- Explain what a force is and know that it is measured in newtons
- Find the resultant of forces acting along one line
- State Newton's first law and explain inertia with examples
- State Newton's third law and identify action–reaction pairs
When a bus brakes suddenly, the passengers lurch forward. When you push off a wall on roller skates, you roll backwards. Both everyday events are explained by Isaac Newton's laws of motion. In this lesson you will meet two of them: the first law and the third law.
What is a force?
A vector quantity that measures how one object acts on another. A force has a size, a direction and a point where it acts. Its unit is the newton (N), and it is measured with a spring balance (force meter).
- A force can speed an object up or slow it down (kicking a ball, braking).
- A force can change the direction of motion (a footballer heading the ball).
- A force can change an object's shape (a spring stretches, modelling clay is squashed).
Resultant force
Usually several forces act on an object at once. The single force that has the same effect as all of them together is called the resultant (net) force. Forces in the same direction add up; for forces in opposite directions, subtract the smaller from the larger, and the resultant points the way of the larger force.
Murad pulls a box to the right with a force of 60 N. a) Elvin helps him and pulls the box to the right with 40 N. b) Elvin pulls the box to the left with 40 N. Find the resultant force in each case.
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b) The forces are opposite: R = 60 N − 40 N = 20 N, to the right (towards the larger force).
Inertia and Newton's first law
The ancient Greek thinker Aristotle believed that an object moves only while a force keeps acting on it. Galileo Galilei showed with experiments that, without friction, a ball rolling on a smooth surface would never stop. What stops moving objects is friction and air resistance.
If the resultant force on an object is zero, the object either stays at rest or keeps moving in a straight line at constant speed. This tendency of objects to keep their velocity is called inertia.
- When a bus brakes, the passengers keep moving forward at their old speed because of inertia, so they lurch forward.
- A seat belt stops your body from flying forward by inertia when a car stops suddenly.
- When you shake wet hands, the water drops carry on moving by inertia and fly off.
Newton's third law: action and reaction
Forces always come in pairs. When you push a wall with your hand, the wall pushes you back with the same force — on roller skates you would roll backwards. This is an interaction.
- F₁₂force exerted by object 1 on object 2
- F₂₁force exerted by object 2 on object 1
Two objects act on each other with forces that are equal in size and opposite in direction.
On an ice rink, Aysel (mass 40 kg) pushes Murad (mass 60 kg) with a force of 120 N. What force does Murad exert on Aysel? Which of them gets the greater acceleration, and how many times greater?
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The forces are equal, but Aysel's mass is smaller, so her acceleration is larger.
The accelerations are inversely proportional to the masses: 60 kg / 40 kg = 1.5 times.
(With a = F / m from the next lesson: Aysel gets 3 m/s² and Murad gets 2 m/s².)
- When you walk, your foot pushes the ground backwards and the ground pushes you forwards.
- A swimmer pushes water backwards with their hands, and the water pushes the swimmer forwards.
- A rocket pushes hot gases downwards, and the gases push the rocket upwards.
Key points
- A force measures how one object acts on another; it is a vector and is measured in newtons (N).
- Forces in the same direction add up; opposite forces are subtracted.
- First law: with zero resultant force, an object stays at rest or moves in a straight line at constant speed.
- Mass is a measure of inertia.
- Third law: objects act on each other with equal and opposite forces, which act on different objects.
Check yourself
10 questions. Every correct answer earns XP.