Inertia & Action-Reaction

Inertia is the tendency of an object to keep its current state of motion, and it is greater for larger mass. When a bus brakes hard, passengers lean forward because their bodies keep wanting to move ahead. The law of action and reaction says that if one body pushes another, an equal force comes back the other way, and the two forces act on different bodies. On this page, switch between inertia and action-reaction modes to see the bus stop and the wall push.

Newton's First Law (Inertia)

🚌 Intuition — A Bus Suddenly Stops

①Passengers lurch forward when a moving bus brakes hard

②The body keeps wanting to move forward

③This is inertia — bigger mass means bigger inertia

Visualizing It

Newton's Third Law (Action-Reaction)

🤜 Intuition — Push a Wall, the Wall Pushes You

①Pushing a wall hurts your hand — the wall pushes back

②The two forces have equal magnitude, opposite direction

③They act on different bodies (NOT the same body!)

Summary

Action-Reaction Law

F_action = −F_reaction

Equal size, opposite direction; acts on two different bodies

Law of Inertia

External force = 0 → stays at rest or in uniform motion

Bigger mass = greater inertia

🎯 Exam Points

①Inertia: tendency to keep the current motion

②Bigger mass → greater inertia

③Action-reaction always come in pairs

④Equal magnitude, opposite direction

⑤They act on different objects (net force is not 0!)

Examples and Unit Test

Example 1

When a moving bus suddenly stops, a standing passenger lurches forward. Explain why, using inertia.

Inertia is the tendency of an object to keep its original state of motion.

Even when the bus stops, the passenger's body keeps moving forward because of inertia, so it lurches forward.

▸ The passenger lurches forward because of the inertia that keeps the body in motion

Inertia is greater for larger mass, and with no outside force the state of motion is kept.

Example 2

When a person pushes a wall with a hand, the hand hurts too. Explain this with the law of action and reaction, and state the sizes and directions of the two forces and which objects they act on.

When the hand pushes the wall (action), the wall pushes the hand back with the same size in the opposite direction (reaction).

The two forces are equal in size and opposite in direction, and they act on different objects (the hand and the wall).

▸ Action = reaction: equal size, opposite direction, acting on different objects (hand and wall)

Action and reaction always come in a pair and act on different objects, so they do not cancel.

Unit Test

Which statement about inertia and the law of action and reaction is correct?

①The smaller the mass, the greater the inertia

②Action and reaction act together on one object

③Action and reaction are equal in size and opposite in direction

④If the outside force is zero, a moving object soon stops

⑤Inertia appears only in objects at rest

▸ ③ Action and reaction are equal in size and opposite in direction

The greater the mass, the greater the inertia (① wrong), and action and reaction act on different objects (② wrong).

With zero outside force a moving object keeps uniform straight-line motion (④ wrong), and inertia appears in objects both at rest and in motion (⑤ wrong); action and reaction are equal in size and opposite in direction (③ correct).

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