Newton's Laws of Motion
Senior 1 · Physics
Newton's Laws of Motion
Lesson 1 of 5
24 min
A bus from Nyabugogo to Huye is moving fast when the driver brakes hard. The standing passengers lurch forward, and a bag slides off a seat onto the floor. A few minutes later the bus pulls away quickly and the same passengers stagger backwards. Nobody pushed them either time. So what moved them?
More than 300 years ago Isaac Newton answered this question with three laws that still describe almost every motion you see, from a football to a rocket. This lesson is about the first law, and about the property that explains the passengers: inertia.
Definition
Inertia is the tendency of a body to stay at rest, or to keep moving with the same speed in the same straight line. A body with inertia resists any change in its state of motion.
Mass () is the measure of a body's inertia. SI unit: the kilogram (kg). The greater the mass, the harder it is to start the body moving, to stop it, or to change its direction. Example: a 12 000 kg lorry has far more inertia than a 15 kg bicycle.
You need: a plastic cup, a stiff card (a playing card or a piece of a cereal box) and a coin.
Figure 1.1: When the card is flicked away quickly, the coin does not go with it. It drops straight down into the cup.
Observation: a sharp flick sends the card flying, but the coin drops into the cup. A slow pull drags the coin along with the card.
Explanation: the coin was at rest, and its inertia keeps it at rest. A quick flick lets friction from the card act on the coin for only a tiny moment, too short to get it moving. Once the card has gone, nothing holds the coin up, so its weight pulls it down into the cup. A slow pull lets friction act for longer, so the coin is dragged along.
Important
Safety. Use a plastic cup, not glass, and flick the card away from other people's faces. Pick up coins from the floor straight away so nobody slips.
Hang two identical tins from a beam on strings of the same length, one empty and one full of sand. Give each the same gentle push with one finger, then try to stop each one with your finger.
| Empty tin (small mass) | Tin full of sand (large mass) | |
|---|---|---|
| Starting it moving | easy: it swings away quickly | hard: it hardly moves |
| Stopping it | easy | hard: it pushes back on your finger |
| Inertia | small | large |
The sand-filled tin has more mass, so it has more inertia: it resists both starting and stopping. Both tins are hanging, so this is not about which one is heavier to lift; it is about how hard each one is to speed up or slow down.
Definition
The resultant force on a body is the single force that has the same effect as all the forces acting on it together. Forces in the same direction add; forces in opposite directions subtract.
If the resultant force is zero, the forces are balanced. If it is not zero, they are unbalanced.
Newton's first law of motion: a body stays at rest, or keeps moving with a constant velocity (the same speed in the same straight line), unless a resultant external force acts on it.
A free-body diagram shows one body on its own with every force that acts on it drawn as an arrow from its centre. The arrow points the way the force acts, and its length shows its size.
Figure 1.2: In both diagrams the forces are balanced. The book stays at rest; the car keeps moving at a constant velocity. Balanced forces do not mean “no motion”; they mean “no CHANGE in motion”.
Look at diagram (b) carefully. The car is moving, yet the resultant force on it is zero. The engine's driving force is used only to cancel friction and air resistance. If the driver presses harder on the accelerator, the driving force becomes bigger than the resistance, the forces become unbalanced, and the car speeds up.
Worked example 1: a car at constant velocity
A car travels along the Kigali–Musanze road at a steady . The driving force from its engine is . What is the total resistive force (friction plus air resistance) on the car?
The velocity is constant, so by the first law the resultant force is zero:
The resistive force is , acting backwards.
Worked example 2: is the box going to move?
Two pupils push a heavy box of books across the classroom floor. Keza pushes to the right with and Mugisha pulls to the right with . Friction on the box is to the left. Find the resultant force and say what the box does.
The forces are balanced. If the box was at rest it stays at rest; if it was already sliding, it keeps sliding at a constant velocity.
Worked example 3: comparing inertia
A bicycle has a mass of and a loaded lorry has a mass of . How many times more inertia does the lorry have?
The lorry has 800 times the inertia of the bicycle, which is why a lorry needs a much larger braking force and a much longer distance to stop.
Warning
A moving body does not need a force to keep it moving. A ball rolling on the ground slows down because friction and air resistance act on it, not because "the force of the kick runs out". On ice, or in space, where there is almost no friction, a moving body keeps going.
Tip
Remember it: no resultant force, no change. At rest stays at rest; moving stays moving. Mass measures how strongly a body refuses to change.
Try it: in PhET: Forces and Motion: Basics (version 2.6.0), open the Friction screen, set friction to none, push the crate and then let go. Watch the speed stay the same.
Newton's first law of motion (Khan Academy). Watch for why a moving object does not need a force to keep moving, and what really makes things slow down.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the experiments, diagrams, worked examples, Rwandan applications and practice exercises.
