Kinetic vs Potential Energy (With Examples)
Kinetic and potential energy explained: formulas, units, everyday examples, energy conversion on roller coasters and pendulums, and a quick energy calculator.
Newton’s Three Laws of Motion, Explained With Everyday Examples
Newton’s first, second and third laws of motion explained simply, with everyday examples, the F = ma formula, worked problems and common misconceptions.
Key takeaways
- First law: objects keep doing what they’re doing unless a net force acts on them.
- Second law: net force equals mass times acceleration, F = ma.
- Third law: forces come in equal and opposite pairs acting on different objects.
On this page
In 1687, Isaac Newton published three laws that describe how things move. More than three centuries later, they still explain almost everything you see in daily life: why you lurch forward when a bus brakes, why heavy trolleys are harder to push and how rockets launch.
Newton’s first law: inertia
An object stays at rest, or keeps moving at a constant velocity, unless a net force acts on it.
Objects resist changes to their motion. This resistance is called inertia, and the more mass something has, the more inertia it has.
Everyday examples:
- When a bus brakes suddenly, your body keeps moving forward. That’s why seatbelts matter.
- A tablecloth can be whipped out from under plates, which tend to stay put.
- A hockey puck glides a long way on ice because there is very little friction to slow it down.
Newton’s second law: F = ma
The net force on an object equals its mass times its acceleration.
F = ma, where:
- F is the net force, in newtons (N)
- m is mass, in kilograms (kg)
- a is acceleration, in metres per second squared (m/s²)
A bigger force produces a bigger acceleration, and a bigger mass needs a bigger force for the same acceleration.
Everyday examples:
- An empty shopping trolley is easier to speed up than a full one.
- A football accelerates much more than a bowling ball when kicked with the same force.
Worked example
A 1,200 kg car accelerates at 3 m/s². What net force is acting on it?
F = ma = 1,200 × 3 = 3,600 N
Worked example: rearranging
A net force of 50 N acts on a 10 kg box. What is its acceleration?
a = F ÷ m = 50 ÷ 10 = 5 m/s²
Newton’s third law: action and reaction
For every force, there is an equal and opposite force.
When object A pushes on object B, object B pushes back on A with a force of the same size in the opposite direction. The two forces act on different objects, which is why they don’t cancel out.
Everyday examples:
- When you walk, your foot pushes backwards on the ground, and the ground pushes you forwards.
- A rocket pushes exhaust gases down; the gases push the rocket up.
- When you jump off a small boat, the boat moves backwards.
The laws working together
Think of a vehicle stuck in sand being pulled free by a second vehicle using a stretchy recovery strap. The strap stores energy as it stretches, then pulls on the stuck vehicle (second law: the net force produces acceleration), while the stuck vehicle pulls back on the strap equally (third law). JeepSouq’s desert recovery gear guide shows why doing this safely matters: those forces are large.
Summary table
| Law | Statement | Key idea | Example |
|---|---|---|---|
| First | Objects keep their motion unless a net force acts | Inertia | Lurching forward when a bus brakes |
| Second | F = ma | Force causes acceleration | Pushing a full vs empty trolley |
| Third | Equal and opposite forces | Forces come in pairs | A rocket launch |
Where Newton’s laws apply
Newton’s laws work brilliantly for everyday speeds and sizes. For objects moving close to the speed of light, Einstein’s relativity is needed, and for very tiny particles, quantum mechanics takes over.
Ready to use them? Try our projectile motion calculator, which applies these laws to anything thrown or launched.
Frequently asked questions
What is the unit of force?
The newton (N). One newton is the force needed to accelerate 1 kg at 1 m/s².
Why don’t action and reaction forces cancel out?
Because they act on different objects. Forces only cancel when they act on the same object.
Is weight a force?
Yes. Weight is the force of gravity on a mass: W = mg, where g is about 9.8 m/s² on Earth.
Sources
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