Class 5 Science | Chapter 2: Friction in Action

1. Detailed Introduction

Imagine you are rolling a toy car on a smooth marble floor. It goes quite far, doesn't it? Now, try rolling the same toy car with the exact same push on a grassy lawn or a rough sandy path. What happens? It stops very quickly! But why does it stop when no one is touching it?

The invisible force that slows down moving objects and eventually stops them is called Friction. Friction acts like a natural brake that comes into play whenever two surfaces touch and slide over each other.

If Object Moves Forward ➔ Direction of Motion (Forward) ← Friction Force (Opposite/Backward)
Fig 1: Friction Always Opposes the Direction of Motion
What is Friction Force?
Friction is a resistive force that always acts in the opposite direction to the direction in which an object wants to move or is moving.

How Does Friction Happen?

No surface is perfectly smooth. Even a glass table top or a mirror that looks completely flat has microscopic ups and downs (called irregularities, ridges, and grooves) when viewed under a microscope. When one surface slides over another, these tiny ridges get locked into each other. To keep things moving, we have to apply force to unlock these microscopic structures.

Types of Surfaces and Friction

  • Rough Surfaces: Surfaces like sandpaper, concrete, brick, and carpets have large irregularities. They create High Friction.
  • Smooth Surfaces: Surfaces like ice, wet tiles, glass, and oiled metal have very small irregularities. They create Low Friction.

Friction in Action: Is it a Friend or an Enemy?

Friction is known as a necessary evil because it helps us in some ways but causes problems in others:

As a Friend: It helps us walk safely on the road without slipping, allows brakes to stop a fast bicycle, and lets you write on paper with a pencil.

As an Enemy: It wears out the soles of your shoes, creates unwanted heat in machines, and slows down moving vehicles, making them use more fuel.

2. Solved Examples (10)

  1. Why do the soles of old shoes become completely smooth after a few months?
    Answer: Constant walking creates friction between the ground and the shoe soles. Over time, this friction wears out the material, making it smooth.
  2. Why do car tyres have deep lines and patterns (treads) carved into them?
    Answer: These deep grooves increase the rough irregularities on the tyre's surface. This increases friction with the road and prevents cars from skidding.
  3. Why do people slip easily on a wet bathroom floor?
    Answer: Water fills up the microscopic rough gaps on the floor tiles, making the surface extremely smooth. This reduces friction, causing us to lose our grip and slip.
  4. Why does a matchstick catch fire when rubbed against the side of a matchbox?
    Answer: The side of the matchbox is made rough. Rubbing the matchstick head creates high friction, which produces enough heat energy to ignite the chemical.
  5. Why do we apply oil or grease to the moving chains of our bicycles?
    Answer: Oil acts as a lubricant. It covers the rough irregularities of the metal links, reducing friction so the bicycle operates smoothly with less effort.
  6. How does writing with chalk on a blackboard work?
    Answer: The blackboard surface is rough. When we rub chalk against it, friction tears off tiny chalk particles, which stick to the blackboard grooves, making the words visible.
  7. Why do gymnasts rub a white powder (coarse substance) on their hands before performing?
    Answer: The powder absorbs sweat and increases the roughness of their hands. This increases friction, giving them a firm grip on the metal bars.
  8. Why do heavy boxes become easier to push when we put wheels underneath them?
    Answer: Rolling friction (created by wheels) is much weaker and smaller than sliding friction (created by dragging flat boxes).
  9. Why do doors make squeaking noises, and how can they be fixed?
    Answer: The metal parts of the hinges rub against each other, creating dry friction and noise. Adding a few drops of coconut oil reduces friction and stops the noise.
  10. Why do meteors burn up when they enter the Earth's atmosphere?
    Answer: Meteors travel at extreme speeds. When they enter the atmosphere, high friction between the meteor surface and air particles generates intense heat, burning them up.

3. Practice Exercise (15 Questions)

Click on the "Show/Hide Answer" button underneath each question to view its solution.

Q1. What is the fundamental cause of friction between two surfaces?

The microscopic interlocking of surface irregularities (roughness) causes friction.

Q2. In which direction does friction act relative to the movement of an object?

Friction always acts in the exact opposite direction to the movement.

Q3. Which type of surface produces more friction: a smooth glass pane or concrete?

Concrete produces much higher friction because its surface is highly rough.

Q4. What name is given to substances used to reduce friction?

They are called Lubricants (e.g., oil, grease, graphite powder).

Q5. Why is it dangerous to drive a car fast on a rainy day?

The layer of rainwater drastically reduces friction between the road and tyres, increasing the risk of sliding.

Q6. Why is friction referred to as a "necessary evil"?

It is necessary because we cannot walk or write without it, but an evil because it damages machinery and wastes energy.

Q7. What happens to our hands when we rub them together vigorously during winter?

Friction between our palms converts kinetic energy into heat energy, making our hands feel warm.

Q8. Which is easier to perform: sliding a book on a table or rolling it on cylindrical pencils?

Rolling it on pencils is easier because rolling friction is significantly less than sliding friction.

Q9. Why do car brakes fail if oil accidental spills on the brake pads?

Oil destroys the friction between brake pads and wheel discs, preventing the vehicle from stopping.

Q10. Why do we sprinkle fine talcum powder on a carrom board before playing?

The fine powder coats the rough grooves of the wooden board, reducing friction so the striker can glide smoothly.

Q11. Does friction produce heat? Give one real-world evidence.

Yes. The heating up of a mixer-grinder jar after running for a few minutes is a clear example.

Q12. Why do ice skaters use shoes with sharp metal blades beneath them?

The thin blades cut smoothly into the ice, minimizing contact surface area to keep friction very low for smooth gliding.

Q13. What would happen if there was absolutely zero friction on Earth?

Once an object started moving, it would never stop. No one would be able to walk, write, or hold any item securely.

Q14. Why are the bodies of aeroplanes designed like birds (streamlined)?

A streamlined shape cuts smoothly through the air, minimizing fluid friction (air drag).

Q15. Can friction ever be completely eliminated down to zero?

No, because no surface can ever be 100% molecularly smooth. Some level of friction always remains.

4. Interactive Science Quiz (15 Questions)

Select your options and check your target results instantly!

1. Friction is a force that always ______ movement.

2. Which of the following surfaces generates the least amount of friction?

3. Tyres are treaded to ______ friction.

4. Oil, grease, and powder are examples of:

5. Which friction is much smaller and easier to handle?

6. Rubbing two stones together produces:

7. A friction force is created because surfaces have microscopic:

8. Why do soles of your old shoes lose grip?

9. Air drag is another term used for:

10. Sprinkling powder on a carrom board makes the striker move faster because:

11. A matchstick ignites because rubbing generates:

12. Streamlined shapes are given to planes and fish to save fuel by overcoming:

13. Which type of shoes provide maximum safety from slipping on muddy grounds?

14. If you push a toy car across a carpet, it stops quickly due to:

15. Friction is called a "necessary evil" because: