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Why do you lurch forward when a bus brakes suddenly? Why does a rocket shoot upward when its exhaust gases blast downward? Why can a heavy lorry not stop or start as nimbly as a bicycle? The answers lie in two of the three laws of motion set out by Isaac Newton more than three hundred years ago — laws so reliable that they still guide everything from car safety to spaceflight. This lesson, part of Topic P2 (Forces) of OCR Gateway Science A, covers Newton's First Law (about balanced forces and inertia) and Newton's Third Law (about forces coming in equal and opposite pairs). Newton's Second Law, which links force, mass and acceleration, has its own lesson next.
By the end of this lesson you should be able to state Newton's First Law and explain it using the idea of resultant force, describe inertia and inertial mass, state Newton's Third Law, and identify action–reaction force pairs.
Newton's First Law states that an object will stay at rest, or keep moving at a constant velocity (constant speed in a straight line), unless a resultant force acts on it. In other words, an object's motion only changes if there is a non-zero resultant force.
This law has two halves, depending on the object's starting state:
So a zero resultant force does not mean "no motion" — it means "no change in motion". To change an object's speed or direction, a resultant force is always needed.
Exam Tip: Newton's First Law: with zero resultant force, an object stays at rest or continues at constant velocity. The key idea is that a resultant force is needed to change motion, not to maintain it. A moving object does not need a forward force to keep moving at steady speed.
A common misconception, going right back to the ancient Greeks, is that a constantly applied force is needed just to keep something moving. In fact, once moving, an object would continue forever at constant velocity if no resultant force acted. The reason a rolling ball slows and stops on Earth is friction (and air resistance) — a resultant force opposing its motion. Remove those forces (as in deep space) and the object would keep going indefinitely.
When the forces on an object are balanced, you can confidently say its velocity is not changing. When they are not balanced — there is a resultant force — the object will accelerate (speed up, slow down or change direction) in the direction of that resultant force.
Inertia is the tendency of an object to keep doing what it is already doing — to stay at rest if it is at rest, or to keep moving if it is moving. It is the reason you are thrown forward when a car brakes sharply: your body "wants" to keep moving at the old speed, and only the seatbelt provides the force to slow you with the car. Inertia is also why a massive object is hard to get moving, and once moving is hard to stop.
The more mass an object has, the more inertia it has, and so the harder it is to change its motion (to start it, stop it, speed it up, slow it down or change its direction). This is why a loaded lorry needs a much bigger force to accelerate or to stop than a bicycle does.
Higher tier: Inertial mass is a measure of how difficult it is to change the velocity of an object. It is defined as the ratio of the force needed to the acceleration produced:
inertial mass=accelerationforce
This comes straight from Newton's Second Law (F=ma, so m=F/a). An object with a large inertial mass needs a large force to give it even a small acceleration — it strongly "resists" changes in its motion.
Exam Tip: Inertia is the tendency to resist a change in motion; the greater the mass, the greater the inertia. (Higher) inertial mass =accelerationforce measures exactly how hard it is to change an object's velocity.
Newton's Third Law states that whenever two objects interact, they exert equal and opposite forces on each other. The forces are often described as action and reaction: for every action force, there is an equal and opposite reaction force.
The crucial detail — and the one examiners test hardest — is that the two forces in a Newton's Third Law pair act on different objects. If object A pushes on object B, then object B pushes back on object A with a force that is:
For example, when you push on a wall, the wall pushes back on you with an equal and opposite force — which is why you can feel the wall resist, and why pushing hard on a wall while on a skateboard sends you rolling backward.
Exam Tip: A Newton's Third Law pair is equal in size, opposite in direction, the same type of force, and acts on two different objects. The phrase "on different objects" is essential — if both forces acted on the same object they would simply cancel, and nothing could ever accelerate.
To name a Newton's Third Law pair correctly, describe each force as "the force of X on Y", then swap X and Y for its partner. Some standard examples:
| One force (action) | Its Third-Law partner (reaction) |
|---|---|
| The force of your foot pushing back on the ground when walking | The force of the ground pushing you forward |
| The force of a swimmer's hands pushing the water backward | The force of the water pushing the swimmer forward |
| The force of a rocket pushing exhaust gas downward | The force of the exhaust gas pushing the rocket upward |
| The gravitational pull of the Earth on the Moon | The gravitational pull of the Moon on the Earth |
| The force of a hammer on a nail | The force of the nail on the hammer |
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