is zero, acceleration is zero.
3. Example 1 (Rest → Motion): A book lying on a table will not move by itself. You need to apply a force to push it. Without a force, it stays at rest.
4. Example 2 (Motion → Rest): When you stop pedalling a bicycle, the force of friction brings it to rest. If there were no friction (no net force), the bicycle would continue moving forever at constant velocity.
5. Galileo’s Contribution: In ancient times, it was wrongly believed that a continuous force was needed to keep an object moving. In the 17th century, Galileo Galilei used thought experiments to show that if all impediments (like friction) are removed, a body moving on a horizontal plane would continue to move indefinitely without any applied force. Newton built on this idea to formally state his First Law.

1. Statement: “When a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is proportional to the net force and inversely proportional to the mass of the object.”
2. Step 1 — Force proportional to acceleration (for fixed mass): From experiments, if mass m is constant, then acceleration a ∝ F.
3. Step 2 — Acceleration inversely proportional to mass (for fixed force): If force F is constant, then a ∝ 1/m.
4. Step 3 — Combining both relations: a ∝ F/m → F = k × m × a, where k is a proportionality constant.
5. Step 4 — Choosing SI units: The unit of force (newton) is defined so that k = 1. Therefore: F = m × a
6. Definition of 1 Newton: One newton is the force that produces an acceleration of 1 m s⁻² in an object of mass 1 kg. That is: 1 N = 1 kg × 1 m s⁻² = 1 kg m s⁻²

1. Statement: “Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object.” These forces are called action-reaction pairs.
2. Key Note: The two forces are equal in magnitude and opposite in direction, but they act on two different objects. Therefore, they cannot cancel each other (cancellation only happens when two forces act on the same object).
3. Example 1 — Rowing a canoe: The canoeist pushes water backward with the paddle (action). The water pushes the paddle and canoe forward with an equal force (reaction). Action acts on water; reaction acts on the canoe. Both are equal in magnitude but do not cancel — the canoe moves forward.
4. Example 2 — Walking: When you walk, your foot pushes the ground backward (action). The ground exerts an equal frictional force on your foot in the forward direction (reaction). This reaction pushes you forward. Without friction, your foot would slip and you could not walk.
5. Conclusion: Newton’s Third Law applies to all forces — contact (friction, normal) and non-contact (gravity, magnetic, electrostatic).

1. Objective: To investigate how the force of friction varies with the nature of surfaces.
2. Setup: A stack of 4 coins (secured with adhesive tape) is launched using a stretched rubber band. The rubber band is stretched to the same mark (point C) every time. This ensures the same initial force each time.
3. Procedure: The experiment is repeated on different surfaces — wooden table top, laminated table top, and polished marble/tile floor. The distance the coin stack travels before stopping is recorded.
4. Observation: The stack travels the shortest distance on the wooden surface (rough) and the longest distance on the polished marble (smooth).
5. Explanation: On rough surfaces, the force of friction is greater — it reduces the coin stack’s velocity quickly, so it stops in a shorter distance. On smooth surfaces, friction is smaller, so the velocity decreases more slowly and the coins travel farther.
6. Conclusion: The force of friction depends on the nature of surfaces in contact — rougher surfaces produce greater friction. A spring balance can be used to directly measure the friction force on each surface, confirming the conclusion.

Why are the accelerations different? Even though the forces are equal (Newton’s Third Law), the masses are very different. By a = F/m, the lighter bullet gains much greater acceleration than the heavy gun. The gun “recoils” slowly while the bullet shoots forward fast.

1. Setup: Box 1 (mass m₁) and Box 2 (mass m₂) are connected by a string on a frictionless horizontal surface. A horizontal force F is applied on Box 1 to the right.
2. Tension as Internal Force: Box 1 pulls Box 2 forward through the string with tension T. By Newton’s Third Law, Box 2 pulls Box 1 backward with the same tension T. T is an internal force within the two-box system.
3. Treating as a System: When we consider both boxes together as a single system, internal forces (T on Box 1 and T on Box 2) cancel each other out. Only external forces matter for the system’s overall acceleration.
4. Applying Newton’s Second Law to the System: The only external horizontal force is F. Total mass of system = m₁ + m₂.
5. Derivation: a = F / (m₁ + m₂)
6. Physical Meaning: The system of two connected boxes accelerates exactly like a single object of mass (m₁ + m₂) under force F. This approach greatly simplifies analysis of connected objects.