Summary & Quick Quiz ചലനനിയമങ്ങൾ
The whole chapter on one page: the fifteen key boxes, every formula with its unit, the mistakes to avoid, flashcards, a scored quiz and your study checklist.
1. The chapter in one picture
Read it clockwise from the top left: forces add up to a resultant; only an unbalanced external force changes motion (first law); how much it changes the momentum is the second law; a big force for a short time is impulse; and forces always come in action–reaction pairs (third law).
2. The key boxes of the textbook
When several forces act on an object at the same time, the total force they produce is the resultant force. Same direction: add; opposite directions: subtract (take one direction as +). – page 5
Forces whose resultant is zero. They cannot move a body at rest, nor change the speed or direction of a moving body. – page 5
Forces whose resultant is not zero. They can move a body at rest and change the speed or direction of a moving body. – page 5
An unbalanced external force can change an object’s state of rest or motion, its direction or speed, or create a tendency to do so; the change is in the direction of the unbalanced force. – page 5
All internal forces are balanced forces – that is why pushing an autorickshaw from inside cannot move it. – page 5
To stop a moving object, an unbalanced external force must act opposite to its motion. No unbalanced external force is needed to keep an object moving uniformly in a straight line. – page 6
An unbalanced external force is needed to change an object’s state of motion or rest. – page 6
Every object continues in its state of rest or of uniform motion in a straight line unless acted on by an unbalanced external force. Force is what changes, or tends to change, that state. – page 6
The tendency of an object to continue in its state of rest or motion: inertia of rest and inertia of motion. Inertia increases with mass. – page 7
The product of the mass and the velocity of a moving object. A vector, in the direction of the velocity. – page 8
The rate of change of momentum of an object is directly proportional to the unbalanced external force on it, and the change is in the direction of the force. With the newton as unit, k = 1. – page 9
A large force acting for a very short time is an impulsive force; force × time is its impulse. – page 11
The impulse of a force equals the change in momentum it produces. Same Δp, longer t → smaller F. – page 11
For every action there is an equal and opposite reaction. – page 12
They act at the same time, on different objects, and only in pairs: if there is an action, there is a reaction. – page 12
3. Formulas & units
📐 Formulas & units card
| Quantity | Formula | SI unit | Remember |
|---|---|---|---|
| Resultant of forces along a line | F = F1 + F2 + … | newton (N) | take one direction +, the opposite −: 100 N east and 150 N west → 100 + (−150) = −50 N, i.e. 50 N west |
| Momentum | p = mv | kg m/s | vector, same direction as v; at rest p = 0 |
| Change in momentum | Δp = mv − mu = m(v − u) | kg m/s | a ball bouncing back: mv − mu = −2mu (not 0!) |
| Rate of change of momentum | (mv − mu)/t | kg m/s2 = N | equals the force |
| Second law | F = ma, a = (v − u)/t | N (1 N = 1 kg m/s2) | 1 N gives 1 kg an acceleration of 1 m/s2; negative F = opposite to motion |
| Impulse | I = Ft = mv − mu | N s (= kg m/s) | same Δp: longer t → smaller F |
| Third law | F12 = −F21 | N | equal size, opposite direction, on different bodies |
| Speed conversion | km/h × 5/18 = m/s | m/s | 36 km/h = 10 m/s, 54 km/h = 15 m/s, 72 km/h = 20 m/s, 90 km/h = 25 m/s, 144 km/h = 40 m/s |
| Equations of motion | v = u + at, s = ut + ½at2, v2 = u2 + 2as | m/s, m | used with F = ma (Let us assess Q12, Q13, Q16) |
| Mass | g ÷ 1000 = kg | kg | 200 g = 0.2 kg, 60 g = 0.06 kg |
- Adding forces in opposite directions without signs. 100 N east and 150 N west is not 250 N: 100 + (−150) = −50 N, i.e. 50 N towards the west.
- Forgetting to convert km/h to m/s (× 5/18) or g to kg (÷ 1000) before using a formula.
- Taking the change in momentum of a rebounding ball as 0. A 200 g ball hitting a stick at 20 m/s and coming back at 20 m/s: 0.2 × (−20) − 0.2 × 20 = −8 kg m/s.
- “Action and reaction cancel.” They act on different bodies, so they never cancel.
- “A moving object needs a force to keep moving.” A force is needed only to change the motion (Galileo, first law); friction is what stops things.
- Mixing up mass, inertia and weight; thinking inertia depends on speed. Inertia depends only on mass.
- Dropping the negative sign, or not explaining it. −5000 N means the force acts opposite to the direction of motion (a braking force).
- Mixing up units: momentum kg m/s, force N = kg m/s2, impulse N s.
- Saying a body in uniform circular motion has balanced forces. Its direction keeps changing, so an unbalanced force acts (a satellite: gravity).
4. Flashcards
Click a card to flip it.
5. Scored quiz
Answer all fourteen, then press Check my answers. Each wrong answer shows the page to revise. For number answers type only the number (use a minus sign where the answer is negative).
6. Your study checklist
Ticks come from “Mark this page as completed” at the bottom of each page.
Next: the model test paper ▶ – attempt it on paper with a timer first, then check with the solved paper.