Force & Newton's Laws

Why does a ball stop rolling? Why do you lurch forward when a bus brakes? Why does it take more effort to push a loaded cart? Sir Isaac Newton answered these questions with three elegant laws.

Newton's 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 direction, unless acted upon by an unbalanced force.

Example: When a bus suddenly stops, you lurch forward because your body wants to keep moving (inertia).

Inertia depends on mass -- heavier objects are harder to start or stop.

Newton's Second Law (F = ma)

F = ma (Force = mass x acceleration). Unit: Newton (N). 1 N = 1 kg x 1 m/s^2.

Example: A 5 kg box pushed with 20 N: a = 20/5 = 4 m/s^2

Example: Force to accelerate a 1,200 kg car at 3 m/s^2: F = 1200 x 3 = 3,600 N

Newton's Third Law (Action-Reaction)

For every action, there is an equal and opposite reaction.

Example: When you push against a wall, the wall pushes back equally. When a rocket expels gas downward, the gas pushes the rocket upward.

Important: Action and reaction act on different objects -- they do not cancel out.

Friction

Friction opposes motion between two surfaces in contact.

• Static friction keeps objects at rest (must be overcome to start moving)
• Kinetic friction acts on sliding objects

Friction depends on surface roughness and the normal force (weight pressing surfaces together).

Nepal Connection: In hilly Nepal, friction prevents vehicles from sliding on steep roads. During monsoon, wet roads reduce friction, making driving more dangerous.

Key Takeaways

• First Law: Objects resist changes in motion (inertia)
• Second Law: F = ma connects force, mass, and acceleration
• Third Law: Every action has an equal opposite reaction on a different object
• Friction opposes motion and depends on surface type and weight