SEE Science (Force) Notes

                                                         

                                                  

Force

1)    In which direction does the force of gravity act?                                                                       
Ans: The direction of force of gravity is always acting towards the center of heavenly body.

2)    Define null point.
Ans: The space between the gravitational fields of two heavenly bodies, at which the resultant gravity is zero, is called null point.

3)    A satellite does not need any energy to revolve around the earth, why?
Ans:  While a satellite is revolving around the earth, the centripetal force due to gravity of the earth is equal to the centrifugal force due the motion of the satellite. So, a satellite does not need any energy to revolve around the earth.

4)    In which condition do feather and coin dropped from the same height fall at the same speed? What is the acceleration of the feather and the coin at that instant? Justify your answer.
Ans:  A feather and a coin dropped from the same height falls at the same speed at the condition of free fall. At that condition of free fall, both coin and feather gain same acceleration of 9.8m/s². Because the acceleration produced in the freely falling bodies, whatever their masses, are same in the absence of air resistance.

5)    Write one effect seen on the ocean or sea due to the gravitation of the moon and the sun.
Ans:  Tides in sea and ocean are due to the gravitation of the moon and the sun.

6)    Where does the weight of the body become maximum in Nepal?
Ans:  The weight of the body become maximum at the Terai region of Nepal because as we know that, the weight of the body depends upon the acceleration due to gravity (g) and the value of 'g' is inversely proportional to square of the radius of the earth (i.e. g  1/R² ). The radius of the earth at the Terai region is less than that of mountain region as the earth is not a perfect square.

7)    Why does a sheet of paper fall later on the earth surface than that the paper ball of equal weight if thrown from a certain height?
Ans:   A sheet of paper fall later on the earth surface than that the paper ball of equal weight if thrown from a certain height because the sheet of the paper will experience a larger air resistance due to its larger surface area but the paper ball has less surface area so that air interferes less on paper ball than that of paper sheet.

8)    Where does the weight of the body become maximum on the surface of the earth?
Ans:  As we know that, the weight of the body depends upon the acceleration due to gravity (g) and the value of 'g' is inversely proportional to square of the radius of the earth (i.e. g  1/R²). The polar radius of the earth is less than the equatorial radius as the earth is not a perfect square. Hence, the weight of the body becomes maximum at the pole on the surface of the earth.

9)    What are the two factors that affect the force of gravitation?
Ans:  The two factors that affect the force of gravitation are:-

                a)    Product of masses of any two bodies and

                b)    Square of the distance between their centers

10)  The weight of the object is more at polar region than equatorial region, why?
Ans:  As we know that, the weight of the body depends upon the acceleration due to gravity (g) and the value of 'g' is inversely proportional to square of the radius of the earth (i.e. g  1/R²). The polar radius of the earth is less than the equatorial radius as the earth is not a perfect square. Hence, the weight of the body is more at the polar region than at the equatorial region.

11)  When a person jumps from the aeroplane with parachute can land safely on the earth surface but it is not possible on the moon, why?
Ans: When a person jumps from the aeroplane with parachute can land safely on the earth surface because on the earth there are layers of different gases. When a parachutists jump out of the aeroplane, parachute opens and expands due to which air exert large resistance on the parachute so that parachute fall down slowly and uniformly. But it is not possible on the moon because there is the absence of atmosphere on the moon. So, upthrust of air is not acted on parachute and falls with acceleration equal to acceleration due to gravity and strict with more force on the surface of moon.

12)  Write two differences between gravity and gravitation.

S.N	Gravitation	S.N	Gravity
a)	It is the mutual force of attraction between any two object in the universe.	a)	It is the force of attraction of the heavenly body to any object towards its center.
b)	It does not measure the weight of body.	b)	It measures the weight of the body.
c)	It is universal force.	c)	It is not universal.

13)  What is free fall and weightlessness?
Ans:  When a body is falling freely towards the center of the earth under the influence of the gravitational pull of the earth without any external resistance, the fall of the body is called free-fall.

Weightlessness of a body is the state in which the body feels that it is not being attracted by any force.

14)  Why satellites not fall while revolving around Earth?                                                           
Ans: Satellites not fall while revolving around sun due to the balance of two factors:                                           
     a) The gravitational pull between the Earth and the satellite i.e. centripetal force.                                                               
        b) Velocity or the speed at which it would travel in a straight line i.e. centrifugal force.

15)  What effect will be on acceleration due to gravity if radius of the earth is decreased?
Ans: The magnitude of the acceleration due to gravity increase if radius of the earth is decreased by keeping its mass constant.

16)  What is gravitational constant? Write its S.I. unit and value?
Ans:  Universal gravitational constant 'G' can be defined as the force of gravitation that is exerted between two unit masses which are separated from one another by a unit distance.

Its S.I. unit is Nm²/kg² and its value is 6.67 10^-11 Nm²/kg².

17)  At what condition the value of acceleration due to gravity is approximately zero, while getting down from parachute?
Ans:  When a parachute falls down, it is pulled downward by a certain force of gravity. Air also provides resistance and opposes the downward movement of parachute. When air resistance is equal to the pulling force of gravity of the earth, the acceleration of the falling parachute is approximately zero, while getting down from parachute.

18)  What is gravity?
Ans:  The force of attraction of the heavenly body to any object towards its Center is called gravity.

19)  We do not get hurt, when we jump with parachute?
Ans:  On the earth there are layers of different gases. When a parachutists jump out of the aeroplane, parachute opens and expands due to which air exert large resistance on the parachute so that parachute fall down slowly and uniformly. Hence, the parachutists do not get hurt, when they jump with parachute out of an aeroplane.

20)  Define acceleration due to gravity (g). Write the S.I. unit of weight of an object?
Ans: The uniform acceleration produced in a freely falling body due to the gravitational pull of the earth towards its Center is called acceleration due to gravity.

The S.I. unit of weight of an object is Newton (N).

21)  State Newton's Universal Law of Gravitation.
Ans: It states that, "everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers."

22)  A piece of stone and a feather are dropped from the same height at lunar surface, which one will reach on the surface fast and why?
Ans:  As we know that there is no atmosphere in the lunar surface. When a piece of stone and a feather are dropped from the same height at lunar surface, they fall simultaneously due to free fall. Because acceleration produces in the freely falling bodies, whatever their masses, are the same in the absence of air resistance.

23)  The probability of getting hurt is more when a man jumps from greatest height, why?
Ans:  As we know that force is directly proportional to the acceleration due to gravity (i.e. F  g). If a person jumps from a certain height, his acceleration goes on increasing towards the earth surface. More the acceleration, with more force he strict on the earth surface. Hence, the probability of getting hurt is more when a man jumps from greatest height.

24)  What is gravitation?
Ans:  The mutual force of attraction between any two bodies in this universe is called gravitation.

25)  What is the difference between fall of parachute on the earth and that of the moon?
Ans: Difference between fall of parachute on the earth and that of the moon are:

S.N.	Fall of parachute on the earth	S.N.	Fall of parachute on the moon
a)	Parachute acquires upthrust of air so fall is with uniform velocity.	a)	Upthrust of air is not acted on parachute and falls with acceleration equal to acceleration due to gravity.
b)	Free-fall is not experienced.	b)	Free-fall and weightlessness is experienced.

26)  What is the effect of gravity on the falling object?
Ans: When object is falling, gravity provides necessary acceleration on it which is always acting towards the center of the heavenly body. 



27)  What is the value of acceleration due to gravity at equatorial region of the earth? Write in S.I. unit?
Ans:  The value of acceleration due to gravity at

           a)    Equatorial region:- 9.78 m/s²

           b)    Polar region:- 9.83 m/s²

28)  The acceleration due to gravity in the moon is 1.67m/s². What does it mean?
Ans: It means that when the object falls towards the surface of the moon under the action of gravity, the velocity of the falling object increases at the constant rate of 1.67m/s in each second during its fall if the air resistance is neglected.

29)  What is gravitational field and gravitational field intensity?
Ans: The space around a mass over which it can exert gravitational force on other masses is called the gravitational field of the mass.

The gravitational field intensity at a point in the gravitational field is defined as the force experienced by unit mass placed at that point.

30)  A feather and a coin fall simultaneously on the surface of moon, why?
Ans:  As we know that on the surface of the moon, there is no air resistance (i.e. no atmosphere). If there is no air resistance, the acceleration produces in the freely falling bodies, whatever their masses, are the same. Therefore, if a feather and a coin are dropped simultaneously on the surface of moon, their speed increases with the same rate. As a result they reach the ground together.

31)  It is difficult to lift a large stone on the surface of the earth but easy to lift small one, why?
Ans: To lift a load, we have to apply the force equal to the weight of the load. The load of an object depends on the force of gravity and its mass. The force of gravity is more in the large stone as it has more mass then in small stone. Due to this, large stone has more load than small stone. As a result, it is difficult to lift a large stone on the surface of the earth but easy to lift small one.

32)  Stone of 1 kg and 5 kg masses are dropped from the same height at lunar surface, which one will reach on the surface fast and why?
Ans:  When two stone of 1 kg and 5 kg masses are dropped from the same height at lunar surface, both will reach on the surface simultaneously. It is because at the lunar surface, there is no air resistance (i.e. no atmosphere). If there is no air resistance, the acceleration produces in the freely falling bodies, whatever their masses, are the same.

33)  Differences between mass and weight.

S.N	Mass	S.N	Weight
a)	Mass is the total amount of matter contained in a body.	a)	Weight of an object is the force with which the object is attracted towards the Center of the heavenly body or the earth.
b)	It is scalar quantity.	b)	It is vector quantity.
c)	It is measured in kilogram (kg).	c)	It is measured in Newton (N).

34)  What is the effect of earth's gravity?
Ans: All the bodies present inside the gravitational field of a planet fall towards the surface of the planet due to the effect of gravity of that planet. Gravity produces an acceleration on the falling bodies.