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Force and Pressure Exercise Solutions | KalviGavani

Samacheer Kalvi 8th Science Chapter 2 Force and Pressure exercise solutions with MCQs, fill in blanks, true or false, match the following, short and detailed answers, common mistakes, and exam checklist.

Updated 12 June 202611 min read

Exercise Solutions Available

Step-by-step answers for all textbook questions in this chapter

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I. Choose the Best Answer (MCQs)

Q1. If we apply a force against the direction of motion of a body, then the body will

  • (a) Stop moving (b) Move with increased speed (c) Move with decreased speed (d) Move in a different direction
Answer(a) Stop moving A force applied opposite to the direction of motion decelerates the body and eventually stops it.

Q2. Pressure exerted by a liquid is increased by

  • (a) The density of the liquid (b) The height of the liquid column (c) Both (a) and (b) (d) None of the above
Answer(c) Both (a) and (b) Liquid pressure depends on both density and height (P = h x rho x g). Increasing either increases the pressure.

Q3. Unit of pressure is

  • (a) Pascal (b) N m^-2 (c) Poise (d) Both (a) and (b)
Answer(d) Both (a) and (b) Pascal (Pa) and N/m^2 are equivalent units for pressure. Poise is the unit of viscosity.

Q4. The value of the atmospheric pressure at sea level is

  • (a) 76 cm of mercury column (b) 760 cm of mercury column (c) 176 cm of mercury column (d) 7.6 cm of mercury column
Answer(a) 76 cm of mercury column Atmospheric pressure at sea level supports a 76 cm column of mercury, equivalent to 1 atm or approximately 1.013 x 10^5 Pa.

Q5. Pascal's law is used in

  • (a) Hydraulic lift (b) Brake system (c) Pressing heavy bundles (d) All the above
Answer(d) All the above Pascal's law has applications in hydraulic lifts, brake systems, and hydraulic presses for compressing bundles.

Q6. Which of the following liquids has more viscosity?

  • (a) Grease (b) Water (c) Coconut oil (d) Ghee
Answer(a) Grease Viscosity is the resistance to flow. Grease is the thickest and most viscous among the given options.

Q7. The unit of viscosity is

  • (a) N m^2 (b) Poise (c) kg m s^-1 (d) No unit
Answer(b) Poise Viscosity is measured in Poise (P), named after Jean Louis Marie Poiseuille.

II. Fill in the Blanks

1. The pressure of a liquid column increases with the depth of the column. 2. Hydraulic lift works under the principle of Pascal's Law. 3. The property of surface tension of a liquid surface enables the water droplets to move upward in plants. 4. A simple barometer was first constructed by Torricelli.

III. True or False

Q1. Force acting on a given area is called pressure.
AnswerTrue — Pressure = Force / Area.
Q2. A moving body comes to rest due to friction alone.
AnswerTrue — Friction opposes the relative motion between surfaces and eventually stops a moving body.
Q3. A body will sink if the weight of the body is greater than the buoyant force.
AnswerTrue — When weight exceeds buoyant force, the net force is downward and the body sinks.
Q4. One atmosphere is equivalent to 1,00,000 newton force acting on one square metre.
AnswerTrue — 1 atm = 1.013 x 10^5 N/m^2, approximately equal to 10^5 N/m^2.
Q5. Rolling friction is slightly greater than the sliding friction.
AnswerFalse — Rolling friction is much less than sliding friction. That is why wheels are used to reduce friction.
Q6. Friction is the only reason for the loss of energy.
AnswerTrue — Friction converts kinetic energy into heat energy, causing energy loss.
Q7. Liquid pressure decreases with the decrease of depth.
AnswerTrue — As depth decreases, P = h x rho x g decreases proportionally.
Q8. Using barometers, one can measure the height of a building.
AnswerFalse — Barometers measure atmospheric pressure, not building height. Altimeters or other instruments measure height.
Q9. Surface tension causes the spherical nature of a water drop.
AnswerTrue — Surface tension pulls water molecules inward, minimizing surface area. A sphere has the least surface area for a given volume.
Q10. Viscosity depends on the pressure of a liquid.
AnswerTrue — Viscosity is influenced by both temperature and pressure of the liquid.

IV. Arrange in Increasing Order

Q1. Rolling friction, static friction, sliding friction Answer: Static friction > Sliding friction > Rolling friction

Q2. Arrange surfaces (cotton cloth, glass plate, paper, card board, silver plate) so a marble covers the greatest distance. Answer: Glass plate, silver plate, paper, cotton cloth, card board (least friction to most friction).

V. Match the Following

Match 1:

Column IColumn IIAnswer
i(image-based)i — e
ii(image-based)ii — c
iii(image-based)iii — b
iv(image-based)iv — a
v(image-based)v — d

Match 2:

Column IColumn IIAnswer
i(image-based)i — b
ii(image-based)ii — d
iii(image-based)iii — e
iv(image-based)iv — a
v(image-based)v — c

VI. Analogy

Q1. Knot in a thread : ____ friction :: ball bearing : ____ friction Answer: Static, Rolling

Q2. Downward force : weight :: Upward force offered by liquid : ____ Answer: Buoyant force

VII. Problems

Q1. A stone weighs 500 N. Calculate the pressure if it contacts a surface of area 25 cm^2.

Solution
  • F = 500 N
  • A = 25 cm^2 = 25 x 10^-4 m^2
  • P = F/A = 500 / (25 x 10^-4) = 2,00,000 N/m^2 (or 2 x 10^5 Pa)

Q2. In a hydraulic lift, input piston area = 10 cm^2, output piston area = 3000 cm^2. A 100 N force is applied to the input piston. Calculate the output force.

Solution
  • Input pressure = F1/A1 = 100 / (10 x 10^-4) = 10^5 Pa
  • By Pascal's law: P is equal throughout
  • Output force F2 = P x A2 = 10^5 x 3000 x 10^-4 = 3 x 10^4 N (30,000 N)

VIII. Assertion and Reason

Q1. Assertion: Sharp knives are used to cut vegetables. Reason: Sharp edges exert more pressure.
Answer(a) Both true and reason correctly explains the assertion. Sharp edges have a smaller area, so pressure (P = F/A) is greater.
Q2. Assertion: Broad straps are used in bags. Reason: Broad straps last for long life.
Answer(b) Both true, but reason does not explain the assertion. The correct explanation: Broad straps distribute weight over a larger area, reducing pressure on the shoulder.
Q3. Assertion: Water strider slides easily on water surface. Reason: Water strider experiences less buoyant force.
Answer(b) Both true, but reason does not explain the assertion. The real reason is the surface tension of water, not buoyant force.

IX. Short Answers

Q1. Give two examples that a force changes the shape of a body. Answer: 1. Squeezing a sponge changes its shape. 2. Pulling a rubber band makes it longer.

Q2. Give two examples that a force changes the static condition of a body. Answer: 1. A resting rubber ball moves when force is applied. 2. A stationary football moves when a player kicks it.

Q3. Taking out paste from a toothpaste tube highlights which physical property? Answer: Pascal's law — pressure applied on the paste is transmitted equally in all directions.

Q4. What do you feel when you touch a nail immediately after hammering? Why? Answer: The nail feels hot because friction between the hammer and nail converts kinetic energy into heat energy.

Q5. How does friction arise between surfaces in relative motion? Answer: Friction arises from the interlocking of microscopic irregularities (bumps and depressions) between the two surfaces in contact.

Q6. Name two instruments that measure fluid pressure. Answer: Manometer and Pressure gauge.

Q7. Define one atmosphere. Answer: The pressure exerted by a 76 cm mercury column at sea level is defined as one atmosphere (1 atm). It equals approximately 1.013 x 10^5 Pa.

Q8. Why are heavy bags provided with broad straps? Answer: Broad straps provide a larger contact area with the shoulder. Since P = F/A, increasing area reduces the pressure, making the bag more comfortable.

Q9. How does surface tension help a plant? Answer: Surface tension causes capillary action in xylem vessels, enabling water absorbed by roots to rise upward through narrow vessels against gravity.

Q10. Which has greater viscosity, oil or honey? Why? Answer: Honey has greater viscosity because it is thicker than oil. Viscosity is the internal resistance to flow; greater viscosity means more frictional force between liquid layers.

X. Detailed Answers

Q1. "Friction is a necessary evil" — explain. Answer: Friction is essential in daily life: we hold objects, walk on roads, write with pens, apply brakes in vehicles, light matchsticks, sew clothes, and fix nails — all due to friction.

However, friction also has disadvantages: it wears out surfaces (like screws, gears, shoe soles), wastes energy by requiring excess effort to overcome it, and generates unwanted heat in machines.

Because it is both indispensable and harmful, friction is termed a "necessary evil."

Q2. Give the different types of friction and explain each with an example. Answer:

  • Static friction: Acts on bodies at rest. Example: A knot in a thread stays tied; objects at rest on Earth remain stationary.
  • Kinetic friction: Acts during motion. It is subdivided into:
  • Sliding friction: When one body slides over another. Example: A block sliding on a surface.
  • Rolling friction: When one body rolls over another. Example: Wheels on vehicles. Rolling friction is always less than sliding friction.

Q3. Describe an experiment proving friction depends on the nature of a surface. Answer: Place drops of different liquids (coconut oil, honey, water, ghee) on separate glass plates. Tilt each plate to let the liquid slide. Water flows fastest, coconut oil moderately, and ghee very slowly. This shows each liquid experiences different frictional forces between its own layers (viscosity), proving friction depends on the nature of the substance/surface.

Q4. Explain how friction can be minimised. Answer: Two main methods: 1. Using lubricants — Substances like grease, coconut oil, or graphite fill gaps in irregular surfaces, creating a smooth layer that prevents direct contact of rough surfaces. 2. Using ball bearings — Since rolling friction is smaller than sliding friction, ball bearings (lead shots in cycle hubs) replace sliding with rolling, thereby reducing friction.

Q5. Describe an experiment proving that liquid pressure increases with depth. Answer: Punch three holes at different heights on the side of a plastic bottle. Pour water in and observe the flow. Water from the lowest hole exits with the greatest force and travels the farthest; water from the topmost hole exits with the least force. This confirms that liquid pressure increases with depth.

Common Mistakes

  • Confusing force with pressure: Force (N) is a push or pull; pressure (Pa) is force per unit area.
  • Saying rolling friction is greater than sliding friction: Rolling friction is always less than sliding friction.
  • Confusing barometer with manometer: Barometers measure atmospheric pressure; manometers measure fluid/gas pressure differences.
  • Saying Pascal's law only applies to liquids: Pascal's law applies to confined fluids (both liquids and gases).
  • Mixing up buoyant force with surface tension: Water striders float due to surface tension, not buoyancy.
  • Forgetting to convert units in pressure calculations: Always convert cm^2 to m^2 (multiply by 10^-4) before calculating pressure.

Exam Checklist

  • [ ] Can you define force, pressure, and distinguish between them?
  • [ ] Can you calculate pressure using P = F/A with proper unit conversions?
  • [ ] Can you state and apply Pascal's law?
  • [ ] Can you list and explain the three types of friction (static, sliding, rolling)?
  • [ ] Can you explain why "friction is a necessary evil"?
  • [ ] Can you explain surface tension and viscosity with examples?
  • [ ] Can you describe how liquid pressure varies with depth and density?
  • [ ] Can you define atmospheric pressure and explain the barometer?
  • [ ] Can you solve numerical problems on hydraulic lifts using Pascal's law?
  • [ ] Can you explain why sharp knives cut better and broad straps feel comfortable?

Frequently Asked Questions

Force acting per unit area: Pressure = Force / Area.