Class 8 Force and Pressure - important Q and A

Force and Pressure Q&A

Comprehensive physics reference covering force basics, motion effects, pressure, and practical applications

1) Force: Basics

1. What is force?
   Force is a push or a pull that arises due to interaction between two objects.
2. Give three examples of applying force.
   Pushing a table, pulling a drawer, kicking a football.
3. Will a body at rest start moving on its own?
   No. An external force is needed to change the state of rest.
4. Can a moving object stop without force?
   No. A force (like friction or an applied opposing force) is needed to stop it.
5. When does force come into play?
   When two objects interact (contact or non-contact).
6. Name two effects a force can produce.
   It can change the state of motion (speed/direction) and/or change shape/size.
7. What are common ways we exert force in daily life?
   Push, pull, lift, squeeze, kick, twist.
8. What does it mean that force has magnitude and direction?
   To fully specify a force, we must state how strong it is and the direction it acts.
9. What is the SI unit of force?
   Newton (N).
10. Define 1 newton.
    The force that produces an acceleration of 1 m/s² in a 1 kg mass.
11. What device measures force?
    Spring balance (it stretches a spring and reads force on a scale).
12. Does changing the direction of force change its effect?
    Yes. Changing direction, magnitude, or both can change the outcome on the object.
13. Can a force act perpendicular to motion and not change speed?
    Yes. A perpendicular force can change direction without changing speed (idealized case).
14. Give examples where force changes shape.
    Squeezing a balloon, stretching a rubber band, molding clay, rolling dough.

2) Effects of Force on Motion

1. What does "change in state of motion" mean?
   Change in speed, change in direction, or both.
2. What happens if force acts along the direction of motion?
   Speed increases (acceleration).
3. What happens if force acts opposite to the direction of motion?
   Speed decreases (deceleration), and it may stop.
4. What happens if force acts at an angle to motion?
   Both speed and direction can change.
5. Why does a rolling ball slow down on its own?
   Due to frictional force opposing motion.
6. Why does a heavier object often need a bigger push to start moving?
   More mass requires greater force for the same change in speed.
7. If the same force is applied to a car and a loaded truck, who speeds up more?
   The car, because it has less mass.

3) Balanced and Unbalanced Forces

1. What are balanced forces?
   Equal and opposite forces acting on an object, net force equals zero.
2. Do balanced forces change motion?
   No. They do not change the state of rest or uniform motion, but may deform objects.
3. What are unbalanced forces?
   Forces whose net result is not zero; they cause change in motion and can change shape.
4. In tug of war, what happens when both teams pull equally?
   The rope remains stationary (balanced forces).
5. In tug of war, why does the weaker team move towards stronger team?
   Net unbalanced force acts towards the stronger team.
6. How do unbalanced forces combine in the same direction?
   They add up.
7. How do unbalanced forces combine in opposite directions?
   They subtract; the net force is towards the larger force.

4) Newton's Third Law and Interaction Pairs

1. State Newton's third law in simple words.
   For every action, there is an equal and opposite reaction.
2. Do forces always occur in pairs?
   Yes. If A pushes B, B pushes A equally and oppositely.
3. Then why does a small body show greater effect in a collision with a large body?
   Forces are equal, but the same force produces more acceleration in the smaller mass.
4. How does a rocket go up?
   Hot gases are pushed down (action), rocket gets pushed up (reaction).

5) Types of Forces

1. What are contact forces?
   Forces that act when objects are in physical contact (e.g., muscular force, friction, air resistance/drag).
2. What is muscular force?
   Force produced by muscles of humans/animals (e.g., lifting, pulling carts).
3. What is frictional force?
   A force that opposes motion between surfaces in contact.
4. What are non-contact forces?
   Forces that act without direct physical contact (gravitational, magnetic, electric).
5. Give an example of gravitational force.
   Earth pulling objects down; fruits falling from trees.
6. Give examples of magnetic force.
   Magnets attracting iron objects; like poles repel, unlike poles attract.
7. Give an example of electric (electrostatic) force.
   Charged glass rod attracting a charged ebonite rod of opposite charge; like charges repel, unlike attract.
8. Can a single isolated force exist?
   No. Forces arise from interactions, so they come in pairs.

6) Pressure: Basics

1. What is thrust?
   The component of force acting perpendicular (normal) to a surface.
2. Define pressure in simple words.
   Pressure is thrust per unit area.
3. Write the formula for pressure.
   P = F/A (when F is perpendicular to the surface).
4. SI unit of pressure?
   Pascal (Pa) = N/m².
5. Does pressure depend on direction?
   Pressure acts normal (perpendicular) to surfaces and is considered a scalar in basic context.
6. If the same force acts on smaller area, what happens to pressure?
   Pressure increases.
7. If the same force acts on larger area, what happens to pressure?
   Pressure decreases.
8. Why do school bags have wide straps?
   To spread force over larger area and reduce pressure on shoulders.
9. Why does a sharp knife cut better than a blunt knife?
   The sharp edge has smaller area, giving higher pressure for the same force.
10. Why are nails and pins pointed?
    Small tip area increases pressure, making them easier to drive in.

7) More Applications of Area and Pressure

1. Why do trucks/buses have double rear wheels?
   To spread the load over larger area and reduce pressure on the road.
2. Why do dams and building foundations have wide bases?
   To distribute weight and reduce pressure on the ground, improving stability.
3. Why use sleepers under railway tracks?
   To increase contact area and reduce pressure on the ground.
4. Why do tanks/bulldozers use caterpillar tracks?
   Large area reduces pressure on soft ground, preventing sinking.

8) Pressure in Solids vs Fluids

1. Can force on solids be applied in any direction?
   Yes, but for pressure we consider the normal (perpendicular) component.
2. In fluids at rest, how must force be applied for pressure?
   At right angles to the surface; fluids cannot sustain shear (tangential) stress at rest.
3. Does pressure always act perpendicular to surfaces?
   Yes, in the basic treatment of pressure in fluids and contact surfaces.

9) Liquid (Hydrostatic) Pressure

1. What is hydrostatic pressure?
   Pressure exerted by a liquid at rest at a point inside it.
2. On what does liquid pressure at a depth depend?
   Depth (h) and liquid density (ρ). P (relative) = ρ g h.
3. Does hydrostatic pressure increase with depth?
   Yes, it increases as depth increases.
4. Does hydrostatic pressure depend on the shape of the container?
   No. At the same depth in the same liquid, pressure is the same regardless of container shape.
5. Do liquids exert pressure in all directions?
   Yes. At a point, pressure acts equally in all directions.
6. What happens if you make holes at the same level on a can filled with water?
   Jets go out equally far—pressure is same at same horizontal level.
7. If holes are at different heights, which jet goes farther?
   The lowest hole (greater depth, greater pressure).
8. Do different liquids at the same depth have the same pressure?
   No. It depends on density. Higher density → higher pressure at same depth.
9. What principle explains pressure transmission in liquids?
   Pressure applied to a confined fluid is transmitted equally in all directions (Pascal's principle).
10. Name devices based on hydraulic transmission of pressure.
    Hydraulic jack, car hoist, hydraulic brakes.

10) Atmospheric Pressure

1. What is atmospheric pressure?
   Pressure exerted by the air (atmosphere) around Earth due to its weight.
2. Why don't we feel crushed by atmospheric pressure?
   Our body fluids and internal air spaces exert nearly equal pressure outward; the pressures balance.
3. Does atmospheric pressure change with altitude?
   Yes, it decreases as altitude increases.
4. Why do high-altitude climbers carry oxygen?
   Air becomes thinner (lower pressure and fewer oxygen molecules per breath), making breathing difficult.
5. Why do airplanes have pressurized cabins?
   To maintain air pressure at a comfortable level for passengers and crew.
6. How do straws and syringes work?
   Sucking reduces pressure inside; higher outside atmospheric pressure pushes liquid up into the straw/syringe.
7. How do suction cups stick to walls?
   Air is expelled from behind the cup; outside atmospheric pressure presses it against the surface.
8. Why can a hot-water–rinsed plastic bottle crush inward after sealing?
   Hot air escapes; after sealing and cooling, inside pressure becomes lower than outside, so atmospheric pressure crushes the bottle.

11) Concept Connections and Reasoning

1. If two equal and opposite forces act on a block, what is the net force?
   Zero (balanced forces), so no change in motion (but shape may change).
2. If a block is pushed by 30 N right and 10 N left simultaneously, what is the net force and direction?
   20 N to the right (unbalanced).
3. Why must a cyclist keep pedaling on a level road to maintain speed?
   To balance resistive forces like friction and air drag.
4. Why is it harder to move a loaded trolley than an empty one?
   More mass requires more force to accelerate.
5. Why is it easier to walk on rough ground than on ice?
   Greater friction on rough ground provides better grip and prevents slipping.
6. Why does a person wearing high heels exert more pressure than wearing flat shoes?
   Smaller contact area of heels increases pressure for the same weight.

12) Numericals (Simple)

1. A force of 50 N acts normally on an area of 0.5 m². Find the pressure.
   P = F/A = 50/0.5 = 100 Pa.
2. A person of weight 600 N stands on two feet, each 200 cm². Find pressure on the ground.
   Total area = 2 × 200 cm² = 400 cm² = 0.04 m². P = 600/0.04 = 15,000 Pa.
3. A box rests on a table with base area 0.25 m². Its weight is 250 N. Calculate pressure.
   P = 250/0.25 = 1,000 Pa.
4. A sharp knife has an edge area of 2×10^-5 m². If 40 N is applied, find pressure at the edge.
   P = 40 / (2×10^-5) = 2,000,000 Pa = 2 MPa.
Pressure at 4 m is double that at 2 m (since P ∝ h).

13) Concept Checks and True/False

1. True/False: Pressure depends only on force.
   False. It also depends on area (P = F/A).
2. True/False: At the same depth, pressure is different at different horizontal positions.
   False. At the same level, pressure is the same.
3. True/False: Balanced forces can deform objects.
   True. Even if net force is zero, shape can change (e.g., squeezing a ball from both sides).
4. True/False: Atmospheric pressure increases with altitude.
   False. It decreases with altitude.
5. True/False: For the same force, doubling the contact area halves the pressure.
   True. P ∝ 1/A.

14) Higher-Order and Everyday Applications

1. Why do camels have broad feet for desert travel?
   Large area reduces pressure on sand, preventing sinking.
2. Why are knives sharpened regularly?
   To reduce edge area and maintain higher pressure for easy cutting.
3. Explain why a balloon bulges more when more water is poured into a vertical tube sealed with a balloon at the bottom.
   Greater height of water increases pressure at the bottom, stretching the balloon more.
4. A sealed plastic bottle collapses after hot water is poured out and the cap is tightened. Explain.
   Inside air pressure becomes lower upon cooling; outside atmospheric pressure crushes it inward.
5. Why do divers feel pressure on ears under water?
   Hydrostatic pressure increases with depth and acts on body surfaces including the eardrum.
6. Why is it hard to remove a well-seated suction hook from a wall?
   Low pressure behind/inside the cup and higher atmospheric pressure outside holds it firmly.
7. Why does a cyclist lean while turning?
   To balance sideways forces and maintain a resultant force towards the center of the curve (changing direction).
8. Why can a lighter person be pushed back in a collision with a heavier person even though forces are equal?
   Same force causes higher acceleration on the smaller mass.

15) Mixed Practice (Match/Fill/Short)

1. Match: (a) Muscular force (b) Friction (c) Gravity (d) Magnetism – Opposes motion between surfaces → (b) – Pulls objects towards Earth → (c) – Produced by animals/humans → (a) – Acts between poles without contact → (d)
2. Fill in the blanks: Pressure = ________ / ________.
   Thrust / Area.
3. Fill in: For liquids, pressure increases with ______ and ______ of the liquid.
   Depth; density.
4. Name two hydraulic pressure applications.
   Hydraulic jack, hydraulic brakes.
5. Two reasons we don't feel atmospheric pressure.
   Our internal pressure balances it; pressure acts equally from all directions.

16) Short Explanations

1. Why do soft soils need wider foundations?
   To spread load and reduce pressure to avoid sinking/settling.
2. Why do swimmers feel "heavier" out of water than in water?
   In water, buoyant force (related to displaced fluid) partly supports weight.
3. Does the bottom area of a tank affect pressure at the base due to the liquid column?
   No. At a given depth, pressure depends on ρ(density of liquid), g(acceleration due to gravity), h(depth/ height of liquid column), not base area.
4. Why is it easier to cut ripe tomatoes with a very sharp knife?
   Small contact area increases pressure, overcoming resistance easily.
5. Why is it harder to breathe at high altitudes?
   Lower air pressure means fewer oxygen molecules per breath.

17) Challenge Questions

1. Two blocks rest on a table: Block A area 0.02 m², weight 200 N; Block B area 0.05 m², weight 300 N. Which produces more pressure?
   A: P = 200/0.02 = 10,000 Pa. B: P = 300/0.05 = 6,000 Pa. Block A produces more pressure.
2. Explain why a blunt nail bends instead of penetrating wood when hammered lightly.
   Large tip area gives low pressure, insufficient to penetrate; the force deforms/bends the nail instead.
3. A hydraulic lift has small piston area 0.01 m² and large piston area 1.0 m². If 200 N is applied on the small piston (ignoring losses), what force appears on the large piston?
   Pressure is the same: P = 200/0.01 = 20,000 Pa, so Force_large = P × A_large = 20,000 × 1.0 = 20,000 N.

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