When a moving bus brakes hard, the standing passengers lurch forward. The bus has stopped, but their bodies try to keep going at the speed they had. This tendency of an object to keep its state of motion is inertia, and the greater the mass, the greater the inertia.
Push hard on a wall and your palms hurt. The moment your hands push the wall, the wall pushes back on your hands with a force of the same size; this is the law of action and reaction.
Choosing inertia in the diagram shows passengers pitching forward in a stopped bus; choosing action and reaction shows the force of the hands on the wall and of the wall on the hands, equal in size and opposite in direction.
Because the two forces act on different objects, they do not cancel each other out.
The Law of Inertia (Newton's First Law)
🚌 When a Bus Stops Suddenly
①When a moving bus brakes hard, passengers lurch forward
②Their bodies tend to keep the motion they shared with the bus
③The tendency of an object to keep its state of motion is called inertia, and greater mass means greater inertia
④When a stopped bus suddenly pulls away, passengers lurch backward instead
Seeing It in Pictures
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Passengers in a bus that brakes hard (inertia) and a person pushing a wall (action and reaction)
The Law of Action and Reaction (Newton's Third Law)
🤜 Push a Wall and the Wall Pushes You
①Push a wall hard and your hands hurt, because the wall pushes your hands too
②The two forces are equal in size, opposite in direction, and always appear at the same moment
③The wall receives the action and the hands receive the reaction: they act on different objects
④A rocket pushes gas backward, and the gas pushes the rocket forward
Summary
Law of action and reaction
Faction = −Freaction
Equal in size, opposite in direction, each acting on a different object
Law of inertia
Net force on an object = 0 → it stays at rest or keeps moving in a straight line at constant speed
The greater the mass, the greater the inertia
🎯 Common Test Points
①Inertia is an object's tendency to keep its state of motion
②The greater the mass, the greater the inertia
③Action and reaction always appear together as a pair
④The two forces are equal in size and opposite in direction
⑤Action and reaction act on different objects, so they do not add up to zero; do not confuse them with two balanced forces on one object
Examples and Unit Review
Example 1
Use inertia to explain why standing passengers lurch forward when a moving bus stops suddenly.
1
Inertia is an object's tendency to keep its original state of motion.
2
Even though the bus stops, the passengers' bodies keep going at the speed they had with the bus, so they lurch forward.
▸ Because of inertia: their bodies tend to keep moving forward as before
A seat belt holds you back against exactly this inertia.
Example 2
When a person pushes a wall, their hands hurt. Explain this with the law of action and reaction, giving the size and direction of the two forces and the object each acts on.
1
When the hands push on the wall (action), the wall pushes back on the hands with a force of the same size in the opposite direction (reaction).
2
The two forces are equal in size and opposite in direction; the action acts on the wall and the reaction on the hands.
▸ Equal in size and opposite in direction; the action acts on the wall, the reaction on the hands
Your hands hurt because of the reaction from the wall, and the harder you push, the larger that reaction becomes.
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
1
The greater the mass, the greater the inertia (① wrong), and action and reaction act on different objects (② wrong).
2
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).