Chapter 1 Physical Environment Long Answer Type Questions
Question 1. The intervening distance between two objects of masses m, and m, is r. The masses are attracting each other with a force F. What is the name of force F?
Answer:
Given:
The intervening distance between two objects of masses m, and m, is r.
The masses are attracting each other with a force F.
State the law in connection with the force. State the effect of the mass of the objects and the distance separating them on F. The force F is known as the Gravitational force of attraction.
It is governed by the universal law of Gravitation. The law states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The mathematical expression of the law is, Here, m, and m, are the mass
of the two objects, separated by a distance r. G is the universal gravitational constant. From this expression, we can conclude that by keeping the distance between the two objects the same,
if the mass of the object(s) is increased, the gravitational force between them is also increased. For example, keeping the distance between them fixed,
if the mass of the two objects is doubled, the gravitational force is increased by 4 times. Similarly, we find that by keeping the mass of the two objects the same,
if the distance between them is increased, the gravitational force between them is decreased. For example, keeping the mass of the two objects the same,
if the distance between them is doubled, the gravitational force becomes or(½)² 1/4th of the former.
WBBSE Class 8 Force Active Without Contact long answer questions
Question 2. Show that the force of gravity on unit mass is equal to the acceleration due to gravity.
Answer:
Given:-
The force of gravity on unit mass is equal to the acceleration due to gravity.
We know that Force = mass x acceleration or, F=mxa and 77 electrons before electrification. 5 number of electrons have been transferred from ‘Q’ to ‘P’ due to friction.
When F is the force of gravity on an object of Mm mass “m”, then, F-G So comparing equations 1. and 2. we can write, m.a = G Mm/R² or, a = G M/R² Or, a=g [since,g=GM/R²]
So, we can show that the force of gravity on unit mass is equal to the acceleration due to gravity, denoted by “g”.
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3. State the laws of free-falling bodies. Two bodies of different masses are allowed to fall freely. If the heavier body touches the ground in 5s, how much time will the lighter body take to touch the ground?
Answer:
Given:
Two bodies of different masses are allowed to fall freely.
If the heavier body touches the ground in 5s, how much time will the lighter body take to touch the ground
Falling Bodies
Due to gravity, ail bodies lying within a certain distance above the earth, come down when they are released.
It is seen, usually, that a lighter body, for example., a piece of paper, descends more slowly than a heavier object for example., a piece of stone.
This is because when a body Is falling on the earth, the air resists its motion. When the heavier stone is falling,
Its weight prevails over the resistance offered by air So it comes down more quickly than the lighter object, which cannot overcome the resistance of air appreciably.
But, we should note that in absence of air, when there is no resistance, all bodies of different masses come down at the same time.
In fact, Galileo, the famous Italian scientist, demonstrated this phenomenon through an experiment in public.
From the top of the leaning tower of Pisa, Galileo simultaneously released two spheres of equal volume but of different masses (one made of wood and the other made of iron).
One was much lighter than the other. The people gathered there to witness this experiment saw that both spheres touched the ground almost at the same time.
Laws of Freely Falling Bodies
If you drop a stone from a cliff, in addition to gravity, air resistance acts on it. Hence not all bodies fall down freely.
When a body is falling on earth due to gravity in absence of air, there is no resistive force, and it is called a free-falling body.
A body is thus said to be falling freely or in a state of free fall when it is under the influence of gravity alone and no other force acts on it.
When bodies are in free fall, the acceleration due to gravity acting on them is the same and is independent of their masses.
Galileo established three laws for freely falling objects. The laws are:
1. In a vacuum, all bodies starting from rest fall with equal rapidity.
2. In a given time, the velocity acquired by a body falling freely from rest is directly proportional to time.
(This means that the speed of a freely falling body increases with the increase in the time of fall).
3. The distance traversed by a body falling freely from rest is directly proportional to the square of the time. (This means that the distance traversed by a freely falling body increases with the increase in time of fall)
Understanding non-contact forces for Class 8
Newton’s Guinea And Feather Experiment Newton’s Guinea And feather Experiment
Newton’s Guinea and feather experiment Newton’s Guinea and feather experiment proves the 1st law as mentioned above.
Newton in his experiment used a hollow glass tube in which a guinea and a feather were introduced.
When the tube was inverted with air in it, the coin was found to come down earlier than the feather.
After that, the air was drawn out from the tube and again the tube was inverted. Both the guinea and the feather this time were seen to fall at the same time.
So, we find that, in absence of any opposing forces, like air resistance, gravity causes all bodies (heavier or lighter) to fall simultaneously if they start to fall simultaneously from rest.
This means the earth always produces the same acceleration on every object.
10. Common Facts Due To The Action Of Gravity
1. It is our common experience that when an object is thrown vertically upward, its speed decreases with increasing altitude. Ultimately at a certain altitude, the speed becomes zero.
Then the direction of motion is altered and it comes back to the earth again. Acceleration due to gravity is directed towards the centre of the earth. So, while moving upwards, speed decreases with increasing altitude.
When the object starts falling down, acceleration occurs due to gravitational pull, and hence, the speed of fall increases with the time of fall.
2. If the same object is thrown upwards (as a projectile), making an acute angle (0) with the ground, under the action of gravity the direction and speed of the projectile will continuously change as
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Moving along a curved path, the projectile ultimately comes back to the ground after traversing some horizontal distance.
If a stone is thrown with greater and greater force, then a situation will arise when the object will not return to the earth due to balancing between its velocity and gravitational pull by the earth.
It will then start revolving around the earth. During its revolution around the earth, along a path of circular trajectory, its velocity will change continuously due to changes in its direction
4. Artificial satellites: Man-made objects which revolve (or orbit) around the earth in outer space are called artificial satellites.
Aryabhatta (the first artificial satellite successfully launched by India), Bhaskara, Rohini, INSAT-1A etc. are some of the satellites launched by India.
Artificial satellites are “thrown” with a very great speed and they revolve around the earth. Moon is a natural satellite of the earth and also revolves around the earth following the same principle.
5. Escape Velocity: When a body is projected upwards, it comes down to the earth after some time due to the earth’s gravitational attraction.
So, one may think, whether it is possible to throw a body with such a velocity, that the body will not return to the earth again.
For an object to escape from the earth and never return, it must be launched with a velocity, which will take the body beyond the gravitational field of the earth. Such velocity is known as Escape Velocity”.
It Is defined as the least velocity with which a body must be thrown vertically upwards In order that it may just escape the gravitational pull of the earth. The value of escape velocity is estimated to be 11.2 km/s or approximately 7 mile/s.
Both the heavier body and the lighter body shall touch the ground with equal rapidity, ie., in 5s.
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Question 4. Will the weight of any particular object be the same anywhere on the earth’s surface? Is the value of G the same on earth and on the moon? Mathematically it can be shown that, g=G.
Answer:
Here, g is the acceleration due R² to gravity, G is the universal gravitational constant, M is the mass of the earth and R is the distance of an object placed on the earth’s surface from the centre of the earth.
From this expression, we find that the magnitude of “g” at any point on the earth’s surface depends on the distance of that point from the centre of the earth.
The shape of the earth is not perfectly spherical. The radius of the earth is minimum at the poles and maximum at the equator.
Hence, even if we place an object at sea level, the distance from the centre of the earth is not always the same, and it will be different at different places.
We know that the weight of a body is the product of the mass of the body and the acceleration due to gravity.
Hence, as the magnitude of “g” changes, the weight of the object will also change. G is a universal constant. Hence the value of G is the same on earth and in the moon.
Question 5. If an object is thrown upwards, making an acute angle with the ground, what will happen? What do you mean by Universal Gravitational Constant?
Answer:
Given:
An object is thrown upwards, making an acute angle with the ground.
If an object is thrown upwards making an acute angle with the ground, under the action of gravity, the direction and speed will continuously change as shown and the object shall come back to the ground after traversing some horizontal distance.
If m, = m, = 1 kg and r = 1m, then G = F. Thus, Universal Gravitational Constant (G) is defined as the force of attraction between two bodies of masses 1 kg each and separated by a distance of 1m.
In-depth analysis of non-contact forces for Class 8
Question 6. “Repulsion is a surer test for electrification”. Justify.
Answer:
“Repulsion is a surer test for electrification”.
When a charged body attracts a second body, then we can say that either, the second body is uncharged since the former induces an opposite charge in the latter, and as a result, the former attracts the latter;
or, the second body is oppositely charged (since we know that two oppositely charged objects attract each other).
So, merely from the phenomenon of attraction, one cannot be sure whether a body is charged or not.
But if repulsion takes place between two bodies then it is sure that they are electrically charged, for repulsion only occurs between two similarly charged bodies.
Hence we find that repulsion is a surer test of the electrification of a body.
Question 7. What is the effect of the magnitude of charge and distance between the two point charges on the electrostatic force of attraction or repulsion?
Does the value of electrostatic force between two charged bodies change when they are dipped in water?
Answer:
Given:
The effect of the magnitude of charge and distance between the two point charges on the electrostatic force of attraction or repulsion.
According to Coulomb’s law, the magnitude of force exists between two charged particles.
Where, F is the electrostatic force of attraction or repulsion between two particles having the amount of charge q, and q2 on them, separated by a distance “r”.
“k” is a constant whose value depends on the nature of the intervening medium between the two charged particles.
From the above mathematical expression we can conclude that by keeping the distance between the two charged particles the same, if the charge of the particle(s) is increased, the electrostatic force between them is also increased.
For example, keeping the distance between them fixed, if the charge of the two particles is doubled, the magnitude of the electrostatic force is increased by 4 times.
Similarly, we find that by keeping the charge of the two particles the same, if the distance between them is increased, the electrostatic force between them is decreased.
For example, keeping the charge of the two particles the same, if the distance between them is doubled, the magnitude of electrostatic force becomes 1/4th of the former.
K is a constant whose value depends upon the intervening medium between the two point charges. Hence the value of electrostatic force between two charged bodies varies as the bodies are immersed in water.
Question 8. Prove with an experiment that “Like charges (or similar charges) repel each other”.
Answer:
Given:
“Like charges (or similar charges) repel each other”
Apparatus needed: Two ebonite rods, some wool, silk thread and two stands.
Procedure: Let us take an ebonite rod and rub it with wool. It is then suspended freely from some support by a silk thread.
It is then brought near to another ebonite rod which has also been rubbed with wool and suspended from a stand by a silk thread.
Observation: It is observed that the suspended ebonite rod is repelled (i.e. moves away).
Inference: When an ebonite rod is rubbed with wool, it acquires a negative charge. So when two negatively charged ebonite rods are brought closer to each other they repel. This proves that like charges repel each other.
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Question 9. Prove with an experiment that “Unlike charges (or opposite charges) attract each other”.
Answer:
Given:
“Unlike charges (or opposite charges) attract each other”.
Apparatus needed: One ebonite rod, some wool, one glass rod, a piece of silk, silk thread and two stands.
Procedure: Let us take an ebonite rod and rub it with wool. It is then suspended freely from a stand by a silk thread.
It is then brought near to a glass rod which has been rubbed with a piece of silk and suspended from a stand by a silk thread.
Observation: It is observed that the suspended ebonite rod and the suspended glass rod come closer to each other (i.e. attract each other)
Inference: When an ebonite rod is rubbed with wool, it acquires a negative charge. When a glass rod is rubbed with silk, it acquires a positive charge.
So when two oppositely charged rods are brought closer to each other they move towards each other. This proves that opposite charges attract each other.
WBBSE Chapter 1 exercises long answers solutions
Question 10. Two objects one with a mass of 1 kg and the other with a mass of 2 kg are allowed to fall freely from a certain height. How does the weight of a body on the surface of the moon vary compared to that at the surface of the earth?
Answer:
Given:
Two objects one with a mass of 1 kg and the other with a mass of 2 kg are allowed to fall freely from a certain height.
1. The gravitational pull of the earth will be more on which object?
2. Whose acceleration will be more?
Answer:
When the two objects of different mass are allowed to fall freely from the same height, the earth will pull the heavier object with more force.
Because, we know from the law of gravitation, that, other things remain constant, the higher the mass of the body, the more will be the force of gravitation by the earth.
But we also know that the earth always produces the same acceleration on every object. So the acceleration of both objects will be the same. Weight of a body on the surface of the moon=x weight of the body on the surface of the earth.
Question 11. The acceleration due to gravity on earth is 9.8 m/s² . If the radius of the earth is 6.37 x 105 m and the value of G is 6.67/1011 N.M² /kg², then calculate the mass of the earth. Mathematically it can be shown that,
Answer:
Given
The acceleration due to gravity on earth is 9.8 m/s² . If the radius of the earth is 6.37 x 105 m and the value of G is 6.67/1011 N.M² /kg²
g=GM/R²
Here, the acceleration due to gravity, g = 9.8 m/s² the universal gravitational constant,
G = 6.67/1011 N.m² /kg² the radius of the earth, R = 6.37 x 10° m and M is the mass of the earth
So, substituting these values in the above equation, we get,
WBBSE Class 8 Science practice questions on force concepts
Question 12. Two point charges, each having a charge 6.5/ 10′ coulomb are separated by a distance of 50 cm in vacuum (k = 9 x 10°). Calculate the force of repulsion between them. When P is rubbed with Q, Q becomes positively charged. Which of the two materials gains electrons and which one loses electrons?
Answer:
Given:
Two point charges, each having a charge 6.5/ 10′ coulomb are separated by a distance of 50 cm in vacuum (k = 9 x 10°).
Here, q1 = q2 = 6.5/10′ coulomb
r = 50 cm = 0.5 m
k=9x 10°
The electrostatic force of repulsion,
Question 13. Compare the force of gravitation with electrostatic force.
Answer:
Comparing the force of gravitation with electrostatic force:
| Force of Gravitation | Electrostatic force | ||
| 1 | The force of gravitation is attractive in nature. | 1 | The electrostatic force between two charged particles may be attractive or repulsive,
depending on whether the charges on two particles are opposite or similar. |
| 2 | The force of gravitation is existent between any two particles in this universe. | 2 | Electrostatic force exists only between two charged particles or between one charged and an uncharged particle. |
| 3 | The magnitude of the force of gravitation between two specific objects separated by a certain distance is independent of the intervening medium. | 3 | The magnitude of the electrostatic force between two particles depends on the intervening medium between them. |
| 4 | The magnitude of the force of gravitation between two objects separated by a certain distance depends on the mass of the two objects. | 4 | The magnitude of the electrostatic force between two particles depends on the charge of each particle. |
Examples of non-contact forces in daily life for Class 8
Question 14. State the law of conservation of charge.
Answer:
law of conservation of charge:-
A charged thermocol ball is suspended by means of a nylon thread. When a neutral body is brought nearer to it, the thermocol ball is first attracted by it and then flies away from it. Explain the phenomenon.
When two bodies are rubbed together and if one body acquires a positive charge by losing some electrons, the second body acquires a negative charge of equal magnitude by gaining the same number of electrons as those lost by the first body.
Answer:
Thus the total electric charge on both the bodies put together, before and after electrification remains the same. Thus the charge is said to be conserved.
This is known as the law of conservation of charge, according to which charge can only be transferred from one body to another and can neither be created nor destroyed.
When the neutral body is brought nearer to the thermocol ball, an opposite charge is induced in the neutral body and so the thermocol ball is attracted to it.
As soon as the thermocol ball touches the body, both develop the same kind of charge due to conduction and then repel each other. Hence, the thermocol ball now moves away from the body.
Question 15. Enumerate the differences between the mass and weight of a body.
Answer:
The differences between the mass and weight of a body:
| Mass | Weight |
| 1. Mass is the quantity of matter contained in the body. | 2. Weight is the gravitational force of the earth acting on the body. |
| 2. Mass is a constant quantity. | 2. Weight changes from place to place. |
| 3. Mass can never be zero. | 3. Weight may be zero. |
| 4. Its SI unit is Kilogram. | 4. Its SI unit is Newton. |