Force — Class 3 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 3 Science. Pushes, pulls, friction and gravity.
1. A force is a push or a pull
Every force is one of two things: a push away from you, or a pull towards you.
| Push | Pull |
|---|---|
| Closing a door | Opening a drawer |
| Pressing a button | Lifting a bucket from a well |
| Kicking a football | Drawing water with a rope |
Some actions use both. Pedalling a cycle pushes one pedal down while the other comes up.
2. What a force can do
A force can do four different things, and most people remember only the first.
- Start something moving, as when you kick a ball at rest.
- Stop something moving, as when you catch it.
- Change its speed, by pedalling harder or braking.
- Change its direction, as a batsman does when the ball arrives from one side and leaves towards another.
Saying only that "force makes things move" leaves out three of the four. Catching a ball is just as much a force as throwing it.
A force can also change the shape of a thing without moving it anywhere: squeezing a sponge, stretching a rubber band, rolling out dough.
3. Friction
Friction is the force that resists movement between two surfaces touching each other.
Roll a ball on a smooth floor and it travels a long way. Roll the same ball on grass and it stops much sooner. The grass is rough, so it produces more friction.
| More friction | Less friction |
|---|---|
| Grass, sand, carpet | Polished floor, ice |
| A rough sole | A worn, smooth sole |
Friction is not a nuisance. It is what lets you walk without slipping, hold a pencil, and stop a cycle. On a completely frictionless floor you could not even stand up. What friction also does is wear things out and waste effort, which is why machines are oiled.
4. Gravity
Gravity is the pull of the Earth on everything near it, always towards the centre of the Earth.
Throw a ball upward and it comes back down. Rain falls down and never up. A mango drops from the tree to the ground. All of this is gravity.
Gravity is also why things have weight. Mass is how much matter something contains and does not change wherever you go. Weight is how hard gravity pulls on that mass, and it does change: the same person weighs less on the Moon, because the Moon pulls more weakly.
5. Worked examples
Example 1. Is opening a tap a push or a pull?
Turning a tap handle is a push on one side of the handle and a pull on the other. Most everyday turning actions are like this.
Example 2. A ball rolls further on a road than on grass. Why?
The road is smoother, so it produces less friction, and less friction means the ball keeps moving for longer.
Example 3. Give an example of a force stopping motion and one of a force changing direction.
A goalkeeper catching a ball stops its motion. A batsman hitting an oncoming ball changes its direction.
Example 4. Why does a squeezed sponge change shape but stay in the same place?
The force acted on the sponge without being large enough or free enough to move it. A force can change shape as well as motion.
6. Practice
- Give two examples of a push and two of a pull from your own home.
- Name the four things a force can do to a moving object.
- Which stops a rolling ball sooner, a carpet or a polished floor? Why?
- Give two ways friction helps us.
- Why does a ball thrown upward always come back down?
- What is the difference between mass and weight?
- Give one example of a force changing the shape of something.
7. Answers
- Pushes: closing a door, pressing a switch. Pulls: opening a drawer, lifting a bucket.
- Start it, stop it, change its speed, change its direction.
- The carpet, because it is rougher and produces more friction.
- It lets us walk without slipping and lets us hold objects such as a pencil.
- Because gravity pulls everything towards the Earth.
- Mass is the amount of matter and stays the same everywhere; weight is the pull of gravity on it and changes.
- Squeezing a sponge, stretching a rubber band or rolling out dough.
8. Summary
- A force is a push or a pull.
- A force can start motion, stop it, change its speed or change its direction.
- A force can also change the shape of a thing without moving it.
- Friction opposes motion; rough surfaces give more of it than smooth ones.
- Friction is useful as well as wasteful: it lets us walk, hold and brake.
- Gravity pulls everything towards the Earth, which is why things fall.
- Mass stays the same everywhere; weight depends on how hard gravity pulls.
