Work and Energy — Class 4 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Science, Unit 3. Work, energy, simple machines and types of lever. Taught in July and August.
1. Force, and then work
You met force in an earlier class. A force is a push or a pull that moves an object at rest, or stops an object that is moving.
Work sounds like an everyday word, but in science it means something exact. Two conditions must both be met:
- A force must act on the object.
- The object must move from one place to another.
If either condition is missing, no work is done — however tired you feel.
| Activity | Work done? | Why |
|---|---|---|
| Pushing a cart that rolls forward | Yes | Force applied and the cart moved |
| Lifting a school bag off the floor | Yes | Force applied and the bag moved |
| Pushing a wall that does not budge | No | Force applied but nothing moved |
| Holding a heavy bag while standing still | No | Force applied but no movement |
| Studying for an exam | No | No force, no movement |
That last row surprises people. In ordinary speech any effort is "work", but in science the word is reserved for force plus movement.
2. Energy
Energy is the capacity to do work. Energy must be transferred to an object before that object can do any work.
Where does it come from?
- A man pulling luggage gets his energy from food.
- A car moves on energy released by burning fuel.
- An escalator runs on electricity.
Energy comes in several forms — mechanical energy in moving things, chemical energy stored in food and fuel, solar energy from the Sun, plus heat, light and sound energy.
Energy is never created or destroyed. It only changes from one form into another. Sunlight becomes chemical energy in a plant, which becomes energy in your muscles when you eat it.
Do not confuse force with energy. A force is a push or pull. Energy is what makes it possible to apply that force.
3. Renewable and non-renewable sources
Renewable sources are replaced naturally over time, so we can keep using them for a long period. Examples: the Sun, wind and water. We have used them since human life began, for lighting, transport, cooking and heating.
Non-renewable sources are not easily replaced once used. Example: petroleum and the other fuels dug out of the ground. When they are gone, they are gone.
4. Simple machines
A simple machine is a tool that makes work easier. We draw water from a well with a wheel and a rope rather than climbing down for it.
There are six simple machines:
| Machine | What it does | Examples |
|---|---|---|
| Pulley | Changes the direction of a pull | Drawing water from a well, flag hoisting |
| Wheel and axle | A wheel fixed to a small rod so both turn together | Bicycle, steering wheel |
| Wedge | A sharp edge that splits materials | Knife, scissors, axe |
| Inclined plane | A slope, so a load rises gradually | Ramp, staircase |
| Screw | Raises weights and holds objects together | Pencil sharpener, screw-jack, bottle cap, windmill |
| Lever | Multiplies the force we apply | Seesaw, nutcracker, plier |
Rolling a heavy box up a ramp takes less effort than lifting it straight up — the same work spread over a longer, gentler path.
5. The parts of a lever
To understand levers you need three words:
- Load — the object on which the force is applied.
- Effort — the force we apply on the lever.
- Fulcrum — the point about which the lever turns.
6. The three classes of lever
Levers are classified by where the fulcrum sits relative to the load and the effort.
| Class | Arrangement | Examples |
|---|---|---|
| Class I | Fulcrum between effort and load | Seesaw, plier, scissors |
| Class II | Load between fulcrum and effort | Wheelbarrow, nutcracker, bottle opener |
| Class III | Effort between load and fulcrum | Stapler, tongs, broomstick, hockey stick |
A quick way to keep them straight is to ask which of the three is in the middle: fulcrum in the middle is Class I, load in the middle is Class II, effort in the middle is Class III.
7. Worked examples
Example 1. A boy pushes a wall with all his strength and the wall does not move. Is work done?
Solution: No. A force was applied, but the object did not move, so the second condition fails.
Example 2. Name the two conditions for work to be done.
Solution: A force must act on the object, and the object must move from one place to another.
Example 3. What is energy?
Solution: The capacity to do work.
Example 4. Classify as renewable or non-renewable: Sun, petroleum, wind, water.
Solution: Renewable — Sun, wind, water. Non-renewable — petroleum.
Example 5. Which simple machine is a bottle cap an example of?
Solution: The screw.
Example 6. In a seesaw, where is the fulcrum, and which class of lever is it?
Solution: The fulcrum is in the middle, between the two children. It is a Class I lever.
Example 7. A wheelbarrow carries its load between the wheel and the person's hands. Which class of lever is it?
Solution: The load is in the middle, so it is a Class II lever.
8. Practice
- What is a force?
- State the two conditions needed for work to be done.
- Give one example where a force is applied but no work is done.
- Define energy.
- Name three renewable sources of energy.
- Name any four simple machines.
- What is a wedge used for? Give two examples.
- Name the three parts of a lever.
- In which class of lever is the effort between the load and the fulcrum?
- Which simple machine helps us draw water from a well?
9. Answers
- A push or pull that moves an object at rest or stops a moving object.
- A force must act on the object, and the object must move.
- Pushing a wall that does not move, or holding a bag while standing still.
- The capacity to do work.
- The Sun, wind and water.
- Pulley, wedge, inclined plane, screw, lever, wheel and axle (any four).
- To split materials. Knife, scissors, axe.
- Load, effort and fulcrum.
- Class III.
- The pulley.
10. Summary
- A force is a push or pull that starts or stops motion.
- Work is done only when a force acts AND the object moves; effort alone is not work.
- Energy is the capacity to do work, and it changes form rather than being created or destroyed.
- Food, fuel and electricity are everyday sources of energy.
- Renewable sources (Sun, wind, water) are replaced naturally; non-renewable ones such as petroleum are not.
- The six simple machines are pulley, wheel and axle, wedge, inclined plane, screw and lever.
- A lever has a load, an effort and a fulcrum.
- Class I has the fulcrum in the middle, Class II the load, Class III the effort.
