Isaac Newton published three rules in 1687 that explain the motion of every object you can see — from a rolling ball to a falling apple to a spacecraft. Three hundred years later, they are still the foundation of high-school physics. Here is what each one actually says, and how to use them.
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Book free demoNewton's First Law — The Law of Inertia
Newton's First Law
This is called the law of inertia. Inertia is the tendency of an object to resist any change in its state of motion. The more mass an object has, the more inertia it has.
Real examples:
- •Passenger lurching forward when a car brakes — the person's body wants to stay in motion; the car stops but the person's inertia carries them forward.
- •A hockey puck sliding on near-frictionless ice — without friction to stop it, it would keep going forever.
- •A ball resting on a table — gravity pulls down, normal force pushes up. Net force = 0. Ball stays put.
Newton's Second Law — F = ma
Newton's Second Law
This law links three quantities. If you know any two, you can find the third. Rearranging the formula:
F = ma
Find force
a = F/m
Find acceleration
m = F/a
Find mass
A 5 kg box is pushed with a net force of 20 N. What is its acceleration?
Write what you know
Rearrange F = ma for acceleration
Substitute and solve
What net force is needed to accelerate a 1,200 kg car at 3 m/s²?
Write what you know
Apply F = ma directly
Free-body diagram problems — step by step
A 10 kg block is pushed right with 50 N. Friction acts left with 20 N. Find acceleration.
Find net force (taking right as positive)
Apply F = ma
Newton's Third Law — Action and Reaction
Newton's Third Law
Notice: the two forces act on different objects. They never cancel each other out (because they are on different bodies).
Real examples:
- •Walking: Your foot pushes backwards on the ground; the ground pushes forward on you — that forward push is what moves you.
- •Rocket propulsion: The rocket pushes exhaust gas downward; the gas pushes the rocket upward.
- •Swimming: You pull the water backward; the water pushes you forward.
Weight vs Mass
Many students mix these up in calculations.
| Property | Mass | Weight |
|---|---|---|
| What it is | Amount of matter | Gravitational force on an object |
| Unit | kg | Newtons (N) |
| Changes in space? | No — constant everywhere | Yes — depends on gravity |
| Formula | — | W = mg (g = 9.8 m/s² on Earth) |
What is the weight of a 60 kg person on Earth?
Use W = mg, where g = 9.8 m/s²
- First law: no net force = no change in motion (rest or constant velocity).
- Second law: F = ma. Net force, not just applied force.
- Third law: equal and opposite forces — but on different objects.
- Weight (N) = mass (kg) × g (9.8 m/s²).
- Draw a free-body diagram before every force problem — it prevents sign errors.
Practice Problems
- 1
A 3 kg book rests on a table. What is the normal force from the table? (g = 9.8 m/s²)
Hint: The book is not accelerating — use the first law: N = W = mg.
- 2
A net force of 15 N acts on a 3 kg object. What is its acceleration?
Hint: Use a = F/m.
- 3
A 70 kg person stands on a scale in an elevator accelerating upward at 2 m/s². What does the scale read?
Hint: F_net = ma; the scale reads N = m(g + a).
- 4
Identify the third-law pair when a ball hits a wall.
Hint: Ball pushes on wall; wall pushes on ball — same magnitude, opposite direction.
- 5
Two forces act on a 4 kg box: 30 N right and 10 N left. Find the acceleration.
Hint: Calculate net force first.
Building on Newton's Laws
These three laws lead directly into kinematics equations, friction problems, inclined planes, and eventually energy and momentum. If physics is feeling like separate formulas rather than a connected story, book a free demo session and we will build the logical chain from the ground up.
