Summary & Quick Quiz പ്രവൃത്തിയും ഊർജവും
The whole chapter on one page: the eleven key boxes of the textbook, every formula and unit you need, 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: work is force × displacement; doing work against gravity stores potential energy; energy is the capacity to do work; mechanical energy is potential + kinetic; energy only changes form (conservation); power tells how fast the work is done; and almost all our energy comes from the Sun – through food, fuels, rain and solar cells.
2. The key boxes of the textbook
When a force acts on an object and the object is displaced in the direction of the force, the force is said to have done work on it. No displacement → no work (holding a sack still, pushing a rock that does not move). – page 5
Work = force × displacement in the direction of the force. Unit: N × m = newton metre = joule (J), after James Prescott Joule. – page 5
1 J is the work done when a force of 1 N moves an object through 1 m in the direction of the force. – page 5
Lifting a mass m through a height h needs a force mg upwards, so W = mg × h. It depends only on the height, not on the path (straight up or along a ramp). – page 6
Work is positive when the displacement is in the direction of the force (your push on a trolley, gravity on a falling coconut) and negative when it is opposite (friction, gravity on a stone thrown up). A force at right angles to the motion does no work. – page 6
Energy is the capacity to do work. SI unit: joule. Food energy in calories: 1 cal = 4.2 J, 1 kcal = 4200 J (1000 cal = 1 kcal). – page 7
The energy an object has because of its motion, position or configuration. Two kinds: potential energy and kinetic energy. – page 7
Energy due to position (coconut on the tree, water in a tank or dam) or configuration (compressed spring, drawn bow, bent pole of a pole vaulter). h is measured from the chosen zero level (the ground). – page 7
Energy due to motion. Mass doubled → KE doubled; velocity doubled → 4 times; tripled → 9 times; halved → ¼. – page 8
Energy can neither be created nor destroyed; it can be changed from one form into another without any loss or gain. Work and energy are both scalars with the same unit; energy is used up when work is done. – page 9
Power is the work done per unit time – the rate of doing work. Unit J/s = watt (W), after James Watt. 1 kW = 103 W, 1 MW = 106 W, 1 HP = 746 W. – page 10
3. Formulas & units
📐 Formulas & units card
PE + KE = constant (no friction) · v2 = u2 + 2as · work done = energy transferred
| Quantity | Formula | SI unit | In base units | Scalar / vector |
|---|---|---|---|---|
| Work പ്രവൃത്തി | W = F s; against gravity mgh | joule (J) = N m | kg m2/s2 | scalar |
| Energy ഊർജം | Ep = mgh, Ek = ½mv2 | joule (J) | kg m2/s2 | scalar |
| Power പവർ | P = W ÷ t | watt (W) = J/s | kg m2/s3 | scalar |
| Force ബലം | F = ma; weight = mg | newton (N) | kg m/s2 | vector |
| Displacement സ്ഥാനാന്തരം | – | metre (m) | m | vector |
| Conversion | Remember |
|---|---|
| Food energy | 1 cal = 4.2 J · 1 kcal = 1000 cal = 4200 J (840 kJ = 200 kcal) |
| Power | 1 kW = 1000 W · 1 MW = 1000 kW = 106 W · 1 HP = 746 W |
| Electrical energy | 1 unit = 1 kWh = 1000 W × 3600 s = 3.6 × 106 J |
| Before substituting | g → kg (÷ 1000) · kJ → J (× 1000) · minutes → seconds (× 60) · km/h → m/s (× 1000 ÷ 3600, e.g. 72 km/h = 20 m/s) |
| g | use the value the question gives: 9.8 m/s2 or 10 m/s2; on the Moon 1.6 m/s2 |
| Rearranged | h = Ep ÷ (mg) · v = √(2Ek ÷ m) · t = W ÷ P · F = W ÷ s · falling from rest: v2 = 2gh |
| Graphs | work = area under a force–displacement graph · KE against height (thrown up) = falling straight line |
- Saying work is done when holding a load still or pushing a wall. No displacement → no work, however tired you feel.
- Using the slanted length of a ramp instead of the vertical height in mgh. Work against gravity depends only on h (the barrel: 1000 J either way).
- Forgetting to square v in ½mv², or squaring the ½ or the m. Only v is squared: ½ × 6 × 12² = 3 × 144 = 432 J.
- Forgetting g – using kg as a force, or mixing g = 9.8 and 10. Weight = mg; use the g the question gives.
- Not converting kJ → J, g → kg, minutes → seconds, km/h → m/s before substituting.
- Thinking energy is “lost” in a fall. It changes form – PE becomes KE; friction turns some of it into heat, but the total stays the same.
- Confusing power with energy or work. Power is the rate; more work does not always mean more power (Babu and Raju, Q14: both 30 W).
- Treating work or energy as vectors. They are scalars – add them directly: 18 J + 15 J = 33 J (Q7), whatever the directions.
4. Flashcards
Click a card to flip it.
5. Scored quiz
Answer all sixteen, then press Check my answers. Each wrong answer shows the page to revise. For number answers type only the number.
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.