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State Board · Class 8 · General Science

Lesson 3: Force and Pressure

5 concepts · 40 practice questions with answers · Free to read

This lesson is also in: मराठी

Contact and non-contact forces, friction

From daily life

Pulling a cart, kicking a football and a magnet pulling a pin are all forces. In the first two you touch the object. The magnet pulls without touching. Also, a slippery bathroom floor shows that without friction we cannot even walk steadily.

Simple definition

A force (बल) is a push or a pull. Friction (घर्षण) is the force that opposes the motion of one surface over another.

Contact forcesmuscularfrictionNon-contact forcesgravityNSmagneticelectrostatic

In the figure : In the figure contact forces need touching: muscular force and friction. Non-contact forces act without touching: gravitational, magnetic and electrostatic forces.

Rule 1: Contact and non-contact forces — Contact forces need touching: muscular force and friction. Non-contact forces act from a distance: gravitational, magnetic and electrostatic force.

  • Easy : Pushing a door: muscular force (contact).
  • Medium : A fruit falling from a tree: gravitational force (non-contact).
  • Exam-type : A comb rubbed on dry hair picks up paper bits: electrostatic force (non-contact).
PQ

In the figure : In the figure friction is caused by the bumps of the two surfaces catching on each other. It is greater on the rough surface P than on the nearly smooth surface Q.

Rule 2: Friction — Friction opposes motion. It helps us walk and grip things. It is reduced by a banana peel or mud (slippery surfaces).

  • Easy : Brakes of a cycle use friction to stop the wheel.
  • Medium : Oil on a chain reduces friction and the cycle moves easily.
  • Exam-type : A wet soap makes the floor slippery by reducing friction; step carefully.
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Contact and non-contact forces, friction

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Remember

Contact: muscular, friction. Non-contact: gravity, magnet, electrostatic. Friction opposes motion.

Common mistake

Thinking friction is always bad. Without it we cannot walk or hold things.

Shortcut

Memory clue: "Friction = Fights the motion".

Spot the difference

PointContact forceNon-contact force
Touching neededYesNo
ExamplesMuscular force, frictionGravity, magnetic, electrostatic

Practice questions

Easy
  1. 1What is a force?
  2. 2Name two contact forces.
  3. 3Name two non-contact forces.
Medium
  1. 4What does friction do to motion?
  2. 5Why does friction cause surfaces to heat up? Give a use of friction.
  3. 6Why is friction greater on a rough surface?
Challenging
  1. 7Why do we spread sand on a slippery path?
  2. 8Why are tyres made with grooves and not completely smooth?
Show answers (try them yourself first)
  1. 1A push or a pull.
  2. 2Muscular force and friction
  3. 3Gravitational and magnetic force
  4. 4It opposes it.
  5. 5Bumps catch on each other and energy turns to heat. Use: walking or brakes.
  6. 6Its bumps catch on each other more.
  7. 7Sand makes the surface rough and increases friction.
  8. 8Grooves give a good grip (friction) on the road, especially when wet.

Balanced and unbalanced forces, inertia

From daily life

When a bus suddenly starts, you lean back. When it suddenly stops, you lean forward. Your body wants to keep doing what it was doing. This stubbornness of objects has a name.

Simple definition

Inertia (जडत्व) is the tendency of an object to remain in its existing state of rest or motion.

20 N20 Nbalancednet 0 N15 N10 Nsame directionnet 25 N12 N30 Noppositenet 18 N

In the figure : In the figure equal and opposite forces are balanced (net 0). Forces in the same direction add (15 + 10 = 25 N). Opposite unequal forces subtract (30 − 12 = 18 N) towards the larger one.

Rule 1: Balanced and unbalanced forces — Balanced forces: net force zero, no change in motion. Unbalanced forces change speed or direction. Forces in the same direction add; opposite forces subtract and the net acts towards the larger. Force is a vector (it has magnitude and direction).

  • Easy : Two children push a box with 20 N and 5 N in the same direction: net = 20 + 5 = 25 N.
  • Medium : In tug of war, team A pulls 400 N and team B 350 N: net = 400 − 350 = 50 N towards A.
  • Exam-type : A box on the floor with push 40 N and friction 40 N has net force 0: it keeps moving at the same speed or stays at rest.
bus stops suddenlybus starts suddenly

In the figure : In the figure inertia of motion makes passengers lean forward when a moving bus stops suddenly. Inertia of rest makes them lean backward when a bus at rest starts suddenly.

Rule 2: Types of inertia — Inertia of rest: an object at rest tends to stay at rest. Inertia of motion: a moving object tends to keep moving. Directional inertia: an object tends to keep its direction (a stone flies off tangentially from a whirling string).

  • Easy : Dust flies off a carpet when you beat it: inertia of rest.
  • Medium : A cyclist continues to move forward for a bit even after he stops pedalling: inertia of motion.
  • Exam-type : A heavier object has more inertia: it is harder to start or to stop.
P4 mQ9 mR17 mS?

In the figure : In the figure the smoother the track, the farther the ball goes. On a track with no friction it would never stop: an object continues in its state of rest or motion unless an external force acts on it.

Rule 3: Law of inertia — An object continues in its state of rest or of uniform motion in a straight line unless an external force acts on it. Friction is the reason a rolling ball stops.

  • Easy : A hockey puck slides far on smooth ice.
  • Medium : A ball rolls farther on a polished floor than on a carpet.
  • Exam-type : Wearing a seat belt keeps the body from moving forward by inertia when the car stops suddenly.
Remember

Net force zero: balanced. Same direction: add; opposite: subtract. Inertia: stay as it is.

Common mistake

Saying a moving object needs a force to keep moving. It needs a force only to change its motion.

Shortcut

Memory clue: "Inertia = Stay as you are": Lazy object.

Spot the difference

PointBalanced forcesUnbalanced forces
Net forceZeroNot zero
Effect on motionNo changeSpeed or direction changes

Practice questions

Easy
  1. 1What are balanced forces?
  2. 2What is inertia?
  3. 3Name the three types of inertia.
Medium
  1. 4What is the net force of 20 N and 5 N in the same direction?
  2. 5Net force of 50 N and 30 N in opposite directions?
  3. 6Why do passengers lean forward when a bus stops suddenly?
Challenging
  1. 7Why is a seat belt important in a car?
  2. 8A box is pushed with 45 N and friction is 45 N. Is it accelerating? Explain.
Show answers (try them yourself first)
  1. 1Forces whose net force is zero.
  2. 2The tendency to remain in the existing state of rest or motion.
  3. 3Of rest, of motion and directional.
  4. 425 N
  5. 550 − 30 = 20 N in the direction of the 50 N.
  6. 6Inertia of motion keeps their bodies moving forward.
  7. 7It stops the body from moving forward by inertia during a sudden stop.
  8. 8No; net force = 45 − 45 = 0, so the motion does not change.

Pressure

From daily life

A boy standing on one foot in wet sand sinks deeper than when standing on both feet. The same weight, but a smaller area presses harder. That pressing 'per unit area' is pressure.

Simple definition

Pressure (दाब) is the force acting per unit area. Pressure = force ÷ area.

FPFQ

In the figure : In the figure, for the same force F, a smaller area gives a larger pressure. Nail P with its point down goes in deep, while nail Q with its head down hardly goes in.

Rule 1: Pressure and its units — The SI unit is the pascal (Pa) = N/m2. 1 bar = 105 Pa. Pressure is a scalar. A smaller area gives a larger pressure (nail, knife); a broad area reduces it (a camel's broad feet, skis).

  • Easy : A knife with a sharp edge cuts easily because its edge area is very small.
  • Medium : A camel does not sink in sand because its broad feet reduce the pressure.
  • Exam-type : Heavy trucks have many wide tyres so that the pressure on the road is smaller.
60 N0.20 m²

In the figure : In the figure pressure = force ÷ area. A weight of 60 N on 0.20 m2 gives 60 ÷ 0.20 = 300 N/m2 = 300 pascal.

Rule 2: Calculating pressure — P = F ÷ A. Always write the force in newton and the area in square metre to get pascal.

  • Easy : A force of 100 N on 0.5 m2: P = 100 ÷ 0.5 = 200 Pa.
  • Medium : A 400 N child standing on two feet of total area 0.04 m2: P = 400 ÷ 0.04 = 10,000 Pa. On one foot (0.02 m2): 400 ÷ 0.02 = 20,000 Pa, double.
  • Exam-type : 3 bar = 3 × 105 = 300,000 Pa. A 600 N person on a heel of area 0.0004 m2: P = 600 ÷ 0.0004 = 1,500,000 Pa = 1.5 × 106 Pa.
Live figure

Pressure

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Remember

P = F ÷ A, unit pascal. Small area: more pressure. Large area: less pressure.

Common mistake

Using area in cm2 without converting to m2. 1 m2 = 10,000 cm2.

Shortcut

Memory clue: "P = F over A": pointed things press more.

Spot the difference

SituationAreaPressure
Nail pointSmallLarge
Nail headLargeSmall
Camel's feetLargeSmall

Practice questions

Easy
  1. 1What is pressure?
  2. 2What is the SI unit of pressure?
  3. 3Is pressure a vector or a scalar?
Medium
  1. 4Why does a sharp knife cut better?
  2. 5Find the pressure of 200 N on 0.4 m2.
  3. 6Why do skis help in walking on soft snow?
Challenging
  1. 7A box of weight 120 N rests on a face of area 0.30 m2. Find the pressure. If turned onto a face of 0.10 m2, what is it now?
  2. 8Convert 2.5 bar to pascal.
Show answers (try them yourself first)
  1. 1Force acting per unit area.
  2. 2Pascal (N/m2)
  3. 3Scalar
  4. 4Its small edge area gives high pressure.
  5. 5200 ÷ 0.4 = 500 Pa
  6. 6They increase the area and reduce the pressure.
  7. 7120 ÷ 0.30 = 400 Pa; 120 ÷ 0.10 = 1,200 Pa (three times).
  8. 82.5 × 105 = 250,000 Pa

Liquid, gas and atmospheric pressure

From daily life

Holes punched at different heights in a water bottle shoot water jets of different strength. A mountain climber finds it hard to breathe on a peak because the air above is thinner. Both are about pressure that changes with depth or height.

Simple definition

Atmospheric pressure (वातावरणीय दाब) is the pressure exerted by the air layer around the earth. Liquids and gases together are called fluids.

ABC

In the figure : In the figure liquid pressure increases with depth. The jet from the lowest hole C is the strongest and that from the highest hole A is the weakest.

Rule 1: Liquid pressure — Liquid pressure increases with depth and is the same at one level. Fluids press equally in all directions.

  • Easy : The walls of a dam are thicker at the bottom because the pressure is higher at depth.
  • Medium : Your ears feel pressure when you dive deep in a swimming pool.
  • Exam-type : A water tank is placed on the roof so that taps downstairs get water with high pressure.
A10 cmB10 cmC5 cm

In the figure : In the figure, at the same depth the liquid pressure is the same whatever the shape of the vessel. A and B (10 cm) are equal, and C (5 cm) has less.

Rule 2: Same depth, same pressure — The pressure depends on depth, not on the shape or width of the vessel. A narrow and a wide vessel with the same depth have the same pressure at the bottom.

  • Easy : Two vessels filled to 10 cm have equal pressure at the bottom.
  • Medium : A vessel filled to 5 cm has half the depth, so less pressure than one filled to 10 cm.
  • Exam-type : Depth is what matters: an ocean diver at 20 m feels more pressure than a swimmer at 2 m.
A0 mB1500 mC4000 m

In the figure : In the figure atmospheric pressure decreases with height. It is the greatest at sea level (A) and the least at the peak (C).

Rule 3: Atmospheric pressure — 1 atm = 101 × 103 Pa = 101,000 Pa. It falls with height because the air gets thinner.

  • Easy : A mountaineer carries oxygen on a high peak because the air is thin.
  • Medium : 101 × 103 Pa is about 1.01 bar since 1 bar = 105 Pa.
  • Exam-type : Ears may 'pop' when a bus climbs a hill because outside pressure changes.
Remember

Liquid pressure: more with depth, same at one level. Atmospheric pressure: less with height.

Common mistake

Thinking a wider vessel gives more pressure. Only depth matters.

Shortcut

Memory clue: "Water: deeper, harder. Air: higher, thinner".

Spot the difference

PointLiquid pressureAtmospheric pressure
Changes withDepth (increases)Height (decreases)
ValueSame at one level1 atm = 101,000 Pa

Practice questions

Easy
  1. 1What happens to liquid pressure with depth?
  2. 2What happens to atmospheric pressure with height?
  3. 3What is the value of 1 atm?
Medium
  1. 4Which hole's jet is strongest and why?
  2. 5Why are dam walls thicker at the bottom?
  3. 6Why do climbers find it hard to breathe on peaks?
Challenging
  1. 7Vessel X is filled to 12 cm and Y to 8 cm. Which has greater bottom pressure?
  2. 8Two vessels of different shapes are filled to the same depth. Compare the bottom pressures and give the reason.
Show answers (try them yourself first)
  1. 1It increases.
  2. 2It decreases.
  3. 3101 × 103 Pa
  4. 4The lowest hole, because the depth is greatest there.
  5. 5The water pressure is highest at the bottom.
  6. 6Atmospheric pressure and air are less there.
  7. 7X, as it has greater depth.
  8. 8Equal, because pressure depends on depth, not on shape.

Buoyancy, Archimedes' principle and relative density

From daily life

A heavy iron ship floats but a small iron nail sinks. A bucket of water feels lighter inside a well. Water is pushing up on things.

Simple definition

Buoyant force (उत्प्लावक बल) is the upward force exerted by a fluid on an object placed in it.

PW = 6 NB = 4 NQW = 2 NB = 5 NRW = 3 NB = 3 N

In the figure : In the figure an object sinks if B is less than W (P), rises if B is greater than W (Q), and stays where it is if B equals W (R).

Rule 1: Float or sink — The buoyant force acts upwards and depends on the volume of the object and the density of the liquid. The object floats if the buoyant force is greater than its weight and sinks if it is less.

  • Easy : A wooden block floats because the buoyant force is greater than its weight.
  • Medium : A steel nail sinks because its weight is greater than the buoyant force.
  • Exam-type : A steel ship has a hollow shape which displaces a lot of water, so the buoyant force is large and it floats.
4.5 Nin air3 Nin water

In the figure : In the figure a stone weighs less under water: 4.5 N in air and 3.0 N in water. The loss, 4.5 − 3.0 = 1.5 N, is the buoyant force.

Rule 2: Apparent weight loss — An object seems lighter in a liquid. Buoyant force = weight in air − weight in the liquid.

  • Easy : A stone weighs 6 N in air and 4 N in water: buoyant force = 6 − 4 = 2 N.
  • Medium : An object weighs 10 N in air and the buoyant force is 3 N, so it weighs 10 − 3 = 7 N in water.
  • Exam-type : If the buoyant force equals the weight, the object stays at the same depth.
Bw

In the figure : In the figure of Archimedes' principle the buoyant force B equals the weight w of the fluid displaced, that is, the water pushed out of the can into the beaker.

Rule 3: Archimedes' principle and relative density — Archimedes' principle: the buoyant force on an object equals the weight of the fluid it displaces. Relative density (सापेक्ष घनता) = density of the substance ÷ density of water; it has no unit.

  • Easy : If 2 N of water is pushed out, the buoyant force is 2 N.
  • Medium : Aluminium density 2,700 kg/m3 and water 1,000 kg/m3: relative density = 2,700 ÷ 1,000 = 2.7.
  • Exam-type : Ice has relative density about 0.92, less than 1, so it floats on water.
Remember

Buoyant force = weight of fluid displaced. Relative density = density ÷ water density. Less than 1 floats.

Common mistake

Thinking big objects always sink. Shape and displaced water decide.

Shortcut

Memory clue: "Displace, Weigh, Float": Buoyancy = weight of displaced fluid.

Spot the difference

ConditionBuoyant force vs weightResult
B less than WLessSinks
B greater than WGreaterRises, floats
B equals WEqualStays in place

Practice questions

Easy
  1. 1What is buoyant force?
  2. 2State Archimedes' principle.
  3. 3What is relative density?
Medium
  1. 4What is the unit of relative density?
  2. 5An object weighs 8 N in air and 5 N in water. Find the buoyant force.
  3. 6Will a substance of relative density 0.8 float or sink in water?
Challenging
  1. 7Find the relative density of a metal of density 8,000 kg/m3.
  2. 8A steel ship floats but a steel nail sinks. Explain with displaced water.
Show answers (try them yourself first)
  1. 1The upward force exerted by a fluid on an object.
  2. 2Buoyant force equals the weight of the fluid displaced.
  3. 3Density of the substance ÷ density of water.
  4. 4It has no unit.
  5. 58 − 5 = 3 N
  6. 6It floats, because it is less than 1.
  7. 78,000 ÷ 1,000 = 8
  8. 8The ship's hollow shape displaces a lot of water, giving a buoyant force equal to its weight; the nail displaces little.

Quick revision

  • P = F/A; pascal. Pressure increases with depth.
  • Buoyant force = weight of displaced fluid.
  • Relative density has no unit.
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Quick revision

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