- Science
- Classes 9-10
- 5 min read
Newton's laws of motion
Learn the three laws of motion: inertia, force equals mass times acceleration, and equal and opposite reactions, with worked sums.
Let us begin
The big idea
Why do you lurch forward when a bus brakes suddenly? Why is a loaded cart harder to push? Sir Isaac Newton explained these with three simple laws of motion.
First law (inertia): an object stays at rest, or keeps moving in a straight line at constant speed, unless an unbalanced force acts on it. Inertia is the tendency of an object to resist a change in its state of motion. The greater the mass, the greater the inertia. This is why passengers lurch forward when a bus brakes.
Momentum is mass times velocity: p = m × v. Its unit is kg m/s.
Second law: the rate of change of momentum of an object is proportional to the unbalanced force. For a body of constant mass this gives F = m × a. Force (F) is in newtons (N), mass (m) in kilograms (kg) and acceleration (a) in metres per second squared (m/s²). One newton is the force that gives a mass of 1 kg an acceleration of 1 m/s². For the same force, a heavier object accelerates less.
Third law: to every action there is an equal and opposite reaction. The two forces act on two different bodies at the same time. When a swimmer pushes the water backward, the water pushes the swimmer forward. A balloon let go flies forward as air rushes out backward.
Because the action and reaction act on different objects, they do not cancel each other. A gun recoils backward when a bullet is fired forward, and the gun moves less because its mass is much more.
Worked examples
See how it works
Find the force
A force acts on a 5 kg box and gives it an acceleration of 2 m/s². F = m × a = 5 × 2 = 10 N.
Find the acceleration
A force of 20 N acts on a 4 kg trolley. a = F ÷ m = 20 ÷ 4 = 5 m/s².
Find the mass
A force of 30 N gives an object an acceleration of 6 m/s². m = F ÷ a = 30 ÷ 6 = 5 kg.
Find the momentum
A 2 kg ball moves at 3 m/s. p = m × v = 2 × 3 = 6 kg m/s.
Try it at home
Balloon rocket
You will need
- A long balloon
- A drinking straw
- About 3 metres of string
- Sticky tape
- Two chairs
What to do
- Thread the string through the straw. Tie the string between two chairs, pulled tight, about 3 m apart.
- Blow up the balloon, but do not tie it. Pinch its neck closed with your fingers.
- Tape the balloon to the straw along its side while keeping the neck pinched.
- Pull the balloon to one end of the string, then let go of the neck.
- Watch the balloon shoot along the string. The air rushes backward (action) and the balloon moves forward (reaction).
Ask a grown-up: An adult should help set up the string and tie the chairs apart. Keep the pieces of a burst balloon away from small children and never put them in the mouth.
Fun fact
The unit of force, the newton, is named after Sir Isaac Newton. A small apple weighs roughly one newton.
Practice sheet
Check what you learned
Practice sheet: Newton's laws of motion (Classes 9-10, Science)
- State Newton's first law of motion.
- What is inertia?
- Why do passengers in a bus lurch forward when the bus stops suddenly?
- Which has more inertia: a 2 kg stone or a 20 kg stone?
- What is the formula for momentum?
- State Newton's second law in the form of a formula.
- A force acts on a 5 kg box and gives it an acceleration of 3 m/s². Find the force.
- A force of 24 N acts on a 6 kg body. Find the acceleration.
- A force of 40 N gives a body an acceleration of 8 m/s². Find its mass.
- Find the momentum of a 1500 kg car moving at 10 m/s.
- State Newton's third law.
- Why does a rocket move forward when hot gases rush out of its back?
Answer key
Answers: Newton's laws of motion
- An object remains at rest or in uniform motion in a straight line unless an unbalanced force acts on it.
- The tendency of an object to resist a change in its state of rest or motion.
- Their bodies tend to keep moving forward because of inertia, while the bus slows down.
- The 20 kg stone, because more mass means more inertia.
- p = m × v.
- F = m × a (force = mass × acceleration).
- F = 5 × 3 = 15 N.
- a = 24 ÷ 6 = 4 m/s².
- m = 40 ÷ 8 = 5 kg.
- p = 1500 × 10 = 15 000 kg m/s.
- To every action there is an equal and opposite reaction.
- The rocket pushes the gases backward (action). The gases push the rocket forward (reaction).