ADDITIONAL QUESTIONS AND ANSWERS:
NUMERICALS :
1. Calculate the force of attraction between two bodies, each of mass 100 kg and separated by a distance of 1 metre.
(Take G = 6.673 x 10−¹¹ N m² kg².)
Ans: F = Gm1 x m2/ d2
= 6.673 x 10−11 x 100 x 100/1²
= 6.673 x 10−11 x 104
= 6.673 X 10−7 N
2. A stone dropped from a bridge strikes the water after 5 seconds. Calculate the height of the bridge above the surface of water.
Given u = 0 m/s, t = 5 s, g = 9.8 m/s², find s
Ans: s = u t + ½ g, t2 = 0 x 5 + ½ x 9.8 x 5²
= ½ x 9.8 x 25 = 122.5 m
3. A force of weight 10 kg mass acts on an object of mass 49 kg. Calculate the acceleration produced (g = 9.8 m/s²)
Ans: The force is equal to the weight of the 10 kg mass = 10 x 9.8 = 98 N
4. The mass of the object is 49 kg, We have to find ‘a’
Ans: a = F/m = 98/ 49 = 2 m s −2
5. A cricket ball dropped from the top of a tower strikes the ground
after 4 s. (g = 9.8 m /s²)
Calculate
A) Height of the tower above the ground.
B) Velocity with which the cricket ball strikes the ground.
Ans:
A) Height of the tower above the ground; given u = 0 m/s, t = 4s, g = 9.8 m/s²;
s = ut+ ½ at 2
= 0 x 4 + ½ x 9.8 x 42
= 78.4 m
B) Velocity with which the cricket ball strikes the ground.
v = u + at = 0 + 9.8 x 4 = 39.2 m s
6. A bullet fired from a gun vertically upwards reaches a height of 160 m. (Take g = 9.8 m/s²)
Calculate the velocity with which the bullet was fired:
Given s = 160 m, v = 0 m/s.
Ans: The velocity with which the bullet was fired:
acceleration of the object= −9.8 m s for upward motion.
(acceleration is −ve when it opposes the motion)
v² = u² + 2 as
0 = u² + 2 x (−9.8) x 160
u² = −2 x (−9.8 ) x 160 = 3136
u = √ 3136 = 56 m/s
The time taken by the bullet to reach that height.
V= u + at ⇒ 0 = 56 + (-9.8) x t
⇒ 9.8 t = 56 ⇒ t = 5.71 s
VERY SHORT ANSWERS:
1. Name the two forces which act on a body immersed in a fluid.
Ans: The up thrust and the weight of the body.
2. The instrument used to determine the density of liquid.
Ans: Hydrometer.
3. The acceleration of a body in a free fall.
Ans: Acceleration due to gravity.
4. The factor on which the effect of the thrust depends.
Ans: Magnitude of area on which it acts.
5. The force that causes tides on the earth.
Ans: Gravitational force of the moon and the sun.
6. The property of the fluid to exert an up thrust on an object immerse in it.
Ans: Buoyancy.
7. The centripetal force acting on a stone tied to a string and whirl round.
Ans: Tension in the string.
8. Any two instruments based on the Archimedes’ Principle.
Ans: Lactometer, Hydrometer.
9. Pressure.
Ans: Pressure is the force acting per unit area in a perpendicular direction.
10. Relative density.
Ans: Relative density is the ratio of density of a substance to the density
11. Gravitational force.
Ans: Gravitational force is the force of attraction between objects.
12. Centripetal force.
Ans: Centripetal force is the force that keeps the body moving towards the centre when the body is in circular motion.
13. Weight.
Ans: Weight of a body is the force with which the body is attracted towards the earth.
14. Thrust.
Ans: Thrust is the force acting perpendicular to the surface.
15. Define pressure.
Ans: The force exerted per unit area is called pressure.
16. Why is it easier to swim in sea water than in river water ?
Ans: The density of sea water is more due to dissolved salts in it as compared to the density of river water. Hence the buoyant force exerted on the swimmer by the sea water is more which helps in floating and makes swimming easier.
17. Why a truck or a motorbike has much wider tyres ?
Ans: The pressure exerted by it can be distributed to more area, and avoid the wear and tear of tyres.
18. Why are knives sharp ?
Ans: To increase the pressure, area is reduced, as pressure ∝ 1/Area.Hence the pressure or force exerted on a body increases.
19. Why is the wall of dam reservoir thicker at the bottom ?
Ans: The pressure of water in dams at the bottom is more, to withstand this pressure the dams have wider walls.
20. Why do nails have pointed tips ?
Ans: The force exerted when acts on a smaller area, it exerts larger pressure. So the nails have pointed tips.
21. While swimming why do we feel light?
Ans: The swimmer is exerted by an upward force by water, this phenomenon is called buoyancy and it makes the swimmer feel light.
GIVE ONE TERM FOR THE FOLLOWING :
1. Thrust per unit area.
Ans: Pressure.
2. The force that keeps the body, in a circular motion, moving away from the centre.
Ans: Centrifugal force.
3. A body falling due to the gravitational force of the earth.
Ans: Free fall.
4. Force acting perpendicular to the surface.
Ans: Thrust.
5. Mass per unit volume.
Ans: Density.
6. The unit of force.
Ans: Kg ms2 or Newton.
7. The formula used to find the acceleration due to gravity (g).
(a) Fd2 (ii) Fd2 X R2
Ans: G = M x m/ R2
= Mm.
8. The SI unit of:
(a) Weight (b) Pressure.
Ans:
a) Newton (N) or kg m.
b) Pascal (Pa) or N m.
ANSWER THE FOLLOWING :
1. The factors on which the gravitational force depends.
Ans:
(i) Gravitational force is directly proportional to the product of the masses.
(ii) Gravitational force is inversely proportional to the square of the distance between them.
2. Archimedes’ principle.
Ans: When a body is immersed fully or partly in a fluid, it experiences a upward force that is equal to the weight of the fluid displaced by it.
3. Universal law of gravitation.
Ans: Universal law of gravitation states that every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square the distance between them.
4. Importance of the Universal Law of Gravitation.
Ans: Universal law of gravitation:
(a) Explains and determines the force that binds all terrestrial body to the earth.
(b) Explains why the moon revolves round the earth.
(c) Explains the motion of the planets round the sun.
(d) Explains how tides are caused by the moon and the s
5. When a stone is tied to a string and whirled round in a uniform circular motion, there is no change in its speed? But still it is accelerated. Why ?
Ans: When a stone is tied to a string and whirled round, the direction of its velocity changes at every point of its motion. The velocity changes when the direction changes. The change in velocity involves acceleration.
6. When is an object said to be in a free fall?
Ans: An object is said to be in a free fall when it falls to the earth only due to the gravitational force of the earth.
7. Why is the force of attraction of the earth not the same at all places on the earth ?
Ans: The earth is slightly flattened at the poles. The equatorial radius of the earth is more than the polar radius. Since the force of attraction is inversely proportional to the square of the distance, the force of attraction is more at the poles than at the equator.
8. How does thrust differ from pressure ?
Ans: Thrust is the total force acting perpendicular to the surface while pressure is the thrust on a unit area.
9. An object is immersed in water. On what factors does the up thrust on the body depend.
Ans: The up thrust on the body depends upon
(i) the density of the liquid i.e. water and
(ii) the volume of the body.
10. On what factors does the pressure exerted by a solid depend.
Ans: The pressure exerted by a solid is directly proportional to the thrust and inversely proportional to the area on which the thrust acts.
11. How is pressure transmitted in a liquid ?
Ans: Pressure exerted in any confined mass of a liquid is transmitted undiminished in all directions.
12. Why are the tyres of a lorry broader than those of a car.
Ans: A loaded lorry has much more weight than a car. When the tyres are broad, the weight acts on a larger area and the pressure exerted on the road is decreased. This prevents sinking of the tyres.
13. Camels can travel easily through a desert. Give a reason.
Ans: The feet of camels are broad i.e. their area is more. The weight of the camel acts on its feet. Because the feet are broad, the pressure on the sand is reduced. This prevents sinking of its feet in the sand. Therefore it can travel easily in a desert.
14. Why does a sewing needle have a pointed end ?
Ans: A sewing needle has a pointed end so that the force applied on the needle acts on a smaller area i.e. the pointed end, the pressure increases and the needle can easily pierce through the material.
15. The foundation of a wall is broader than the wall. Give a reason.
Ans: The wall exerts force i.e. the weight of the wall on the foundation. When the foundation is broader the force is exerted on a larger area, as a result the pressure decreases and the wall is prevented from sinking.
16. Army tanks have a broad steel belt passing over the wheels. Give reason
Ans: Army tanks are heavy. When the heavy weight of the tank is exerted on the broad belts, the pressure under the belts decreases and the wheels do not sink.
17. Why are school bags provided with broad belts ?
Ans: The weight of a school bag acts on the shoulder of the student. When the belt is broad, the pressure acting on the shoulder, due to the weight of the bag, decreases.
18. When a person carries a pot full of water on his head, he places a cotton pad in between the pot and his head? Why?
Ans: The surface in contact between the head and the pot is small. Therefore a greater pressure is exerted on the head. When a pad is used the area, on which the weight of the pot acts, is increased. Thus the pressure exerted on the head is decreased.
19. Why does a body lose weight when immersed in a liquid ? When does a body float or sink in a liquid ?
Ans: When a body is immersed in a liquid it experiences an up thrust which is equal to the weight of the liquid displaced by the body. Therefore a body loses weight when immersed in a liquid. If the density of a body is greater than the density of the liquid, the body sinks in the liquid. If the density of the body is less than the density of the liquid, the body floats on it.
20. An iron nail sinks in water but floats on mercury. Why ?
Ans: The density of iron is more than that of water but less than that of mercury. Therefore an iron nail sinks in water but floats on mercury.
21. Balloons used to fly high in air are filled with helium. Give a reason.
Ans: The helium filled balloon experiences an up thrust in air which is equal to the weight of the air displaced by the balloon. The density of helium is less than that of air. Hence for a given volume, air will weigh more than helium. Hence the weight of the displaced air or the up thrust is more than the weight of the balloon. Therefore the balloon rises in the air.
22. Why does ice float on water ?
Ans: Ice floats on water because the density of ice is less than that of water.
23. A small nail of iron sinks in water while a huge ship made of iron floats on it. Why
Ans: The up thrust on a body is equal to the weight of the liquid displaced by the body. In the case of a nail the up thrust is less than the weight of the nail and so it sinks in water. The ship is hollow and so it displaces more water. The weight of the water displaced by the immersed part of the ship i.e. the up thrust is more than the weight of the ship. So the ship floats on water.
24. Why does relative density have no unit ?
Ans: Relative density has no unit because it is a ratio, i.e. ratio of the density of a substance to the density of water.
25. What is the importance of the universal law of gravitation ?
Ans: It explains and determines the force that binds all terrestrial bodies to the earth. It explains why the moon revolves round the earth. It explains the motion of the planets round the sun. It explains how the tides are caused by the moon and the sun.
26. What would happen to the moon if the earth’s force of attraction disappears all of a sudden ?
Ans: The moon would travel in a straight line tangential to the circular orbit from the point or instant the force of attraction disappears.
27. Why is the law of gravitation called a universal law ?
Ans: The law of gravitation is called a universal law because it is applicable to all bodies irrespective of whether they are terrestrial or celestial.
28. The weight of an object increases when it is taken from the equator to the poles. Why ?
Ans: The radius of the earth decreases from the equator to the poles. Therefore the force of attraction and hence the acceleration due to gravity increases from the equator to the poles. The weight of a body is directly proportional to the acceleration due to gravity. Since acceleration increases, the weight also increases.
29. At a given place the weight of an object is used as a measure of its mass. Why ?
Ans: Weight is the product of mass (m) and acceleration due to gravity (g). At a given place the value of ‘g’ is constant and therefore the weight is directly proportional to the mass. Hence weight of an object is used as a measure of its mass.
30. What is the weight of a 10 kg mass ?
Ans: Weight = mass x acceleration due to gravity m x g = 10 x 9.8 = 98 N
31. Distinguish Between
(a) Mass and weight.
Ans:
|
Mass
|
Weight
|
|
Mass has only magnitude.
|
Weight has magnitude as well direction.
|
| Mass of an object is constant |
Weight of an object changes from place to place.
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(b) Density and relative density.
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Density
|
Relative density
|
| Density is the mass of a unit volume of a substance |
Relative density is the ratio of density of a substance to the density of water.
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| The unit of density is kg ms-3 |
Relative density has no unit.
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