# WBBSE Class 8 Science Solutions Chapter 1.2 Force Active Without Contract

Detailed explanations in West Bengal Board Class 8 Science Book Solutions Chapter 1.2 Force Active Without Contract offer valuable context and analysis.

## WBBSE Class 8 Science Chapter 1.2 Question Answer – Force Active Without Contract

Multiple Choice Questions Answers : (1 mark for each question)

Question 1.
What is the name of attractive force which act between two bodies in the universe-
i) nuclear force
ii) gravitational force
iii) magnetic force
iv) coulomb attractive force
ii) gravitational force

Question 2.
The value of gravitational acceleration-
i) increases as height increase from the earth
ii) decreases as height increase from the earth
iii) remains constant
iv) none of the above
i) increases as height increase from the earth

Question 3.
In which region of earth the weight of a body is slightly greater?
i) at equator
ii) tropic of cancer/tropic of capricorn
iii) at polar region
iv) none of the above
iii) at polar region

Question 4.
Gravitation is a which type of force?
i) repulsive force
ii) attractive or repulsive force
iii) attractive force
iv) not a force
iii) attractive force

Question 5.
The force involved in falling of an apple from a tree is known as-
i) magnetic force
ii) electrostatic force
iii) contact force
iv) gravitational force
iv) gravitational force

Question 6.
Which of the following is an example of a non-contact force-
i) the force exerted by us to lift a bucket
ii) hit a cricket ball for runs
iii) the force exerted by magnet
iv) push a stationary car
ii) hit a cricket ball for runs

Question 7.
In Newton’s equation F = _____.
i) universal gravitational constant
ii) distance between the two particles
iii) gravitational force
iv) masses of two point object
iii) gravitational force

Question 8.
Mass of a proton is-
i) 9.11 1028 gm
ii) 1.675 1024 gm
iii) 1.6725 1024 gm
iii) 1.6725 1024 gm

Question 9.
An atom is usually electrically neutral because-
i) proton and neuton have same number of charges
ii) neutron and electron have same number of charges
iii) proton and electron have same number of charges
iv) none of the above
iii) proton and electron have same number of charges

Question 10.
Metals are good conductor of electricity because-
i) metals are hard
ii) metals have large number of free electrons
iii) metals have many protons.
iv) none of the above
ii) metals have large number of free electrons

Question 11.
Two types of charges were named by-
i) Franklin
ii) Coulomb
iv) Archimedes
i) Franklin

Question 12.
If electron increases in a body then it would be-
i) negatively charged
ii) positively charged
iii) charge free
iv) none of the above
i) negatively charged

Question 13.
It is safe to do electrical work standing on a woolen stool because it is
i) good conductor
ii) preventive
iv) none of the above
ii) preventive

Question 14.
Which particle revolves around the centre of atom-
i) proton
ii) neutron
iii) electron
iv) none of the above
iii) electron

Question 15.
A semi conductor material is-
i) silicon
ii) glass
iii) copper
iv) none of the above
i) silicon

Question 16.
Which one is good conductor –
i) silicon
ii) graphite
iii) cbonite
iv) none of the above
ii) graphite

State whetherTrue or False : (1 mark for each question)

1. G is called ‘Universal gravitational constant’.
2. In a vacuum place a gold coin will fall faster than a feather.
3. Similar charges attracts each other.
4. CGS unit of force is Dyne.
5. The force with which sun attracts an object near it is called gravitational force.
6. The force with which earth attracts an object near it is called gravitational force.
7. If dry hair is combed static electricity is produced.
8. The negative charged particle of atom is proton.
9. The positive charged particle of atom, is proton.
10. CGS unit of force is Newton.
11. The negative and positive charged particles of an atom has same numerical value.
12. When two materials are rubbed against each other one becomes positively and the other becomes negatively charged.
13. G is called acceleration due to gravity.
14. Hydrogen has no neutron.
15. When an atom loses onc or more electron(s) it becomes positively charged.
16. An atom consists of four types of particles.
1. true
2. false
3. false
4. true
5. false
6. true
7. true
8. false
9. true
10. false
11. true
12. true
13. false
14. true
15. truc
16. false

Fill in the blanks : (1 mark for each question)

1. Galileo established three laws for _______ objects.
2. Of all metals _____ is the best conductor of electricity.
3. Static electricity can be produced by _____.
4. Neutron is not present in the nucleus of ____ atom.
5. Minimum velocity to overcome gravitational pull of the earth is –
6. Same kind of electrical charges ______.
7. In the core of atom lies _____ and _____.
8. Earth pulls every object towards itself through _______ force.
9. Average radius of earth ________.
10. Most metals conduct electricity because they have enough _____ available in them.
11. Force = ______ × ______.
12. _____ is called Universal gravitational constant.
13. The force of gravity on unit mass is numerically equal to the ______ due to gravity.
14. When an object is thrown upward, its speed ______ with aititude.
15. French scientist ______ invented a formula to calculate the force exerted between the two charged particles.
16. If glass is rubbed with silk, glass becomes ____ charged and silk becomes ______ charged.
17. The centre of the atom is called ______.
18. Electrons carry ______ charges.
1. free-falling
2. silver
3. rubbing
4. Hydrogen.
5. 11.2 km / sec
6. repel
7. Proton, Neutron
8. gravitational
9. 6370 km
10. free electrons
11. Mass, acceleration
12. G
13. acceleration
14. decreases
15. Charles Augustin de Coulomb
16. positively, negatively
17. Nucleus
18. ‘negative

Match the following : ( 1 mark for each question)

a) Match the column A with column B.

 Column A Column B a) Gràitation is a kind of i) is different in different places. b) SI unit of weight is ii) an applied force c) In the centre of earth iii) it falls same speed. d) Va’ne of acceleration due to gravity iv)  11.2 km / sec e) Velocity of an object changes due to v) is a kind of universal constant. f) If an object falls without any hindrance vi) force g) Minimum velocity to overcome gravitational pull vii) weight of object becomes zero. h) Universal gravitational constant viii) Newton.

a) Gravitation is a kind of force
b) SI unit of weight is Newton.
c) In the centre of earth weight of object becomes zero.
d) Value of acceleration due to gravity is different in different places.
e) Velocity of an object changes due to an applied force.
f) If an object falls without any hindrance it falls at same speed.
g) Minimum velocity to overcome gravitational pull 11 \cdot 2 km / sec.
h) Universal gravitational constant is a kind of universal constant.

b) Match the column A with column B.

 Column A Column B a) Eleetron carries i) attract each other b) Like chăges ii) weight of objects c) Unlike charges iii) attract each other d) Aristotle was a famous philosopher iv) gravitational force e) Any two objects of this universe v) with increasing time of fall f) Spring balance measures vi) repel each other g) The earth pulls all objects with vii) negative charges h) Speed of a free-falling body increases viii) of Greece

a) Electron carries negative charges.
b) Like charges repel each other.
c) Unlike charges attract each other.
d) Aristotle was a famous philosopher of Greece.
e) Any two objects of this universe attract each other.
f) Spring balance measures weight of objects.
g) The earth pulls all objects with gravitational force.
h) Speed of a free-falling body increases with increasing time of fall.

Very Short Questions Answers : (1 mark for each question)

Question 1.
Due to which force all objects fall towards earth?
Due to gravitational force all objects fall towards earth.

Question 2.
With what force one object attracts another object?
With universal gravitation one object attracts another object.

Question 3.
What are the CGS and SI unit of measuring force?
The CGS unit is dyne and SI unit is newton of measuring force.

Question 4.
What like charges do?
Like charges repel each other.

Question 5.
What unlike charges do?
Unlike charges attract each other.

Question 6.
What is produced by rubbing two materials?
Charge is produced by rubbing two materials.

Question 7.
By what unit electric charge is measured?
Electric charge is measured by coulomb.

Question 8.
What is the CGS unit of acceleration?
Of acceleration CGS unit is cm / sec2.

Question 9.
What is the SI unit of acceleration?
Sl unit of acceleration is m / sec2.

Question 10.
Who named the two charges positive and negative?
Renowned scientist Benjamin Franklin gave name to two different charges.

Question 11.
If a glass rod is rubbed with silk what charges occur in which object?
If a glass rod is rubbed with silk positive charge occurs in glass rod and negative charge occur in silk.

Question 12.
If an atom gains electron what charge it acquires?
If atom gains electron is acquires negative charge.

Question 13.
If an atom releases electron of what charge it becomes?
If an atom releases electron it becomes positive charge.

Question 14.
Name a neutral particle in atom?
Neutron is a neutral particle in atom.

Question 15.
What type of charge is in proton?
There is positive charge in proton.

Question 16.
What type of charge is in electron?
There is negative charge in electron.

Question 17.
A fruit falling from tree is an example of which type of force?
Gravetation force.

Question 18.
Water begins to flow towards the ground as soon as we open a tap. This happens due to which force?
Force of gravity.

Question 19.
Who gave us preliminary idea about the nature of electrical charge?
Benjamin Franklin.

Question 20.
What do you mean by free-falling object?
When an object is falling freely towards earth due to gravitation, it is called a freefalling object.

Question 21.
What is the value of universal gravitational constant (G) in C.G.S. unit?
6.67 * 108

Question 22.
What is the value of universal gravitational constant (G) in S.I. unit?
N-m / kg2.

Short Questions Answers : (2 marks for each question)

Question 1.
What is gravitation?
Any two particles in the universe pull each other along a straight line joining them, this force is known as gravitation.

Question 2.
What is gravity?
The force with which earth pulls everything towards its centre is called gravity. The earth’s gravitation is called gravity.

Question 3.
Why G is called ‘Universal gravitational constant’?
The value of G remains same at each place and it is independent of nature of particles, temperature, medium etc. Therefore, it is universal constant and so G is knewn as universal gravitational constant.

Question 4.
What is weight of a body? How it is measured?
The weight of a body is the force with which the earth attracts it. It is measured with spring balance.

Question 5.
What is escape velocity?
The minimum velocity with which an object when thrown upwords from the earth’s surface overcomes gravitational pull of the earth and escapes the earth’s gravity, it is called escape velocity.

Question 6.
On what factors the attraction between two objects depend? Explain how they depend?
The atraction between two objects depend on two factors-i) Mass of two objects : If mass of two objects increase then attraction between them also increase. ii) Distance between two objects: If distance between two objects increase attraction between them decrease.

Question 7.
What is contact force?
Forces which act only when there is physical contact between two interacting objects are known as contact forces.

Question 8.
What are non contact force?
Forces which can act without physical contact between objects are known as non contact forces.

Question 9.
Define electrostatic force.
The force extracted by a charged body on another charged or uncharged body is known as electrostatic force.

Question 10.
Why atom is usually electrically neutral?
Normally, an atom contains the same number of positively charged protons and negatively charged electrons. So an atom is usually electrically neutral.

Question 11.
How many types of electricity is there? Define.
Electricity are of two types: i) Static electricity : When electric charge remains stationary, does not flow it is called static electricity. ii) Current electricity : When electric charges flow from one place to another it is called current electricity.

Question 12.
What Aristotle thought about free falling object?
Aristotle thought that when allowed to fall freely from a certain height, a heavier object touches the ground before a lighter object.

Question 13.
What Galileo proved?
Galileo proved that all material bodies irrespective of their weight, touch the ground at the same time when they are allowed to fall freely and simultaneously from the same height.

Long Questions Answers : (3 marks for each question)

Question 1.
What are the laws of electrostatic attraction and repulsion?
a) Two negatively charged objects repel each other.
b) Two positively charged objects also repel each other.
c) A positively charged object attracts a negatively charged object.

Question 2.
What is triboelectric series?
Some materials are arranged in a series which exchange large amounts of charges when they are rubbed against each other. The one higher up in the order will lose electrons readily and acquire a positive charge and the one lower down the order will gain electrons and get negatively charged. This series is called triboelectric series.

Question 3.
A truck transporting petrol has a metal chain dangling to the ground. Explain the reason.
The tyres of the truck moving on the ground, due to friction acquire the negative charge (due to the movement of electrons from the ground to the tyres). The metal body of the truck near the tyres becomes positively charged by electrostatic induction. The negative charge on the tyre and the positive charge on the metal body may cause sparkling which can ignite the petrol and cause a fire. To avoid it, the truck has a metal chain on its body dangling to the ground so that the positive charge on its metal body gets neutralised by the electrons coming from the ground through the metal chain.

Question 4.
What is free falling object? State the three laws for free falling objects of Galileo.
When an object is falling freely on earth due to gravitation, it is called free falling object.

1. When objects at rest are allowed to fall freely from a certain height they fall down with same speed.
2. Speed of free falling body increases with the increase of time of fall.
3. Distance increases with the time of fall.

Question 5.
Write down mass and charge of proton, neutron and electron.

 Particles mass charge proton 1.6725 × 1024 + 4.8 × 10-10 e.s.u or, neutron 1.6750 × 1024 + 1.6 × 10-19 coulomb neutral electron 9.1 × 1028 – 4.8 × 10-10 e.s.u – 1.6 × 10-19 coulomb

Question 6.
We have seen that bits of paper got attracted to a plastic comb wihich had got charged by rubbing it against dry hair, explain why it happen?
Whenever two bodies are rubbed against each other some of the free electrons move from one body to another. The body which loses electrons gets a positive charge whereas the other body which gains electons (lost by the first body) acquires a negative charge. The static electricity produced in the comb by rubbing with hair, attracts bits of paper.

Question 7.
With an experiment proof that like charges repel.
Rub a glass rod with silk. Suspend it freely with a thread. Rub another glass rod with silk. and bring it close to the suspended glass rod. It will be observed that the suspended glass rod gets repelled and moves away. As same charge is produced in the two glass rod they move apart. This experement proves that like charges repel.

Question 8.
Write Coulomb’s law and explain it.
The force between two charged particles, regarded as point charges Q1 and Q2, a distance apart, is proportional to the product of the charges and inversely proportional to the square of the distance between them.
The formula is, F = k
Where Q1, Q2 represent the amount of charge of the two particles, ‘r’ is the distance between them and ‘ F ‘ is electrostatic force of attraction or repulsion. acting between them. The value of k depends upon the intervening medium between the charged particles.

Question 9.
From electrical point of view explain conductors and non-conductors.
From the electrical point of view, most substances can be classified as conductors are and non-conductors of electricity. Conductors are substances that easily allow electric current to flow through them. Most metals conduct electricity because they have enough free electrons available in their crystal lattice. Non-conductors or insulators are substances that do not allow electric current to flow through them. Some examples of insulators or non-conductors of electricity are glass, ebonite, plastic, silk and wool. They do not conduct electricity because they do not have free electrons.

Question 10.
If charge of two particles are increased two times but the distance between them remains same than state what difference will occur in the attraction of two particles.

∴ In second case the force of attraction would be 4 times greater then the first case.

Question 11.
Define charging by friction. Give example.
Whenever two bodies are rubbed against each other, some of the free electrons move from one body to another. The body which loses electrons gets a positive charge whereas the other body which gains electrons acquires a negative charge.
Example : if a glass rod and a silk cloth are rubbed against each other, the glass rod loses electrons to the silk cloth. The glass rod, therefore gets positively charged and the silk cloth gets negatively charged.

Question 12.
Define charging by conduction. Give example.
When an uncharged conductor is touched with a charged conductor. then the uncharged coductor gets charged and the process is called charging by conduction
Example : If a chargeless thermocol ball is touched with a positively charged glass rod, the ball becomes positively charged by conduction.

Question 13.
Define charging by induction. Give example.
During charging by induction, no physical contact is made between the uncharged and the charged bodies. A charged body is brought very close to (but not touched with it) an uncharged body, and then the nearer end of the uncharged body acquires a charge which is of opposite nature to that of the charged body. The charges so produced on the uncharged conductor are called induced charges.

Question 14.
Describe the atomic structure.
a) An atom consists of three kinds of particles that is, electrons (negatively charged particles), protons (positively charged particles) and neutrons (neutral particles). These are called sub-atomic particles.
b) Normally, an atom contains the same number of positively charged protons and negatively charged electrons. So an atom is ustally electrically neutral:
c) Protons and neutrons together form the core of the atom called the nucleus, which is presant at the centre. Electrons revolve around the nucleus in definite paths called orbits or shells.
d) The electrons in the outer orbits which are away from the nuclers are called free electrons because they can break loose and move about freely inside a suibstance. These free electrons act as charge carriers.
e) If a body gains electrons, it gets negatively charged, and if a body loses electrons, it gets positively charged. In both the cases, the bodies act as charged bodies.

Mathematical Problems : (3 marks for each question)

Question 1.
Mass of an object is 10 g. Find out its weight?
Weight = mass × acceleration due to gravity
= 10 × 980 dyne = 9800 dync

Question 2.
Find out the value of force with which 5 kg mass object is pulled by earth?