Force
1. What is Force?
A force is a push or a pull that acts on an object.
When you push a door to open it or pull a drawer to open it, you are applying force.
Effects of force: A force can:
- Change the speed of a moving object.
- Change the direction of a moving object.
- Change the shape or size of an object.
- Make a stationary object move.
- Stop a moving object.
Worked Example: A footballer kicks a stationary ball. What effects of force do you see?
The force of the kick makes the ball move (stationary to moving), changes its speed (from 0 to some speed), and if the kick is from the side, also changes its direction.
2. Effects of Force in Detail
Effect 1: Change of Speed
- A force can increase the speed of an object (accelerate).
- A force can decrease the speed of an object (decelerate/brake).
Example: Pedalling a bicycle increases its speed. Applying brakes decreases its speed.
Effect 2: Change of Direction
When a force is applied at an angle to the motion, it changes direction.
Example: A batsman hitting a cricket ball changes its direction.
Effect 3: Change of Shape
Force can stretch, compress, bend, or break an object.
Example: Squeezing a sponge changes its shape. Stretching a rubber band makes it longer.
Exam Focus (2 marks): 'A cyclist applies brakes. What effect of force is observed?'
Applying brakes decreases the speed of the bicycle. The force of friction acts opposite to the motion, slowing it down.
3. Types of Forces
Contact Forces (require physical contact)
| Force | Description | Example |
|---|---|---|
| Muscular force | Force exerted by muscles | Lifting a bag, pushing a cart |
| Frictional force | Force opposing motion | Walking, rubbing hands |
| Mechanical force | Force from machines | Hammer driving a nail |
Non-Contact Forces (act without physical contact)
| Force | Description | Example |
|---|---|---|
| Gravitational force | Earth's pull on objects | Falling of an apple |
| Magnetic force | Force from magnets | Attracting iron nails |
| Electrostatic force | Force between charged objects | Balloon sticking to wall |
Common Mistake: Thinking gravitational force only acts on heavy objects. Gravity acts on ALL objects regardless of weight.
4. Gravitational Force
The force of attraction between the Earth and any object (or between any two objects in the universe).
Key points:
- Earth pulls everything towards its centre.
- The force of gravity on an object is its weight.
- Gravity is responsible for objects falling to the ground.
Worked Example: Why does a ball thrown upwards always come back down?
The Earth's gravitational force pulls the ball downwards. Even when the ball is going up, gravity is continuously pulling it back, eventually bringing it down.
5. Frictional Force
Friction is the force that opposes motion when two surfaces are in contact.
Types of friction:
- Static friction: Prevents a stationary object from moving.
- Sliding friction: Opposes sliding motion.
- Rolling friction: Opposes rolling motion (less than sliding friction).
Is friction useful or harmful?
| Useful | Harmful |
|---|---|
| Walking is possible | Wears out shoes |
| Writing with a pen | Heats up moving parts |
| Brakes work | Causes wear in machines |
Exam Focus (3 marks): 'Why do we slip on a wet floor?'
Water reduces friction between the floor and our feet. Less friction means less grip, causing us to slip.
6. Magnetic Force
The force exerted by a magnet. Magnets attract magnetic materials like iron, nickel, cobalt.
Like poles repel, unlike poles attract.
Applications: Compass, magnetic separators, refrigerator magnets, electric motors.
7. Electrostatic Force
The force between charged objects. Like charges repel, unlike charges attract.
Example: Rubbing a balloon on hair charges it. The balloon then sticks to the wall due to electrostatic attraction.
8. Comparison Table: Contact vs Non-Contact Forces
| Aspect | Contact Forces | Non-Contact Forces |
|---|---|---|
| Physical contact | Required | Not required |
| Examples | Muscular, friction, mechanical | Gravitational, magnetic, electrostatic |
| Medium | Needs medium | Can act through vacuum |
| Distance | Acts at short range | Can act at a distance |
9. Self-Test
- Define force. Give two examples.
- List three effects of force with one example each.
- Differentiate between contact and non-contact forces.
- Why does a ball rolling on the ground eventually stop?
- Give an example of electrostatic force from daily life.
- What type of force is involved when a magnet picks up iron nails?
- State two situations where friction is (a) useful (b) harmful.
10. Answers to Self-Test
- Force is a push or a pull. Examples: pushing a cart, pulling a rope.
- (a) Change speed: pedalling a bicycle. (b) Change direction: bat hitting a ball. (c) Change shape: squeezing clay.
- Contact forces require physical contact (pushing a table). Non-contact forces act without contact (magnet attracting a nail).
- The force of friction between the ball and the ground opposes its motion, gradually slowing it down.
- Clothes sticking together in a dryer due to static charge.
- Magnetic force (non-contact force).
- (a) Useful: walking, braking. (b) Harmful: wearing out shoe soles, producing heat in machines.
