**Q ** 1. What is meant by free fall? Two bodies, one of mass 1g and other of mass 1 kg are dropped from the same height in vacuum. Compare the two time intervals in which the two bodies will hit the ground.

**Solution**

Whenever objects fall towards the earth under the influence of gravitational force alone, it is called free fall. Both bodies will hit the ground at the same time as acceleration due to gravity is independent of mass of the falling object.

**Q ** 2. What is the force between the Earth and a body called?

**Solution**

The force between the Earth and the body is known as the force of gravity or gravitational force. It is also called the weight of the body.

**Q ** 3. State Archimedes’ principle. Give two applications based on the principle.

**Solution**

Archimedes’ principle: When an object is wholly (or partially) immersed in a liquid, it experiences a buoyant force (or upthrust) which is equal to the weight of liquid displaced by the object. Buoyant force on an object = weight of liquid displaced by that object Applications of Archimedes’ principle:

1. In determining the relative density of a substance

2. In determining the density of liquids by a hydrometer

**Q **4. Why do buildings have wide foundations?

**Solution**

Buildings have wide foundations to reduce the pressure on the ground, as pressure is inversely proportional to area. If they have narrow foundations,then large pressure will be exerted on the ground by them making the ground susceptible to collapse.

**Q **5. (a) Define weight.

(b) How does the gravitational force between the two bodies change if the distance between them is tripled?

**Solution**

(a) Weight is the gravitational force with which the earth pulls an object towards it. (b)

**Q **6. The dimensions of a wooden block are 2 m × 0.25 m × 0.10 m. If relative density of wood is 0.6, calculate mass of the block in kg. Density of water = 10^{3} kg/m^{3}

**Solution**

V = 2 m × 0.25 m × 0.10 m = 0.05 m^{3} R.D = 0.6

**Q **7. Do all bodies immersed in a given fluid experience the same buoyant force? Explain.

**Solution**

No. According to Archimedes’ principle, bodies with different weight will displace different weight of the fluid. Hence, the buoyant force on different objects will be different.

**Q **8. Give the units for Force, Acceleration due to gravity, Gravitational Constant and Pressure.

**Solution**

Units of Force, Acceleration due to gravity, Gravitational Constant and Pressure are: Force: newton(N) Acceleration due to gravity: m/s^{2} Gravitation Constant: Nm^{2}kg^{-2} Pressure: pascal (Pa)

**Q **9. (a) Name the two factors on which the buoyant force depends.

(b) State the relationship between

the buoyant force on an object and weight of the liquid displaced by it?

**Solution**

(a) The factors on which buoyant force depends are:

(i) Volume of the object immersed.

(ii) density of the fluid

(b) Buoyant force on an object = weight of the liquid displaced by the object

**Q **10. State the source of centripetal force that a planet requires to revolve around the Sun. On what factors does that force depend? Suppose this force suddenly becomes zero, then in which direction will the planet begin to move if no other celestial body affects it?

**Solution**

The source of centripetal force is the gravitational force between the planet and Sun. Gravitational force depends on the mass of the planet, mass of the Sun and distance between them. The planet will begin to move in a straight line tangential to the point of circle in which it was moving at the instant force became zero.

**Q **11. What is S.I Units? State the units of Force and G.

**Solution**

International System of Units, that is, SI unit stands for Système International d’unités and is the modern form of the metric system. Unit of Force, F is newton (N). Unit of universal constant of gravitation, G is N m^{2} kg^{-2}.

**Q **12. What is meant by relative density of a substance? What is its significance? The relative density of mercury is 13.6. What is its density in SI unit? (Density of water=1000 kg/m^{3})

**Solution**

Relative density = Density of the substance/density of water It gives an idea as to how many times a substance is heavier than water 13.6 = Density of mercury/10^{3} Density of mercury = 13.6 x 10^{3} kg/m^{3}

**Q **13. On what factors does the gravitational force between the two bodies depend?

**Solution**

Gravitational force between two bodies depends on:

1. Masses of the two bodies. The greater an object’s mass, the greater its gravitational force.

2. Distance of separation between them. The smaller the distance greater is the gravitational force.

**Q **14. Give two applications of pressure.

**Solution**

1. The edge of knife is made sharp so that with little force it cuts better. This is because, on sharpening the knife the area reduces, and hence the pressure increases.

2. Foundation of dams and building are made on large surface area so as to decrease the effect of force by increasing area on which it acts. Thus, pressure is reduced.

**Q **15. The density of turpentine at 293 K is given as 870 kg/m^{3}. Identify and write the names of substances that sink in turpentine at the same temperature. S. No. Substances Density kg/m^{3} 1 Wood 690 2 Ice 920 3 Rubber 970 4 Paraffin Wax 900 5 Cork 240 6 Bone 1850

**Solution**

Ice, rubber, paraffin wax and bone will sink in turpentine as their densities are larger than turpentine.

**Q **16. Explain the following:

(a) Swimmers are provided with an inflated rubber jacket/tube. Why?

(b) It is easier to swim in sea water than in river water. Why?

**Solution**

(a) Swimmers are provided with an inflated rubber jacket or rubber tube. The jacket tube has low weight and large volume. Hence, it displaces large volume of water. As a result, up thrust due to water increases and the person remains afloat, i.e., there is no chance of drowning of the swimmer in such case.

(b) It is easier to swim in sea water because density of sea water is more due to added salts. Hence, up thrust acting on the swimmer in sea water is more than in fresh water. So, it is easier to swim in sea water.

**Q **17. Ram throws a stone in the pond. It displaced 1.5 kg of water. Calculate the buoyant force acting on the stone. (g = 9.8 m/s^{2})

**Solution**

Mass of water displaced by the stone, m = 1.5 kg We know that, Weight = Mass x Acceleration due to gravity = m x g Weight of the water displaced = 1.5 x 9.8 N = 14.7 N In accordance with Archimedes’ Principle, When an object is wholly or partially immersed in a liquid, it experiences a buoyant force or upthrust which is equal to the weight of liquid displaced by the object. The buoyant force acting on the stone is 14.7 N.

**Q **18. When can the weight of an object be zero?

**Solution**

Weight can be zero when acceleration due to gravity is zero. The value of ‘g’ is maximum on the surface of the Earth and decreases on going inside the surface or above the surface of the Earth. Thus, the weight of a body will be zero at the centre of the Earth.

**Q **19. 1 kg sheet of tin sinks in water but if the same sheet is converted into a box or boat, it floats. Give reason.

**Solution**

The sheet of tin sinks in water because the density of tin is higher than that of water. When the same sheet of tin is converted into a box or a boat, then due to the trapping of lot of ‘light’ air in the box or boat, the average density of the box or boat made of tin sheet becomes lower than that of water and hence it floats in water.

**Q **20. Define thrust and pressure and state the SI units in which they are measured.

**Solution**

THRUST: Force acting perpendicularly on a surface is called thrust. Its SI unit is newton (N). PRESSURE: The thrust acting on unit area of the surface is called pressure. Its SI unit is pascal (Pa).

**Q **21. Why do objects feel lighter when in water than outside water?

**Solution**

When we dip an object in water, there is an upward force known as buoyant force acting on the object produced by the surrounding water in which it is fully or partially immersed, due to the pressure difference of water between the top and bottom of the object. It is this buoyant force that makes the object feel lighter when placed in water than outside water.

**Q **22. (a) Distinguish between mass and weight of an object. (b) How will the weight of a body of mass 100g change if it is taken from equator to the poles? Give reason for your answer.

**Solution**

(a) Mass Weight

1. The quantity of matter contained in a body is called mass of the body.

1. The force with which the earth attracts a body towards its centre is called the weight.

2. Mass is a scalar quantity.

2. Weight is a vector quantity.

(b) W = mg As ‘g’ is greater at the poles than at the equator, the weight of the body increases from equator to poles.

**Q **23. State the relationship between the buoyant force on an object and the weight of the liquid displaced by it.

**Solution**

Buoyant force on an object is equal to the weight of the liquid displaced by the object. This is given by Archimedes’ principle.

**Q** 24. Define thrust and pressure

**Solution**

Thrust: Force acting on a body perpendicular to its surface. Pressure: Thrust per unit area is called pressure.

**Q **25. Which force governs the motion of the Moon around the Earth?

**Solution**

The gravitational force between the Earth and moon governs the motion of the Moon around the Earth.

**Q **26. Why is the pressure on ground more when a man is walking than when he is standing?

**Solution**

Pressure is inversely proportional to area. P = F/A While walking, one foot is in contact with the ground at a time. Surface area of one foot is less than two feet. Therefore, pressure is more than the pressure exerted while standing, as surface area of both feet is more.

**Q **27. A body of weight 20 N floats half submerged in a liquid. What is the buoyant force on the body?

**Solution**

A floating body displaces an amount of fluid equal to its own weight. So, F_{B}=20N

**Q **28. What would be the apparent weight of an iron block of size 5cm x 5cm x 5cm when it is completely immersed in water? (density of iron = 7.8 g/cc)

**Solution**

Volume of the iron block = 125 cc = 1.25 x 10^{-4} m^{3} Mass of the iron block = volume x density = 125 cc x 7.8 g/cc = 975g = 0.975 kg Weight of the iron block = 0.975 kg x 10 m/s^{2} = 9.75 N Upthrust = Weight of water displaced = volume of water displaced x density of water x g = volume of iron bock x density of water x g = (1.25 x 10^{-4} m^{3}) x (1000 kg/m^{3}) x (10 m/s^{2}) = 1.25 N Thus, apparent weight of iron block = 9.75 – 1.25 = 8.5 N

**Q **29. Explain the term ‘gravity’.

**Solution**

The term gravity is used to denote the gravitational force acting on a body due to earth (or moon, sun or any planet). If a body of mass m is at a distance ‘r’ from the earth’s centre, the gravitational force acting on the body by the earth has magnitude of G(mM/r^{2}), where M is the mass of the earth and G is the universal constant of gravitation. It produces acceleration in the body, called gravitational acceleration (g),It is given by the formula:

**Q **30. State any two phenomena related to the Universal Law of Gravitation.

**Solution**

(i) It is the force that binds us to the earth.

(ii) It explains the motion of the moon around the earth.

(iii) It explains motion of planets around the sun.

(iv) It explains the tides due to the moon and the sun. (Any two)