This MCQ module is based on: Non-Contact Forces and Pressure
TOPIC 17 OF 50
Non-Contact Forces and Pressure
🎓 Class 8
Science
CBSE
Theory
Ch 5 — Coal and Petroleum
⏱ ~33 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="chemistry" difficulty="basic"]
5.5 Non-Contact Forces
We have seen that muscular force and friction need physical contact. But some forces act even when the two objects are not touching. A magnet attracts an iron pin from a distance. A falling apple feels the pull of the Earth even though the Earth is thousands of kilometres below. These are called non-contact forces.
Non-Contact Force: A force that one object exerts on another without any physical contact. Three important non-contact forces are magnetic, electrostatic and gravitational.
5.5.1 Magnetic Force
A bar magnet held near a handful of iron filings pulls them towards it without touching. The ends of a magnet are called poles — the north pole (N) and the south pole (S). Two like poles (N–N or S–S) push each other away; two unlike poles (N–S) pull each other together.
Magnets attract only certain materials — mainly iron, nickel and cobalt. Plastic, wood and copper are not attracted. Magnetic force can pass through paper, glass and wood, which is why a magnet can pick up a pin lying under a sheet of paper.
5.5.2 Electrostatic Force
On a cold, dry day, have you ever pulled off a sweater and heard a crackle? Or felt your hair rise after combing it? These are examples of electrostatic force.
🔬 Activity 5.6 — Charged Straw Picks Up PaperL3 Apply
🤔 Predict first: Can a plastic straw — without any magnet — pick up small paper pieces?
- Tear a sheet of paper into very small pieces (rice-grain size).
- Take a clean plastic straw or a plastic comb. Rub it vigorously on your dry hair or a woollen cloth for 20 seconds.
- Bring the rubbed end of the straw close to (but not touching) the paper pieces.
Observation: Many paper pieces jump up and stick to the straw! The straw has become electrically charged by rubbing — it attracts the uncharged paper pieces. This is electrostatic force.
Electrostatic Force: The force exerted by a charged object on another charged or uncharged object, without any physical contact. Like charges repel; unlike charges attract.
When we rub two different materials, tiny particles called electrons move from one surface to the other. The object that gives up electrons becomes positively charged; the one that gains them becomes negatively charged. Charged objects can attract uncharged paper, dust or hair, and can either attract or repel other charged objects depending on the sign of charge.
🔬 Activity 5.7 — Two Balloons Push ApartL3 Apply
🤔 Predict first: If two balloons are rubbed on the same woollen cloth and hung side by side, what will they do?
- Inflate and tie two balloons. Attach a thread to each and hang them from the same support so they dangle next to each other.
- Rub each balloon vigorously on your hair or on a woollen cloth.
- Let them hang freely. Watch carefully.
Observation: The two balloons swing apart! Since they were rubbed the same way, they carry the same type of charge. Like charges repel — so the balloons push each other away even though they are not touching.
5.5.3 Gravitational Force
Why does a ball thrown up come down? Why do we stay stuck to the ground and not float off? The answer is gravity. Every object in the universe with mass attracts every other object with a pull called the gravitational force. It was the English scientist Sir Isaac Newton (1642–1727) who first worked out the laws of this force. The famous (though possibly exaggerated) story says he got the idea by watching an apple fall from a tree.
On Earth's surface, every object is pulled toward the centre of the Earth. The acceleration caused by this pull is called g. On Earth, g ≈ 9.8 m/s². On the Moon, g ≈ 1.6 m/s² — only about one-sixth of Earth's. That is why astronauts on the Moon can leap so high.
Mass vs. Weight
Students often mix up mass and weight, but they are different:
| Mass | Weight | |
|---|---|---|
| Meaning | Amount of matter in a body | Gravitational force acting on that body |
| Unit | kilogram (kg) | newton (N) |
| Depends on location? | No — same everywhere | Yes — changes with g |
| On the Moon | Same as on Earth | About 1/6 of Earth weight |
So a 60 kg astronaut still has a mass of 60 kg on the Moon, but weighs only about 1/6 of their Earth-weight — that is why they appear to "float" when they walk.
Everyday gravity facts: gravity holds the atmosphere around the Earth; keeps the Moon orbiting the Earth; keeps the Earth orbiting the Sun; causes ocean tides; makes rivers flow downhill.
5.6 Pressure
Why is it easier to push a sharp pin into a board than to push your finger into the same board, using the same force? The answer lies in pressure.
Pressure: The force acting per unit area of a surface.
\[ P = \dfrac{F}{A} \]
where F = force in newton (N) and A = area in square metre (m²). The SI unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m².
From the formula: for the same force, a smaller area gives a much higher pressure. That is why:
Sharp Knife Edge
A sharp edge has a very tiny area of contact, so even a small downward force produces high pressure — and the knife cuts cleanly.
Pointed Nail Tip
All the hammer's force is concentrated on the tiny tip area — the nail easily pierces the wood.
Wide Camel Feet
Wide, padded feet spread the camel's weight over a larger area, lowering the pressure on sand so the animal does not sink in.
Broad Building Foundations
A wide base spreads the weight of a tall building over a large area — less pressure on soil, more stability.
Worked Example
A box weighs 200 N. It has a base area of 0.5 m². Find the pressure it exerts on the ground.
Using \( P = F/A \): \( P = 200 / 0.5 = 400 \) Pa. Now if the same box is placed on its edge with a base area of only 0.1 m²: \( P = 200 / 0.1 = 2000 \) Pa — five times the pressure!
5.6.1 Pressure in Fluids
Liquids and gases can also exert pressure. A balloon stays inflated because the air inside pushes outward on the rubber wall. Water at the bottom of a dam pushes hard on the dam wall.
Three important facts about fluid pressure:
- A fluid exerts pressure on the walls of its container and on any object immersed in it.
- Fluid pressure acts in all directions, not just downward.
- Fluid pressure increases with depth. That's why a hole low in a water tank shoots water out farther than a hole near the top, and why deep-sea divers wear pressure suits.
Atmospheric Pressure
The layer of air covering the Earth is called the atmosphere. Air has weight, and this weight presses down on everything at sea level. This pressure is called atmospheric pressure. At sea level it is about 1 atm ≈ 1,01,325 Pa (roughly 10 N pressing on every cm²). We don't feel it because our bodies push back with equal pressure from inside.
🔬 Activity 5.8 — Balloon Pressure DemoL2 Understand
🤔 Predict first: If you blow up a balloon and pinch it hard in the middle, where will it bulge?
- Inflate a balloon (not too much) and tie it.
- Place both your palms flat on the top and bottom of the balloon and press gently.
- Now squeeze the middle with your fingers. Observe the balloon's shape.
Observation: The balloon bulges outward wherever you are not pressing. That's because the air inside pushes equally in all directions — so when you reduce the volume in one place, the pressure pushes the rubber out elsewhere.
5.7 Buoyancy — The Upward Push of Fluids
Dip your hand into a bucket of water. Your hand feels lighter. Push a thermocol block into the water — it pops right back up. What is happening?
Buoyant Force (Upthrust): The upward force exerted by a fluid on any object that is wholly or partly immersed in it. We also call it buoyancy.
Buoyancy acts because fluid pressure increases with depth. The bottom of an immersed object sits in higher-pressure fluid than its top, so the fluid pushes harder from below. The result is a net upward push.
Archimedes' Principle
The Greek thinker Archimedes stated a beautiful rule:
Archimedes' Principle: When a body is fully or partly immersed in a fluid, the fluid exerts an upward force (buoyant force) equal to the weight of the fluid displaced by the body.
Float or Sink?
Whether an object sinks or floats depends on how its weight compares with the upthrust (which equals the weight of the displaced fluid).
- If object's weight < upthrust, the object floats.
- If object's weight > upthrust, it sinks.
- If they are equal, the object rests anywhere inside the fluid (like a fish).
Why Does a Ship Float But a Coin Sink?
A metal coin is denser than water, so the water it pushes aside weighs less than the coin — it sinks. A ship, although made of heavy steel, is mostly hollow. A huge volume of water gets pushed aside by its wide shape. That displaced water weighs more than the ship, so the upthrust matches the ship's weight and it floats. In effect, the ship's average density (including the air inside) is less than the density of water.
🎯 Float-or-Sink Simulator L3 Apply
Enter the object's mass and volume. We calculate its density and compare with water (1000 kg/m³).
Object density: 1000 kg/m³
Fluid density: 1000 kg/m³
Exactly floats (neutral)
📋 Competency-Based Questions
During a science fair, Aarav sets up a display. He rubs a plastic comb on his hair and uses it to attract small paper pieces. Beside the comb, a bar magnet picks up iron nails from a distance. Aarav also keeps a beaker of water in which he drops an iron nail (it sinks) and a thermocol block (it floats).
Q1. L2 Understand Which non-contact force is responsible for the comb attracting paper pieces?
Answer: C. Rubbing charges the comb; a charged object exerts electrostatic force on the uncharged paper.
Q2. L3 Apply A box weighs 500 N and has a base area of 2 m². Find the pressure it exerts on the floor.
Solution: \( P = F/A = 500 / 2 = 250 \) Pa.
Q3. L4 Analyse Why does the iron nail sink in Aarav's beaker while the thermocol block floats?
Answer: The iron nail is denser than water, so its weight is greater than the upthrust (weight of water it displaces) — it sinks. The thermocol has very low density; it displaces water whose weight is more than its own weight, so the upthrust pushes it up and it floats.
Q4. L3 Apply If Aarav's mass on Earth is 40 kg, what will be his mass and weight on the Moon? (Take gEarth=10 m/s², gMoon=1.6 m/s²)
Mass on Moon: same as on Earth = 40 kg. Weight on Moon: W = m × g = 40 × 1.6 = 64 N (compared to 400 N on Earth).
Q5. L5 Evaluate "Pressure only depends on the force applied." Is this statement correct? Justify with an example.
Incorrect. Pressure depends on both force and area (P = F/A). The same 50 N force applied through a pin tip cuts into a surface, but the same 50 N spread over your palm barely dents it. Area matters as much as force.
🔗 Assertion–Reason Questions
Assertion (A): Astronauts on the Moon can jump higher than on Earth.
Reason (R): The value of g on the Moon is about one-sixth of that on Earth.
Answer: A. Weaker gravity means the Moon pulls them back less strongly, so they rise higher.
Assertion (A): A sharp knife cuts better than a blunt one.
Reason (R): A sharp edge has a smaller contact area, so the same force produces higher pressure.
Answer: A. Pressure = Force/Area; smaller area ⇒ higher pressure ⇒ easier cutting.
Assertion (A): A ship made of steel floats in water although steel is denser than water.
Reason (R): The ship's hollow shape displaces a large volume of water, creating an upthrust equal to the ship's weight.
Answer: A. Archimedes' principle — the displaced water's weight provides enough buoyant force to support the ship.
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