Class Ten Science: Force (questions and Answers)

Force

The blog page has been updated on 14 June 2020. More Questions and Answers have been added.  

Force: Force is a pull or push on a body which changes or tends to change the state of rest or uniform motion in a straight line. It's SI unit is Newton and CGS unit is Dyne.

Gravitation: The mutual force of attraction between any two bodies in the universe.

Newton's Universal law of Gravitation: The force of attraction between any two bodies of the universe is directly proportional to the product of their masses and inversely proportional to the square of the distance between them from their centers.

Gravitational constant: The force of gravitation that exists between two bodies of unit mass placed at unit distance from their center. Its value is 6.67×10^-11Nm²Kg^-2.

Consequences of Gravitational Force:

a)      Revolution of planets around the sun

b)      Revolution of satellites around the planets and sun

c)      Motion of galaxy around the galactic centre

d)      Existence of solar system

e)      Revolution of artificial satellites around the earth.

f)       Formation of tides in seas and oceans.

Gravity: The force of attraction by a heavenly body on any body towards its centre.

Effects of Gravity:

a)      Weight of an object.

b)      Flow of water from upstream to downstream.

c)       We can stand and walk firmly on the surface of earth.

d)      Presence of atmosphere.

e)      A body thrown upward fall to the surface of the earth.

f)       Freefall and weightlessness on the earth and other heavenly bodies.

Acceleration due to gravity: The acceleration produced on a freely falling body due to the influence of gravity. It is denoted by g and its average value on the surface of the earth is 9.8m/s².

Free fall: Falling of a body freely under the influence of gravity alone.

Weightlessness: The condition at which a body of certain mass experience no weight in it.

Mass: The quantity of matter present in a body.

Weight: The force of gravity acting on a body. 

Some Important Formulae:

Force (F) = ma (m= mass, a = acceleration)

F= mg (g = acceleration due to gravity)

Gravitational force (F) = Gm₁m₂/d²

where, m1 and m2 are the masses of two bodies, d = distance between them and G=Gravitational constant.

Weight(W)=F= GMm/R²   or W =GMm/(R+h)²

Here, M=mass of heavenly body, m=mass of object, R=radius of heavenly body, h=height of the body from the surface of the heavenly body.

Acceleration due to gravity (g) =GM/R²

g'=(R/R+h)²×g

where, g' = acceleration due to gravity at certain height.

Questions and Asnwers

Group A [1 mark Each]

1. What do you mean by gravitational constant 'G'? Write its unit and Value.

Gravitational constant is the force of gravitation between two bodies each with a mass of one kilogram at a distance of 1 meter apart. It is also called Astronomical constant or Universal gravitational constant. The SI unit of gravitational constant 'G' is Nm²/kg² and value of 'G' is 6.67x 10^-11 Nm²/ kg².

2. What is acceleration due to gravity?

The acceleration produced in a freely falling body without any resisistance towards the surface of the earth as a result of the gravity is called acceleration due to gravity.

3. What is free fall?

The free fall is defined as the fall of an object with the velocity of the acceleration due to gravity without an external resistance.

4. Define the gravitation.

Gravitational force is the force of attraction produced between any two object by virtue of their masses. Everybody in this universe attracts another body towards itself. The mutual forces of attraction between any two bodies is known as force of gravitation.

5. State Newton's law of gravitation.

Newton's universal law of gravitation states that "The gravitational force produced between any two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers."

6. The value of universal gravitational constant is 6.67×10^-11Nm²/kg². What does it mean?

The value of universal gravitational constant is 6.67×10^-11Nm²/kg², it means that the gravitational force between two bodies of 1kg masses placed at a distance of 1m is 6.67×^10-11N.

7. Write down the unit of gravitational constant.

The unit of gravitational constant is Nm²/kg².

8.  What is weightlessness?

Weightlessness is the condition of a body when it completely loses its weight and become weightless.

9. In which condition will acceleration due to gravity of parachute be zero?

Acceleration due to gravity of parachute becomes zero when the force of gravity and the upthrust due to air is equal to each other.

10.  Write the relationship between mass of a body and weight in terms of formula.

The relationship between mass of a body and weight in terms of formula is: (W)=F= GMm/R²

Where,

W = weight of the body, G= Gravitational constant, M = Mass of the earth, m= mass of  object and R = Radius of the earth.

11. In which direction does the force of gravity act?

 The force of gravity acts towards the centre of the earth.

12. Write one effect seen on ocean or sea due to gravitation of the moon and the sun?

Tides are formed on sea and ocean due to the gravitation of the moon and the sun.

13. Where does the weight of a body maximum on the surface of the earth?

Weight of the body is maximum on the poles of the earth.

14. What are the two factors that affect the forces of gravitation?

Two factors that affect the force of gravitation are: Masses of the two bodies and the distance between them from their centers.

15. What is the value of acceleration due to gravity at equatorial region of the earth? Write its SI unit.

The value of acceleration due to gravity at the equatorial region of the earth is 9.78 m/s². Its SI unit is m/s².

Group B [2 Marks Each]

16. Astronaut feel weightlessness in spacecraft, why?

Inside the spacecraft, the gravitational force of attraction on him is balanced by the centrifugal force of the spacecraft. So the net force acting on him is zero. Hence in this condition astonaut inside the spacecraft feels weightlessness.

17. A large stone of 20 kg and small stone of 5 kg are dropped simultaneously from certain height. Which one reaches on the ground first? Why?

Both the large stone of 20 kg and small stone of 5 kg mass will drop simultaneously on the surface. This is because acceleration due to gravity is independent of the mass of the body and the value of acceleration due to gravity is equal on both the stones.

18. What happens to the weight of an object when it is taken from the earth to the moon, why?

Weight of a body is directly proportional to the acceleration due to gravity. The value of acceleration due to gravity is 6 times lesser on the moon than that on the earth. So the weight of an object decreases by 6 times on the moon than on the earth.

19.  The acceleration due to gravity on the surface of Jupiter is 25m/s². What does it mean?

The acceleration due to gravity on the Jupiter is 25m/s². It means that the velocity of a freely falling body towards the surface of Jupiter increases at the rate of 25m/s in every second.

20. Write the reason behind the atmosphere does not escape away from the earth.

The atmosphere does not escape away from the earth because the gravity of the earth binds the atmosphere and prevent it from being escape into the space.

21. Write any two differences between gravity and acceleration due to gravity.

Two differences between gravity and acceleration due to gravity are:

Gravity	Acceleration due to gravity
1. Gravity is the force with which the earth or havenly body pulls an object towards its center.	Acceleration due to gravity is the acceleration produced in a freely falling body due to the gravity of the earth.
Its SI unit is Newton	Its SI unit is m/s2.

22. Why an iron ball and a feather fall together in a vacuum?

All freely falling bodies fall with the same acceleration of 9.8m/s2. In vacuum there is no resistance, so an iron ball and a feather travel equal distance in equal time and therefore reach the ground together in a vacuum.

23. The value of acceleration due to gravity on moon is 1.67m/s2. What does it mean?

The acceleration due to gravity on moon is 1.67m/s². It means that the velocity of a freely falling body towards the surface of moon increases at the rate of 1.67m/s in every second.

24. Why does the value of g is more in polar region than in the equator?

The value of g at a place is inversely proportional to the square of the radius of the earth at that place. The radius of the earth at the poles is lesser than the radius of equator. Therefore,  the value of g is more at the polar region (9.83m/s²)  than equator (9.78m/s²).

25.  Write two differences between G & g.

Gravitational Constant (G)	Acceleration due to gravity (g)
1. Gravitational constant is the force of gravitation between two bodies each with a mass of 1kg at a distance of 1meter apart.	1. The acceleration developed in an object falling freely without resistance towards the surface of the earth as a result of gravity is called acceleration due to gravity.
2. The value of G remains constant.	2. The value of g changes from place to place.
3. It is a scalar quantity	3. It is a vector quantity

 

26. The weight of a body is more at the polar region than the equatorial region. Why?

The weight of a body at any place is directly proportional to the acceleration due to gravity (g). The value of 'g' is inversely proportional to the square of the radius of the earth. The radius of the earth is less at the polar region than that of the equatorial region. So, the weight of a body is more at the polar region than the equatorial region.

27. Why can't we fall safely with the help of parachute towards the moon's surface?

We can't fall safely on the moon's surface with the help of a parachute because there is no atmosphere, therefore  there is no upthrust due to air which reduces the velocity of falling parachute. Thus, the parachute falls hardly and we get hurt while jumping with parachute on moon's surface.

28. What change occur in the gravitational force between two bodies when mass of each body is double and distance between their centre is halved?

Let us suppose two masses m1 and m2 are kept at a distance of d apart. The gravitational force produced between them is:

Let us suppose that the gravitational force produced between masses m1 and m2 when masses are doubled and the distance between them halved is F₂ given by

F₁=Gm₁m₂/d²

25N=Gm₁m₂/d²

25N=Gm₁m₂/4²

25N =Gm1m2/16

25N×16=Gm₁m₂

400=Gm₁m₂…………………(i)

Again F₂=Gm₁m₂/d²

F₂=Gm₁m₂/2²…………………..(ii)

Putting the value of Gm1m2 from eq. (i) in eq. (ii), we get

F₂=400/4

Or F₂ =100N

 

Therefore, the gravitational force will increase by 16 times when the masses of two bodies are doubled and the distance between them is halved.

29. Under what condition the value of gravitational force and gravitational constant become equal, prove.

OR Prove that F=G

When the masses of each body are 1kg and the distance between them is 1m then the value of gravitational force and gravitational constant become equal.

We know that

F=Gm₁m₂/d²

when m1 = 1kg, m2 = 1kg and d = 1m then the above equation becomes

F=(G×1Nm²kg^-2×1kg×1kg)/1m²

or, F=G   Proved.

 

30. What is the value of acceleration due to gravity at the polar region? Write two differences between free fall and weightlessness.

The value of acceleration due to gravity at the polar region is 9.83m/s². Two differences between freefall and weightlessness are:

Free fall	Weightlessness
1. It is the motion of the body under the influence of gravity alo9ne.	1. It is the condition in which a body experiences that it is not being attracted by any force.
2. It is possible in absence of air.	It is possible in a freely falling body.

31. Why is the weight of an object on the moon 1/6th its on the weight on the earth?

OR

The weight of a body is less on the surface of the moon than the earth, why?

Ans: The mass of the moon is 1/100 times and its radius ¼ times that of earth. As a result, the gravitational attraction on the moon is about only one sixth when compared to the earth. Hence, the weight of an object on the moon is 1/6th its weight on the earth.

32. Write two differences between gravity and gravitational force.

Gravity	Gravitational force
1. It is the force with which a heavenly body such as earth pulls an object towards its centre.	1. It is the force of attraction between two heavenly bodies.
2. It is a vector quantity.	It is a scalar quantity.

33. A stone and a piece of paper are dropped from the same height on the lunar surface, which one will fall faster, why?

Both the stone and the piece of paper will fall simultaneously, when they are dropped from the same height. This is because there is no atmosphere on the moon and thus there is no resistance on the falling stone and paper. Also, the acceleration due to gravity on both of them are equal.

34. Acceleration due to gravity vary from place to place on the earth, why?

The value of g is inversely proportional to the square of the earth's radius. i.e. . The earth is not a perfect sphere. That is, different parts on the earth's surface are at different distance from the center of the earth. This means the value of R (radius of earth) varies from place to place on the earth's surface. Therefore, the value of g differs at various places of the earth.

35. When a feather and a coin are dropped towards the surface of the earth, do they reach the ground together? If this experiment is done in moon, what difference occurs? Write with reason.

When a feather and a coin are dropped towards the surface of the earth, coin reaches the ground earlier than feather. This is because the volume of coin is lesser than feather and its density is more than the feather. The resistance of the air on coin is lesser than that of feather. Due to the air resistance, the feather drops slowly whereas the coin falls faster.

If this experiment is done on moon both the coin and feather falls together. This is because there is no any air resistance in moon and the acceleration due to gravity is equal on both the coin and feather.

36. The effect of gravitation is more in solid than in liquid, why?
32. Why will a sheet of paper fall slower than one that is crumpled into a ball?

Answer:  When a sheet of paper is crumbled into a ball, then its density increases. Hence, resistance to its motion through the air decreases and it falls faster than the sheet of paper.

37. Write the main conclusion of Galileo's 'Feather and coin experiment'?

The main conclusion of Galileo's 'Feather and coin experiment' is that in the absence of air resistance (in vacuum) all the objects whether massive or minute falls together with the same acceleration due to gravity. Fall of an object towards the surface of a heavenly body is independent of the mass of the object.

38. What happens to the weight of an object when it is taken from the earth to the moon? Why?

The weight of an object is reduced to 1/6 of its weight on the moon. This is because the weight of a body is directly proportional to the acceleration due to gravity and the value of acceleration due to gravity is 1/6 times lesser than that of the earth.

39. Why do all planets and satellites revolve around the sun?

All planets and satellites revolve around the sun because of the gravitational forces existing between them.

40. Why do satellites not fall while revolving around the earth?

During the revolution of the satellite around the earth, the gravitational force of the earth and the satellite produces necessary centripetal force while the inertia of the satellite provides centrifugal force. These two forces balances each other. Hence, the satellites do not fall while revolving around the earth.

41. Why does a sheet of paper falls later on the surface of the earth than the paper ball of equal weight if throw from a certain height?

The sheet of a paper falls later on the surface of the earth than the paper ball of equal weight if thrown from a certain height. This is because a sheet of paper has larger surface area and hence upthrust due to air on it is very high as compared to the paper ball of equal weight. Due to this reason the sheet of paper falls later than the paper ball of equal weight.

42. A satellite does not need any energy to revolve around the earth, why?

A satellite does not need any energy to revolve around it because the energy required for the revolution around the earth is provided by the balanced centripetal force by gravity and centrifugal force due to inertia of motion of satellite.

43. What will be the acceleration due to gravity if the radius of the earth is decreased? Why?

The acceleration due to gravity of the earth will increase if its radius is decreased. This is because the value of acceleration due to gravity is inversely proportional to the square of the radius of the earth. i.e. . Thus, if the radius is decreased the value of acceleration due to gravity increases.

44.  Explain on the basis of Newton's law of gravitation that the weight of an object farther from the centre of the earth is lesser.

According to the law of gravitation, from the formula,   weight of an object is inversely proportional to the square of the distance from the centre of the earth. It means as the distance increases there is decrease in the weight of an object. So weight of an object farther from the centre of the earth is lesser.

45. We do not get hurt when we jump with a parachute, why?

When we jump with a parachute on the earth, the upthrust of air act in upward direction and the weight of parachute in downward direction. Upthrust due to air, thus reduces the velocity of the parachute and hence it moves slowly under the action of gravity and hence we do not get hurt.

46. The probability of getting hurt is more when a man jumps from a significant height, why?

Acceleration due to gravity produces on every falling body towards the earth's surface. When a body falls, there is acceleration due to gravity. Due to acceleration, the velocity of body increases and the force also increases. In this way, the probability of getting hurt is more when jumps from significant height.

47. What is the difference between the fall of a parachute on the earth and that on the moon?

On the earth parachute at fist falls with the acceleration the same as acceleration due to gravity i.e. free fall. But after a certain time the upthrust due to air and downward force by parachute balance each other. Due to the balanced forces it attains a constant velocity and falls slowly. But there is no atmosphere on the moon's surface, therefore the parachute falls down freely due to the action of gravity alone.

48. Give two differences between mass and weight.

Two differences between mass and weight are:

Mass	Weight
1. It is the quantity of matter contained in a body.	1. It is the force of gravity applied by the earth to pull any object towards its center.
2. It is constant.	2. It is variable.
3. Its SI unit is Kg	3. Its SI unit is Newton

49. What are the two factors that affect the gravitational force?

The two factors that affect the gravitational force are: (i) Masses of the two bodies and (ii) distance between them from their centers.

50. At what condition, does a coin and a feather fall together? What is the acceleration of the feather and coin at that instant? Justify your answer.

A coin and feather fall together in the vacuum or in the absence of air. The acceleration of feather and the coin at that instant is equal to the acceleration due to gravity. This is because the acceleration due to gravity is independent of the mass of the object.

51. It is difficult to lift a large stone on the earth but easier to lift the smaller one. Why?

The weight of a body is directly proportional to the mass of a body. A large stone has more mass than the smaller one. Hence, its weight is more than the smaller one. So it is difficult to lift a large stone on the earth than the smaller one.

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