## Chapter 1 Physical Environment Short Answer Type Questions

**Question 1. State and explain Newton’s law of universal gravitation.**

**Answer:**

**Newton’s law of universal gravitation**

The force of attraction between any two point objects having masses “m,” and “m² ” separated by a distance “r” acts along the line joining the particles and has the magnitude, F=Gm

G is called the Universal gravitational constant. From the mathematical expression of the law, we find that if the masses of the object are increased, the gravitational force between them is increased.

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.

**Question 2. Why “G” is called the “universal” constant?**

**Answer:**

**“G” is called the “universal” constant:-**

The constant “G” is called “universal” because it does not depend on the mass of the bodies or the medium in which they are placed.

Also, it is not affected by heat, light, magnetic or electric fields etc. It is independent of the presence of other bodies.

**Question 3. What do you mean by gravitation and gravity?**

**Answer:**

**Gravitation And Gravity:-**

Gravitation is the measure of the force of attraction between any two objects in this ball, its density increases (due to a decrease in volume) and hence, resistance to its motion through air decreases.

Thus it falls faster than a sheet of paper. universe. But gravity is the measure of gravitational force with which the earth pulls an object near it towards its centre.

**Question 4. What do you mean by the weight of an object? What is the relation between the weight and mass of an object?**

**Answer:**

**Weight Of An Object:-**

The weight of a body (w) is the force with which the earth attracts it towards its centre.

Mathematically, we can write, w = m.g where, w is the weight of a body, m is the mass of the body and g is the acceleration due to gravity

**Question 5. Can the weight of an object having non-zero mass be zero?**

**Answer:**

At R= 0, that is at the centre of the earth, “g” is equal to zero. Since we know that, the weight of an object (w) = mass of the object (m) x acceleration due to gravity (g), hence, at the centre of the earth, where “g” is zero, “w” will also be zero.

**Read And Learn More WBBSE Solutions For Class 8 School Science Short Answer Type Questions**

**Question 6. Name two contactless forces. Which one is the intrinsic property of a body mass or weight?**

**Answer:**

**Contactless Forces:-**

Gravitational force and electrostatic force are two examples of contactless forces.

The mass of a body is constant, i.e. it remains the same at all places. The mass of a body can never

be zero. Since the value of g changes from place to place, therefore, the weight of a body is not constant.

The weight of a body can even be zero, for example at the centre of the earth. Hence mass is the intrinsic property of a body.

**Question 7. What do you mean by escape velocity?**

**Answer:**

**Escape Velocity:-**

Escape velocity 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 and will not return to earth. (The value of escape velocity is estimated to be 11.2 km/s or approximately 7 mile/s).

**Question 8. What do you mean by 1 coulomb of charge?**

**Answer:**

**1 Coulomb Of Charge:-**

When two point charges containing the same magnitude of the charge is separated by a distance of 1 metre in a vacuum, then if the magnitude of the force exerted by each of the point charges on the other is 9 x 10° Newton, then the magnitude of the charge on each of the point charge is called 1 coulomb.

**Question 9. What is the unit of charge in the CGS and SI units? Define them. What is the relation between them?**

**Answer:**

**Unit Of Charge In the CGS and SI units:-**

The CGS unit of charge is e.s.u. The SI unit of charge is the coulomb.

If the electrostatic force of repulsion between two like point-charges separated by a distance of 1 cm in a vacuum is 1 dyne, then the charge on each point-charge is 1 e.s.u. or 1 statcoulomb.

If the electrostatic force of repulsion between two like point-charges separated by a distance of 1 m in a vacuum is 9 x 10° Newton, then the charge on each point-charge is 1 coulomb. 1 coulomb = 3 × 10° e.s.u.

**Question 10. Why an atom is uncharged although it contains charged sub-atomic particles?**

**Answer:**

** An atom is uncharged although it contains charged sub-atomic particles:-**

We know that all things are made up of atoms. An atom consists of three types of sub-atomic particles electron, proton and neutron. (The only exception is the hydrogen atom which does not have a neutron).

Electron has a unit negative charge in it, a proton has a unit positive charge and a neutron is uncharged.

In an atom, sum total of the positive charge of all the protons is equal to the sum total of the negative charge of all the electrons. So an atom is neutral which means the net charge in an atom is zero.

**Question 11. When an inflated balloon is rubbed against a sweater, then it automatically sticks to the sweater. Why?**

**Answer:**

**When an inflated balloon is rubbed against a sweater, then it automatically sticks to the sweater**

When an inflated balloon is rubbed against a sweater, then static electricity is produced on the surface of both of them.

They are oppositely charged. Hence, the sweater and the balloon attract each other and as a result, the balloon sticks automatically to the sweater without any support.

**Question 12. What do you mean by induction?**

**Answer:**

**Induction:-**

The process by which an uncharged body gets two electrically opposite ends when held near a charged body is called **INDUCTION** and the uncharged body is then said to be induced.

**Question 13. Keeping the mass of the earth constant, if the radius of the earth is reduced by 4%, then how does the value of acceleration due to gravity at the surface of the earth change?**

**Answer:**

**Given:**

Keeping the mass of the earth constant, if the radius of the earth is reduced by 4%.

∴ If the radius of the earth is reduced by 4%, then the value of acceleration due to gravity at the surface of the earth is increased by 8.5%.

**Question 14. A glass rod is rubbed with wool. Again an ebonite rod is rubbed with silk. What will happen now if the piece of silk is brought near to the wool?**

**Answer:**

**Given:-**

A glass rod is rubbed with wool. Again an ebonite rod is rubbed with silk.

When glass is rubbed with wool, wool acquires a positive charge. When ebonite is rubbed with silk, silk also acquires a positive charge.

If now the piece of silk is brought near to the wool, then they will repel each other since like charges repel each other.

**Question 15. ‘P’ contains 50 electrons and 45 protons and ‘Q’ contains 72 electrons and 77 protons after electrification by friction between them.**

**Find the number of electrons and protons in ‘P’ and ‘Q’ before electrification. How many electrons have been transferred between them?**

**Answer:**

**Given:-**

**‘P’ contains 50 electrons and 45 protons and ‘Q’ contains 72 electrons and 77 protons after electrification by friction between them.**

A neutral body contains an equal number of electrons and protons. Hence, ‘P’ should contain 45 protons and 45 electrons before electrification. ‘Q’ should contain 77 protons