A7LAM CSCA ACADEMYCSCA Academy / Physics / Mechanics
CSCA Physics · Lesson 2 of 16Forces and Newton's laws
Forces et lois de Newton · القوى وقوانين نيوتن
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Forces change motion. Newton's three laws explain how, and drawing every force on a diagram is the first step of every problem.
Key ideas
- First law: with no net force, an object keeps its velocity (or stays at rest).
- Second law: F_net = m·a. Force in newtons (N), mass in kg.
- Third law: forces come in equal and opposite pairs, acting on different objects.
- Weight W = m·g. Friction f = μ·N, opposing the motion.
Formulas to know
- F = ma
- W = mg
- f = μN
Worked example
A net force of 20 N acts on a 4 kg box. Find its acceleration.
- a = F / m.
- a = 20 / 4.
Answer: a = 5 m/s²
Practice questions
1. Weight of a 50 kg student (g = 10 m/s²)?
5 N · 50 N · 500 N ✓ · 5,000 N
W = mg = 50 × 10 = 500 N.
2. Which law explains a rocket being pushed up by its exhaust gas?
First law · Second law · Third law ✓ · Law of gravitation
The rocket pushes gas down; the gas pushes the rocket up.
3. A 10 kg box on a floor (μ = 0.3, g = 10 m/s²). Friction force when sliding?
3 N · 30 N ✓ · 100 N · 0.3 N
N = mg = 100 N, so f = 0.3 × 100 = 30 N.