GRAVITATION – INTEXT QUESTIONS
Questions on page 1
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What are the effects of force acting on an object?
Effects of force on an object –
- Moves a body at rest
- Cause s moving object to change its speed.
- Cause moving object to change its direction
- Change shape and size of an object.
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What types of forces are you familiar with?A . Contact force –
- When a force acts on two objects when they are in direct contact with each other, it is said to contact force.
Ex.
- Muscular force (when we use our muscles on any object)
(Ex. Kicking a ball, pushing an object)
- Frictional force (force exerted by a surface when an object moves across it)
(Ex. A vehicle stops when we apply brakes)
- Elastic force (force exerted by an elastic object)
(Ex. Force by rubber)
- Applied force (when a person or an object exerts a force on another object)
(Ex. Book kept on table)
- Spring force (when a stretched or compressed spring exerts a force)
- B . Non-contact force –
When a force is applied between two objects even if the two objects are not in contact, such force is non-contact force.
Ex.
- Gravitational force (force of attraction exerted by any object on the other object)
(Ex. Apple falling from a tree)
- Electrical force (Force due to electric field)
(Ex. Device running on electricity, when an electric current is passed through iron converting it into an electromagnet, it attracts iron and particles towards it.)
- Magnetic force (Force due to magnets)
(Ex. Iron gets attracted towards magnet)
- Static force / Electrostatic force (the force between two objects when their surfaces are rubbed on each other)
(Ex. The charging of the hair and attraction of paper bits towards it
- What do you know about the gravitational force?
- It is the force of attraction between any two objects in the universe.
- Discovered by Sir Isaac Newton (Explained in book PRINCIPIA)
In text question –
Why all apples fall vertically downward and not at an angle to the vertical. Why do they not fly off in a horizontal direction?
- The earth must be attracting the apple towards itself and this attractive force must be directed towards the centre of the earth.
- The direction from the apple on the tree to the centre of the earth is the vertical direction at the position of the apple and thus, the apple falls vertically downwards.
Can you recall –
What are Newton’s laws of motion?
Newton’s first law of motion – An object continues to remain at rest or in state of uniform motion along straight line unless an external unbalanced force acts on it.
Newton’s second law of motion –
The rate of change of momentum is proportional to the applied force and change in momentum occurs in the direction of the force.
Newton’s third law of motion –
Every action force has an equal and opposite reaction force which act simultaneously.
GRAVITATION- INTEXT QUESTIONS
Questions on page 2
Try this.
Tie a stone to one end of a string. Take the other end in your hand and rotate the string so that the stone moves along a circle as shown in figure 1.2 a.
Are you applying any force on the stone?
Yes, we are applying a force on the stone.
In which direction is this force acting?
The force is acting in the direction towards the centre of the circle along which the stone is rotated.
How will you stop this force from acting?
We can stop the force from acting by releasing the string.
What will be the effect on the stone?
The stone will fly off along a straight line tangential (90 degrees) to the circle at the position of the stone when the stone is released.
GRAVITATION- INTEXT QUESTIONS
Questions on page number 4
If the area ESF in figure 1.4 is equal to area ASB, what will you infer about EF?
According to Kepler’s second law, if area of ESF = area of ASB, then, AB and EF are distances covered by the planet in same time.
Use your brain power –
Show that in SI units, the unit of G is Newton m2 kg-2. The value of G was first experimentally measured by Henry Cavendish. In SI units its value is 6.673 x 10-11 N m2 kg-2
According to Newton’s law of gravitation,
F = Gm1 m2/d2
G = Fd2/m1m2
The SI unit of force is Newton (N), distance is measured in metre (m) and mass in kg
Thus,
G = Newton x (metre) 2 / kg x kg
G = Nm2/kg2
GRAVITATION- INTEXT QUESTIONS
Questions on page number 5
Why did Newton assume inverse square dependence on distance in his law of gravitation?
Newton assumed inverse square dependence on distance in his law of gravitation by considering Kepler’s third law which states that, the square of period of revolution of the planet around the Sun is directly proportional to the cube of the mean distance of a planet from the Sun
Use your brain power –
Is there a gravitational force between two objects kept on a table or between you and your friend sitting next to you? If yes, why don’t the two move towards each other?
Yes, there exists a gravitational force between two objects kept on table or between me and my friend sitting beside each other.
This force of attraction can be given by formula –
F = Gm1m2/ d2
From this formula, it is clear that, any two objects attract each other and force between them is dependent on masses of object and distance between them.
Due to small masses, the objects exert very small force as compared to the gravitational force of the earth. Hence, they don’t get attracted towards each other.
Questions on page number 6
GRAVITATION- INTEXT QUESTIONS
Use your brain power –
Assuming the acceleration in Example 2 above remains constant, how long will Mahendra take to move 1 cm towards Virat?
Given: Acceleration (a) = 5.34 × 10-9 m/ s2
≈ 5 × 10-9 m/ s2
Displacement (s) = 1 cm = 1 100 m
Initial velocity (u) = 0 m/s
To find: time (t) =?
Questions on page 7
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Will the velocity of a stone thrown vertically upwards remain constant or will it change with time?
The velocity of the stone thrown vertically upwards decreases with time and at a certain point, it becomes zero.
-
How will it change? Why doesn’t the stone move up all the time? Why does it fall down after reaching a certain height?
When a stone is thrown vertically up, it is acted upon by the gravitational force of the earth which pulls the stone down. Due to this, the velocity of the stone becomes zero after reaching a certain height. Once the velocity becomes zero, the object starts moving vertically downwards.
- What does its maximum height depend on?
The maximum height of the stone depends on the initial force on which the initial velocity depends with which the stone is thrown vertically upwards.
GRAVITATION- INTEXT QUESTIONS
Use your brain power
According to Newton’s law of gravitation, every object attracts every other object. Thus, if the earth attracts an apple towards itself, the apple also attracts the earth towards itself with the same force. Why then does the apple fall towards the earth, but the earth does not move towards the apple?
- The apple attracts the earth with the same force with which the earth attracts the apple.
- According to Newton’s third law, these forces are equal and opposite in direction.
- The acceleration produced in a body is inversely proportional to its mass.
- The mass of the earth is very large as compared to mass of the apple. Therefore, the acceleration of the earth is very small compared to acceleration of apple and we cannot notice that.
- Hence, the apple falls towards the earth but the earth not.
The moon and the artificial satellites orbit the earth. The earth attracts them towards itself but unlike the falling apple, they do not fall on the earth, why?
- Even though the moon and the artificial satellites are attracted towards the earth, they have a specific velocity. This velocity prevents them from falling on the earth. If this velocity was not there, they would have fallen on the earth.
GRAVITATION- INTEXT QUESTIONS
Questions on page 8
Think about it –
-
What would happen if there were no gravity?
- All the objects will not get attracted to the earth and will float off into space away from the earth.
- The moon also will not orbit around the earth.
- The important constituents like oceans, atmosphere would also float into space.
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What would happen if the value of G was twice as large?
- The value of acceleration due to gravity (g) would become double.
- Weight of all objects would become double and it will become difficult to pick the objects as it would require double force.
- The atmospheric air pressure would become double leading to various climatic changes.
- Due to rise in atmospheric pressure, the blood pressure would also increase and our survival would become difficult.
Can you tell –
What would be the value of g on the surface of the earth if its mass was twice as large and its radius half of what it is now?
Given –
Mass of the earth = M’ = 2M
Radius of the earth = R’ = R /2
Find out –
Gravitational acceleration (g’)
Formula –
g = GM / R2
Solution –
We know that,
g = GM / R2
But here, M = 2M and R = R/2
Therefore,
g’ = G2M
(R/2)2
= G x 2M x 4
R2
= GM x 8
R2
But, GM / R2 means g.
Therefore,
g’ = 8 x g
= 8 x 9.8
= 78. 4 m/s2
Thus, the value of g would be 8 times its original value.
Questions on page 9
GRAVITATION- INTEXT QUESTIONS
Think about it –
-
Will the direction of the gravitational force change as we go inside the earth?
There will be no change in the direction of gravitational force. Gravitational force always acts downwards, towards the centre of the earth.
-
What will be the value of g at the centre of the earth?
The value of g at the centre of the earth is zero.
Questions on page 10
Use your brain power –
Will your weight remain constant as you go above the surface of the earth?
No, our weight will not remain constant if we move away from the surface of the earth. Weight is the product of mass and gravitational acceleration. When we move away from the surface of the earth, mass remains the same but the value of gravitational acceleration decreases. Hence, weight decreases.
GRAVITATION- INTEXT QUESTIONS
Suppose you are standing on a tall ladder. If your distance from the centre of the earth is 2R, what will be your weight?
Questions on page 11
GRAVITATION- INTEXT QUESTIONS
Take a small stone. Hold it in your hand. Which forces are acting on the stone? Now release the stone. What do you observe? What are the forces acting on the stone after you release it?
When a stone is held in our hands, it experiences the gravitational force exerted on it by the earth pulling it downward and the force of the hand pushing it upwards.
When the stone is released, it falls down. When the stone is released, the forces acting on the stone are gravitational force and an upward force opposing the gravitational force.
If the medium in which the stone is released is air, frictional force due to air also acts on the stone opposing its motion.
Questions on page 12
GRAVITATION- INTEXT QUESTIONS
Use your brain power –
According to Newton’s law of gravitation, earth’s gravitational force is higher on an object of larger mass. Why doesn’t that object fall down with higher velocity as compared to an object with lower mass?
The acceleration produced in a body under the influence of the force of gravity alone is called acceleration due to gravity. It is given by,
g= GM / R2
The value of g depends upon the mass and radius of the earth.
It does not depend on the shape, size, and mass etc. of the body which is falling. Hence, the object with greater mass and the object with comparatively less mass, both will fall with the same velocity.
GRAVITATION- INTEXT QUESTIONS
Questions on page 14
Space travellers as well as objects in the spacecraft appear to be floating. Why does this happen?
In space, the only force that acts on a spacecraft is the gravitational force of the earth. Therefore, the spacecraft is in a state of free fall.
The velocity in free fall is independent of the properties of the object. This velocity is the same for all objects in the space craft and travellers.
This leads to weightlessness. Due to which everything appears to be floating.
GRAVITATION- INTEXT QUESTIONS
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