Exercise Solutions (From Screenshot Pages 21-22)

Q1: Fill in the blanks.

1. The force exerted by the Earth to pull objects towards its center is called Gravitational Force.
2. The SI unit of pressure is Pascal (Pa).
3. The upward force exerted by a fluid on an immersed object is called Buoyant Force.
4. The density of water at 4°C is 1000 kg/m³ or 1 g/cm³.
5. Pascal's principle is used in Hydraulic Jack and Hydraulic Brake.

Q2: Match the pairs.

(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)

• Muscular force - Contact force
• Magnetic force - Non-contact force
• Pressure - Force / Area
• Atmospheric pressure - 101 x 10³ Pa
• Archimedes' Principle - Buoyant force

Q3: Give reasons.

1. It is easier to cut vegetables with a sharp knife.
   Reason: A sharp knife has a very small surface area at its cutting edge. When force is applied, this small area results in a very high pressure (Pressure = Force / Area), allowing the knife to easily penetrate and cut through vegetables. A blunt knife has a larger area, thus exerting less pressure for the same force, making cutting difficult.
2. Dams are built with thicker walls at the base.
   Reason: The pressure exerted by a liquid increases with its depth. As the depth of water in a dam is greatest at its base, the water exerts immense pressure on the walls at that level. To withstand this enormous pressure and prevent the dam from collapsing, the walls are constructed to be much thicker and stronger at the base.
3. A ship floats on water.
   Reason: A ship, despite being made of dense materials like steel, floats on water due to Archimedes' Principle. Its hollow design ensures that it displaces a large volume of water. The buoyant force exerted by this displaced water is equal to the weight of the displaced water. If this buoyant force is equal to or greater than the total weight of the ship (including its cargo and air), the ship floats. The average density of the ship (including the air in its hull) is less than the density of water.
4. Atmospheric pressure decreases with increasing altitude.
   Reason: Atmospheric pressure is caused by the weight of the air column above a point. As altitude increases, the amount of air above that point decreases, and the air also becomes less dense. Consequently, the weight of the air column above decreases, leading to a reduction in atmospheric pressure.
5. A rubber sucker sticks to a smooth surface.
   Reason: When a rubber sucker is pressed against a smooth surface, the air between the sucker and the surface is expelled, creating a partial vacuum or low-pressure area inside. The higher atmospheric pressure acting on the outside surface of the sucker then pushes it firmly against the smooth surface, causing it to stick.

Q4: Answer the following questions.

1. What are the types of force? Explain with examples.
   Answer: Forces are mainly of two types:
   • Contact Force: Acts when there is direct physical contact between objects.
     • Muscular Force: Force exerted by muscles (e.g., pushing a table).
     • Mechanical Force: Force exerted by machines (e.g., a car engine pushing the car).
     • Frictional Force: Force opposing motion between surfaces in contact (e.g., brakes stopping a bicycle).
   • Non-Contact Force: Acts without direct physical contact.
     • Gravitational Force: Earth's pull on objects (e.g., an apple falling from a tree).
     • Magnetic Force: Force between magnets or magnetic materials (e.g., a magnet attracting iron filings).
     • Electrostatic Force: Force between charged objects (e.g., a charged comb attracting paper bits).
2. Explain the concept of pressure.
   Answer: Pressure is the force acting perpendicularly on a unit surface area. It is calculated as Pressure = Force / Area. The effect of a force depends not only on its magnitude but also on the area over which it is distributed. A smaller area of contact results in higher pressure for the same force, while a larger area results in lower pressure. The SI unit of pressure is Pascal (Pa).
3. What is atmospheric pressure? Why is it important?
   Answer: Atmospheric pressure is the pressure exerted by the weight of the column of air in the Earth's atmosphere on all surfaces. It is important for various daily phenomena and applications, such as drinking with a straw, the working of syringes, and the ability of rubber suckers to stick to surfaces. It also plays a role in weather patterns and altitude sickness.
4. State Archimedes' Principle and its applications.
   Answer: Archimedes' Principle states that 'When an object is partially or wholly immersed in a fluid, a force of buoyancy acts on it in the upward direction. This force is equal to the weight of the fluid displaced by the object.' Applications:
   • Ships and Submarines: Their design ensures they displace enough water to generate buoyant force equal to their weight, allowing them to float.
   • Hydrometers: Used to measure the density of liquids based on how much they sink or float.
   • Hot Air Balloons: Rise because the hot air inside is less dense than the surrounding cooler air, creating an upward buoyant force.
5. Explain Pascal's Principle and its applications.
   Answer: Pascal's Principle states that 'pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel.' This means that pressure applied at one point in a confined fluid is transmitted equally throughout the fluid. Applications (Hydraulic Systems):
   • Hydraulic Jack: A small force applied to a small piston creates pressure, which is transmitted to a larger piston, generating a large force to lift heavy objects.
   • Hydraulic Brake: When the brake pedal is pressed, pressure is transmitted through brake fluid to all wheels, applying equal and strong braking force.
   • Hydraulic Press: Used for compressing materials or shaping metals.

Q5: Solve the following problems.

1. If a force of 100 N is applied on an area of 5 m², calculate the pressure.
   Solution: Given, Force (F) = 100 N Area (A) = 5 m² Pressure (P) = Force / Area P = 100 N / 5 m² P = 20 N/m² P = 20 Pa Answer: The pressure is 20 Pa.
2. A block of wood has a mass of 50 kg and a volume of 0.05 m³. Calculate its density.
   Solution: Given, Mass (M) = 50 kg Volume (V) = 0.05 m³ Density (ρ) = Mass / Volume ρ = 50 kg / 0.05 m³ ρ = 1000 kg/m³ Answer: The density of the block of wood is 1000 kg/m³.
3. If the density of a substance is 800 kg/m³, calculate its relative density. (Density of water = 1000 kg/m³)
   Solution: Given, Density of substance = 800 kg/m³ Density of water = 1000 kg/m³ Relative Density = Density of Substance / Density of Water Relative Density = 800 kg/m³ / 1000 kg/m³ Relative Density = 0.8 Answer: The relative density of the substance is 0.8.