Newton’s Laws – Quick Study Notes
Goal : Understand the three laws in plain English, see how they work in everyday life, and solve a couple of tiny examples.
Tip : Read each section, then try the little practice question before moving on.
1. First Law – Law of Inertia
| What it says | Everyday picture | Quick math |
|---|---|---|
| A body stays at rest or keeps moving in a straight line unless something pushes or pulls on it. | A book on a table won’t move unless you push it. A skateboard rolls forward until friction stops it. | No net force → constant velocity (F_net = 0 ⇒ v = constant) |
Tiny Example
A ball sits on a table.
- Before: No force → ball stays still.
- After: You push → a force appears → ball starts rolling.
2. Second Law – Force = Mass × Acceleration
| What it says | Everyday picture | Quick math |
|---|---|---|
| The more you push (force), the faster an object speeds up. If an object is heavy (large mass), it needs more push to change its motion. | Pushing a shopping cart: a light cart speeds up quickly; a heavy one takes longer. | (F = m a) (force = mass × acceleration) |
Tiny Example
A 2 kg toy car is pushed with a 4 N force.
- Acceleration
a = F/m = 4 N / 2 kg = 2 m/s². - After 3 s, its speed =
a × t = 2 m/s² × 3 s = 6 m/s.
3. Third Law – Action & Reaction
| What it says | Everyday picture | Quick math |
|---|---|---|
| Every push has an equal pull back. When you push on something, it pushes back with the same strength. | Standing on a skateboard: you push the ground backward, the ground pushes you forward. | (F_{12} = -F_{21}) (force on 1 by 2 equals the negative of force on 2 by 1) |
Tiny Example
You push a wall with 50 N.
- The wall pushes you back with 50 N in the opposite direction.
Quick Recap
| Law | Key Idea | Direct Formula |
|---|---|---|
| 1st | Keep doing what you’re doing unless something changes it | (F_{\text{net}} = 0 \Rightarrow v = \text{const.}) |
| 2nd | Push more → speed up faster; heavier → slower change | (F = m a) |
| 3rd | Push → get pushed back equally | (F_{12} = -F_{21}) |
Mini‑Practice Problems
-
First Law
A 5 kg box is on a frictionless floor. What force is needed to keep it moving at a constant speed?
Answer: 0 N (no force needed). -
Second Law
A 10 kg car accelerates at 3 m/s². What net force acts on it?
Answer:F = m a = 10 kg × 3 m/s² = 30 N. -
Third Law
A 2 kg hammer strikes a nail. If the hammer exerts 100 N on the nail, how much force does the nail exert on the hammer?
Answer: 100 N opposite direction.
Things to Remember
- Force is a push or pull.
- Mass is how much stuff is in an object.
- Acceleration is the change in speed per second.
- Newton’s laws are the same everywhere, from a playground to a planet.
Keep these notes handy, read them aloud, and try the practice problems again after a few days to solidify the concepts. Happy studying!