Download FBISE Federal Board 9th class physics chapter 5 Gravitation Exercise conceptual questions pdf.
Unit 5: Gravitation
Exercise Conceptual Questions:
Question 1: Newton’s law of gravitation Suggests that gravitational force acts between any two material objects. Then why do two chairs in your room not attract each other due to this force?
Answer: Because the gravitational force between the two chairs is extremely small as compared to their weights, so they are not attracted towards each other.
Explanation: Since “the gravitational force between any two objects is F= G m1m2/r^2.
In order to be able to notice the gravitational force of attraction between any two objects, at least one of the objects on the earth should have an extremely large mass.Since no object on the Earth has an extremely large mass, we can not notice Such forces. The two objects in the room do not move towards each other due to their small masses, which leads to a very small gravitational force.
Example: Say two chairs of mass 10 kg each are Separated by 1m. The gravitational force between them is F= Gm1m2/d^2 = 6.673×10^-11 x 10 x 10 = 6-673×10^-9 = 0.000000006673N. This force is very small, we can say that the force is not enough. So two chairs do not attract each other.
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Question 2: Why is “G” called a universal gravitational Constant?
Answer: G is called a universal constant because its value is the same for every pair of bodies and everywhere of the universe.
Explanation: The value of “G” does not depend on the nature and size of bodies. It also does not depend on the nature of the medium between two bodies. Its value throughout the universe is constant and is 6.673 x10^-11 Nm^2 /kg^2. That is why G is the same for any two bodies and is called a universal gravitational constant.
Question 3: why is it difficult to measure the value of G?
Answer: It is difficult to measure the value of G.
Reason: i. The value of G is extremely small i.e 6.673 X 10^-11 Nm^2 /Kg^2.
ii. Since the gravitational force between masses on the earth is very small, therefore it is very difficult to measure the value of G.
G = F x d^2 / m1m2
So without knowing the value of F, we cannot find the value of G.
Question 4: Where will your weight be greater, at Murree hills or at karachi? where will your mass be greater?
Answer: i. My weight at Karachi will be greater than my weight at murree hills.
ii. My mass will remain the same at all the places..
Explanation: As we know that → W=mg then W ∝ g
Weight of the body depends upon the value of gravitational acceleration g which changes with altitude,As the distance from the earth increases, the value of the g decreased. Therefore my weight at karachi will be greater as it is at sea level and at murree it will be greater as it is at high altitude. My mass will remain Same, it does not change with change of place. It remain Same at all the places.
Question:5 If a man travels from the north pole to the equator, what will be the effect on his weight during this trip?
Answer: Weight of the man will be greater at the North Pole and will gradually decrease when he travels to the equator.
Explanation: As we know w = mg w ∝ g
Since the poles are near to the Center of the Earth as compared to the equator, therefore, at the poles the value of g is greater than at the equator.That’s why the weight of the man will be greater at the North Pole and will gradually decrease as he moves towards the equator.
Question 6: Earth and an apple on a tree attract each other by the force of gravity. The apple falls on Earth but why does the Earth not move towards the apple?
Answer: Earth and an apple on a tree attract each other by the force of gravity. The apple falls on Earth but Earth does not move towards the apple because the mass of Earth is very large as Compared to apples.
Explanation: From Newton’s 2nd Laws of motion F=ma F/m = a or a = F/m then a ∝ 1/m
As acceleration is inversely proportional to mass i.e ~ greater the mass of the body, smaller will be acceleration produced in it and vice versa. As Earth’s mass is extremely large as compared to apples, it has negligible acceleration towards the apple, So, the apple moves towards the earth and not the earth towards the apple.
Question 7: The mass of the moon is 100 times less than the mass of earth and the radius of the moon is 4 times less than the radius of earth. Show that Value of g is 6 times less than the value of g on Earth.
Answer: Solution of this question is available in the PDF given below and lecture is available on Fazal Academy youtube channel.
8. If mass and radius of earth becomes double then what will be the effect on the value of g on the Surface of the earth?
Answer: If mass and radius of earth becomes double then the value of g reduces to half.
Mathematical proof is available in PDF given below and lecture is available on Fazal Academy youtube channel.
Question 9: Artificial Satellites do not have engines like Cars then how can they keep on moving around the earth?
Answer: Artificial Satellites do not have engines like Cars but they keep on moving around the earth.
Explanation: We know that Centripetal force Keeps a body to move in a Circle
Fc = mv^2/r
This Centripetal force is provided to satellites by the gravitational force of Earth.
Fg = Fc
In Simple words Satellite rotates around the earth by using Earth’s gravitational force as centripetal force.
Also as there is no air in space, it does not have to work against air resistance. Hence it doesn’t lose any energy while rotating.
Question 10: The orbital speed of a satellite orbiting very close to earth (at negligible height) is Vo. What will be the orbital speed of another satellite revolving at a height, equal to the radius of earth ( in terms of orbital speed of a close orbiting satellite ).
The orbital speed of the Satellite will be 1.414 times less than the speed at the earth’s Surface.
Mathematical proof is available in PDF given below and lecture is available on Fazal Academy youtube channel.
Physics Class 9 Chapter 4 Conceptual Questions Federal Board
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