Force and Laws of Motion: Class 9 Study Guide
Forces change motion. Newton's laws explain how and why.
Newton's first law: an object stays at rest or keeps moving in a straight line at constant speed unless an unbalanced force acts on it. This property is inertia, and the mass of a body measures it. A heavier body has more inertia.
Newton's second law: the force on a body equals its mass times its acceleration, F = ma. Newton's third law: to every action there is an equal and opposite reaction.
Momentum
Momentum p = mass × velocity, measured in kg m/s. A force changes momentum: F = change in momentum ÷ time. When two bodies collide with no outside force, their total momentum stays the same. This is the law of conservation of momentum.
| Example | Working |
|---|---|
| F = ma: 5 kg, 3 m/s² | F = 15 N |
| Momentum: 2 kg at 6 m/s | p = 12 kg m/s |
| Same force, double the mass | Half the acceleration |
Third law in action
A rocket pushes gas backwards and the gas pushes the rocket forwards. When you walk, you push the ground backwards and the ground pushes you forwards. The action and reaction forces act on different bodies, so they do not cancel each other.
A and B.
The action.
The reaction, equal and opposite.
Find its net force.
Inertia in everyday life
A passenger lurches forward when a bus brakes suddenly, because the body continues to move. Dust flies off a carpet when it is beaten. A seat belt applies a force that stops you moving forward. Items on a table stay in place if you pull the cloth away very quickly.
Worked examples and common mistakes
Example: A 1000 kg car accelerates at 2 m/s². The force is 2000 N. Example: A 60 kg person on a 40 kg trolley: if they jump off at 2 m/s one way, the trolley moves back at 3 m/s, since 60 × 2 = 40 × v. Common mistakes: thinking that a force is needed to keep an object moving, and adding action and reaction on the same body.
Check your understanding: model answers
1. 20 N. 2. 24 kg m/s. 3. Because of inertia: in a sudden stop the body keeps moving forward, and the belt stops it.
✨ Key Takeaways
- Inertia depends on mass.
- F = ma.
- Action and reaction are equal, opposite and on different bodies.
- Total momentum is conserved in a collision.
💬 Questions for Discussion & Reflection
- Find the force to give a 4 kg body an acceleration of 5 m/s².
- Find the momentum of a 3 kg ball at 8 m/s.
- Why do we wear seat belts?