50278 + Interview Questions in Physics

1.	A mass falls from a height ‘h’ and its time of fall ‘t’ is recorded in terms of time period T of a simple pendulum. On the surface of earth it is found that t = 2T. The entire set up is taken on the surface of another planet whose mass is half of that of earth and radius the same. Same experiment is repeated and corresponding times noted as t' and T'. Then we can say
Answer» A mass falls from a height ‘h’ and its time of fall ‘t’ is recorded in terms of time period T of a simple pendulum. On the surface of earth it is found that t = 2T. The entire set up is taken on the surface of another planet whose mass is half of that of earth and radius the same. Same experiment is repeated and corresponding times noted as t' and T'. Then we can say

Discussion

2.	Explain function of pseudopenis
Answer» Explain function of pseudopenis

Discussion

3.	A body is moving along a rough horizontal surface with an initial velocity 6 m/s. If the body comes to rest after travelling 9 m, then the coefficient of sliding firction will be
Answer» A body is moving along a rough horizontal surface with an initial velocity $6 m / s$ . If the body comes to rest after travelling $9 m$ , then the coefficient of sliding firction will be

Discussion

4.	The load versus elongation graph for four wires made of the same material and having the same length is shown in the figure. The thickest wire is represented by the line
Answer» The load versus elongation graph for four wires made of the same material and having the same length is shown in the figure. The thickest wire is represented by the line

Discussion

5.	28 0.2F capacitor is charge to 600V by a battery,on removing the battery,it is connected with another parallel plate condenser of 1F.The potential decreases to
Answer» 28 0.2F capacitor is charge to 600V by a battery,on removing the battery,it is connected with another parallel plate condenser of 1F.The potential decreases to

Discussion

6.	If f(x)=Ax2+Bx satisfies the conditions f′(1)=8 and 1∫0f(x)dx=83, then which of the following is/are correct ?
Answer» If $f (x) = A x^{2} + B x$ satisfies the conditions $f^{'} (1) = 8$ and $1 \int 0 f (x) d x = \frac{8}{3},$ then which of the following is/are correct ?

Discussion

7.	23.The speed of an electron having a wavelength of 10 to the power minus 10 m is.
Answer» 23.The speed of an electron having a wavelength of 10 to the power minus 10 m is.

Discussion

8.	A spring of spring constant K is hanging from the ceiling of an elevator and an object of mass m is attached to the lower end. Whole system is in equilibrium at rest. The elevator starts moving upward with constant acceleration g. An observer in elevator frame observes that i) Object is performing SHM of amplitude mgK ii) Object comes to rest after displacement of mgK downward from initial position iii) Maximum displacement of object from initial position is 3mgK (downward) iv) Maximum displacement of object from initial position is mgK (upward)
Answer» A spring of spring constant K is hanging from the ceiling of an elevator and an object of mass m is attached to the lower end. Whole system is in equilibrium at rest. The elevator starts moving upward with constant acceleration g. An observer in elevator frame observes that i) Object is performing SHM of amplitude $\frac{m g}{K}$ ii) Object comes to rest after displacement of $\frac{m g}{K}$ downward from initial position iii) Maximum displacement of object from initial position is $\frac{3 m g}{K}$ (downward) iv) Maximum displacement of object from initial position is $\frac{m g}{K}$ (upward)

Discussion

9.	The highest magnetic moment is observed for 1)Cu+ 2)Zn2+ 3)Cr3+ 4) Ti3+
Answer» The highest magnetic moment is observed for 1)Cu+ 2)Zn2+ 3)Cr3+ 4) Ti3+

Discussion

10.	is it correct to take the clockwise work done in the graph postive or negative?What does NTA follow
Answer» is it correct to take the clockwise work done in the graph postive or negative?What does NTA follow

Discussion

11.	A particle of mass m attached to a string of length l fixed at a point such that it can perform circular motion in a vertical plane. The particle is imparted a velocity of √8gl at the lowest position by a sharp hit. Find the ratio of tension in string at the lowermost position to the tension in the string at the topmost position of the vertical circular path. Take g=10 m/s2.
Answer» A particle of mass $m$ attached to a string of length $l$ fixed at a point such that it can perform circular motion in a vertical plane. The particle is imparted a velocity of $\sqrt{8 g l}$ at the lowest position by a sharp hit. Find the ratio of tension in string at the lowermost position to the tension in the string at the topmost position of the vertical circular path. Take $g = 10 {m/s}^{2}$ .

Discussion

12.	A capacitor is made of a flat plate of area A and the second plate having a stair- like structure as shown in figure. The width of each stair is a and the height is b. The equivalent capacitance of the combination is
Answer» A capacitor is made of a flat plate of area $A$ and the second plate having a stair- like structure as shown in figure. The width of each stair is $a$ and the height is $b$ . The equivalent capacitance of the combination is

Discussion

13.	12. How beta rays and cathode rays can be distinguished? Explain how ?
Answer» 12. How beta rays and cathode rays can be distinguished? Explain how ?

Discussion

14.	Two Gentlemen of Verona is written in the first person. A story written in the first person is a first-hand account of events told or narrated through the eyes of a single character, typically the main character. Stories written in the first person are easily identified by the use of the pronoun 'I' rather than 'he or she'. The reader will see phrases such as "I said, I thought," rather than "he said, she thought." Everything is experienced through the eyes of a single character, and all thoughts and observations are limited to that one person. There can be no outside observer. If the narrator does not see or experience an event first-hand, it cannot be a part of the story. All scenes in the story are filtered through this person's unique perception. The third-person is a narrative mode in which both the reader and author observe the situation either through the senses and thoughts of more than one character, or through an overarching godlike perspective that sees and knows everything that happens and everything the characters are thinking. In this mode of narration, the narrator can tell the reader things that the main character does not know, or things that none of the characters know.
Answer» Two Gentlemen of Verona is written in the first person. A story written in the first person is a first-hand account of events told or narrated through the eyes of a single character, typically the main character. Stories written in the first person are easily identified by the use of the pronoun 'I' rather than 'he or she'. The reader will see phrases such as "I said, I thought," rather than "he said, she thought." Everything is experienced through the eyes of a single character, and all thoughts and observations are limited to that one person. There can be no outside observer. If the narrator does not see or experience an event first-hand, it cannot be a part of the story. All scenes in the story are filtered through this person's unique perception. The third-person is a narrative mode in which both the reader and author observe the situation either through the senses and thoughts of more than one character, or through an overarching godlike perspective that sees and knows everything that happens and everything the characters are thinking. In this mode of narration, the narrator can tell the reader things that the main character does not know, or things that none of the characters know.

14.

Two Gentlemen of Verona is written in the first person. A story written in the first person is a first-hand account of events told or narrated through the eyes of a single character, typically the main character. Stories written in the first person are easily identified by the use of the pronoun 'I' rather than 'he or she'. The reader will see phrases such as "I said, I thought," rather than "he said, she thought." Everything is experienced through the eyes of a single character, and all thoughts and observations are limited to that one person. There can be no outside observer. If the narrator does not see or experience an event first-hand, it cannot be a part of the story. All scenes in the story are filtered through this person's unique perception. The third-person is a narrative mode in which both the reader and author observe the situation either through the senses and thoughts of more than one character, or through an overarching godlike perspective that sees and knows everything that happens and everything the characters are thinking. In this mode of narration, the narrator can tell the reader things that the main character does not know, or things that none of the characters know.

Answer»

Two Gentlemen of Verona
is written in the first person. A story written in the first person
is a first-hand account of events told or narrated through the eyes
of a single character, typically the main character. Stories written
in the first person are easily identified by the use of the pronoun
'I' rather than 'he or she'.

The reader will see
phrases such as "I said, I thought," rather than
"he said, she thought." Everything is experienced through
the eyes of a single character, and all thoughts and observations are
limited to that one person. There can be no outside observer. If the
narrator does not see or experience an event first-hand, it cannot be
a part of the story. All scenes in the story are filtered
through this person's unique perception.

The third-person is a
narrative mode in which both the reader and author observe the
situation either through the senses and thoughts of more than one
character, or through an overarching godlike perspective that sees
and knows everything that happens and everything the characters are
thinking. In this mode of narration, the narrator can tell the reader
things that the main character does not know, or things
that none of the characters know.

Discussion

15.	A truck travelling due north at 20m/s turns west and travels at same speed. The change in velocity be
Answer» A truck travelling due north at 20m/s turns west and travels at same speed. The change in velocity be

Discussion

16.	The shear stress at a point in a liquid is found to be 0.03 N/m2. The velocity gradient for the fluid flow is 0.15 s−1. What will be its viscosity (in Poise) ?
Answer» The shear stress at a point in a liquid is found to be $0.03 {N/m}^{2}$ . The velocity gradient for the fluid flow is $0.15 s^{- 1}$ . What will be its viscosity (in $Poise$ ) ?

Discussion

17.	37. The potential at a point x (measured in micrometre) due to some charges situated on the x-axis is given by V(x) = 20/(x^-4) V . The electric field E at x = 4 micrometre is given by
Answer» 37. The potential at a point x (measured in micrometre) due to some charges situated on the x-axis is given by V(x) = 20/(x^-4) V . The electric field E at x = 4 micrometre is given by

Discussion

18.	A particle of mass m is moving in a horizontal circle of radius r under a centripetal force equal to −Kr2 , where K is a constant. The total energy of the particle is
Answer» A particle of mass m is moving in a horizontal circle of radius r under a centripetal force equal to $\frac{- K}{r^{2}}$ , where K is a constant. The total energy of the particle is

Discussion

19.	An air filled capacitor of capacity 10 μF is connected to a battery of 12 V. Now the space between the plates is filled with a liquid of dielectric constant 5. The additional charge that flows to the capacitor is
Answer» An air filled capacitor of capacity $10 μ F$ is connected to a battery of $12 V$ . Now the space between the plates is filled with a liquid of dielectric constant $5$ . The additional charge that flows to the capacitor is

Discussion

20.	Pick out the statement which is incorrect.
Answer» Pick out the statement which is incorrect.

Discussion

21.	A uniform long tubeis bent into a circle of radius R and it lies in a vertical plane. Two liquids of same volume but densities ρ and δ fill half the tube. the angle θ is.
Answer» A uniform long tubeis bent into a circle of radius R and it lies in a vertical plane. Two liquids of same volume but densities ρ and δ fill half the tube. the angle θ is.

Discussion

22.	Amonoenergetic (18 keV) electron beam initially in the horizontaldirection is subjected to a horizontal magnetic field of 0.04 Gnormal to the initial direction. Estimate the up or down deflectionof the beam over a distance of 30 cm (me= 9.11 × 10−19C). [Note: Datain this exercise are so chosen that theanswer will give you an idea of the effect of earth’s magneticfield on the motion of the electron beam from the electron gun to thescreen in a TV set.]
Answer» A monoenergetic (18 keV) electron beam initially in the horizontal direction is subjected to a horizontal magnetic field of 0.04 G normal to the initial direction. Estimate the up or down deflection of the beam over a distance of 30 cm (m_e= 9.11 × 10⁻¹⁹ C). [Note: Data in this exercise are so chosen that the answer will give you an idea of the effect of earth’s magnetic field on the motion of the electron beam from the electron gun to the screen in a TV set.]

Discussion

23.	A cube of ice floats partly in water and partly in kerosene oil. Specific gravity of kerosene oil and that of ice is 0.8 and 0.9 respectively. Ratio of the volume of ice immersed in water to that in kerosene oil is
Answer» A cube of ice floats partly in water and partly in kerosene oil. Specific gravity of kerosene oil and that of ice is $0.8$ and $0.9$ respectively. Ratio of the volume of ice immersed in water to that in kerosene oil is

Discussion

24.	A force of 600 newtons will compress a spring 0.5 metres. What is the spring constant of the spring?
Answer» A force of 600 newtons will compress a spring 0.5 metres. What is the spring constant of the spring?

Discussion

25.	The oscillation of body on a smooth horizontal surface is represented by the equation,x=Acos(ωt)Wherex= displacement at time tω= frequency of oscillationWhich one of the following graphs shows correctly the variation ′a′ with ′t′?
Answer» The oscillation of body on a smooth horizontal surface is represented by the equation, $x = A cos (ω t)$ Where $x =$ displacement at time $t$ $ω =$ frequency of oscillation Which one of the following graphs shows correctly the variation $^{'} a^{'}$ with $^{'} t^{'}$ ?

Discussion

26.	InMillikan’s oil drop experiment, what is the terminal speed ofan uncharged drop of radius 2.0 × 10–5m and density 1.2 × 103kg m–3?Take the viscosity of air at the temperature of the experiment to be1.8 × 10–5Pa s. How much is the viscous force on the drop at that speed?Neglect buoyancy of the drop due to air.
Answer» In Millikan’s oil drop experiment, what is the terminal speed of an uncharged drop of radius 2.0 × 10^–5 m and density 1.2 × 10³ kg m^–3? Take the viscosity of air at the temperature of the experiment to be 1.8 × 10^–5 Pa s. How much is the viscous force on the drop at that speed? Neglect buoyancy of the drop due to air.

Discussion

27.	what is a reducing electrode ?
Answer» what is a reducing electrode ?

Discussion

28.	so4 oxidative state
Answer» so4 oxidative state

Discussion

29.	A block of mass 10 Kg is moving in x direction with a cons†an t speed of 10 m/s it is subjected to a retarding force F=-0.1x joule/metre during its travel from x=20 metre to x=30 metre its final kinetic energy will be
Answer» A block of mass 10 Kg is moving in x direction with a cons†an t speed of 10 m/s it is subjected to a retarding force F=-0.1x joule/metre during its travel from x=20 metre to x=30 metre its final kinetic energy will be

Discussion

30.	A block of mass m placed on a smooth horizontal surface is connected by two elastic strings ab and cd of equal length , Young’s Modulus Y & 2Y, area of cross section A & A2 respectively and a spring of force constant C. Both the strings and spring are in unstretched condition, if the time period of small oscillation of the block along the horizontal surface is. 2π[√mx1c+√mlx2YA+x3cl]. Find x1+x3−x2 ___
Answer» A block of mass m placed on a smooth horizontal surface is connected by two elastic strings ab and cd of equal length , Young’s Modulus Y & 2Y, area of cross section A & $\frac{A}{2}$ respectively and a spring of force constant C. Both the strings and spring are in unstretched condition, if the time period of small oscillation of the block along the horizontal surface is. $2 π [\sqrt{\frac{m}{x_{1} c}} + \sqrt{\frac{m l}{x_{2} Y A + x_{3} c l}}] . F i n d x_{1} + x_{3} - x_{2}$ ___

Discussion

31.	Why rounding off 4.36514 gives 4.37 and not 4.36?
Answer» Why rounding off 4.36514 gives 4.37 and not 4.36?

Discussion

32.	A galvanometer is shunted such that only 111th of the main current flows through it. If G=150Ω the shunt resistance is
Answer» A galvanometer is shunted such that only $\frac{1}{11} t h$ of the main current flows through it. If G=150 $Ω$ the shunt resistance is

Discussion

33.	What is the equivalent resistance between point A and B
Answer» What is the equivalent resistance between point A and B

Discussion

34.	The flow of liquid in which its layer slides over another without mixing, is called
Answer» The flow of liquid in which its layer slides over another without mixing, is called

Discussion

35.	A balloon contains 1500 m3 of helium gas at 27 ∘C and 4 atm . The volume of helium gas at −3 ∘C and 2 atm will be
Answer» A balloon contains $1500 m^{3}$ of helium gas at $27^{\circ} C$ and $4 atm$ . The volume of helium gas at $- 3^{\circ} C$ and $2 atm$ will be

Discussion

36.	19. The electric field at point P distant y upon the perpendicular bisector of long line charge(the upper portion has linear charge density - 2lambda and the lower portion has linear charge density lambda?
Answer» 19. The electric field at point P distant y upon the perpendicular bisector of long line charge(the upper portion has linear charge density - 2lambda and the lower portion has linear charge density lambda?

Discussion

37.	Aplane electromagnetic wave travels in vacuum along z-direction. Whatcan you say about the directions of its electric and magnetic fieldvectors? If the frequency of the wave is 30 MHz, what is itswavelength?
Answer» A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?

Discussion

38.	Why is charge always proportional to potential difference for any random conductor
Answer» Why is charge always proportional to potential difference for any random conductor

Discussion

39.	what will happen if I place a dielectric in between 2 charges in air placed at some dis†an ce apar
Answer» what will happen if I place a dielectric in between 2 charges in air placed at some dis†an ce apar

Discussion

40.	What is a wave? Classify different types of waves.
Answer» What is a wave? Classify different types of waves.

Discussion

41.	The metre bridge is in balance position with P/Q=l1/l2 if yes now interchange the position of galvanometer and cell will the bridge work?if yes what will be the balanced condition
Answer» The metre bridge is in balance position with P/Q=l1/l2 if yes now interchange the position of galvanometer and cell will the bridge work?if yes what will be the balanced condition

Discussion

42.	Find slope of the Straight line passing through the points (−3,8) and (1,4).
Answer» Find slope of the Straight line passing through the points $(- 3, 8)$ and $(1, 4)$ .

Discussion

43.	Two solutions, A and B when taken separately in the same conductivity cell had a resistance of 40 and 60 respectively. If equal volumes of both the solutions are mixed, then the resistance of the mixture will be (Assume that there is no increase in the degree of dissociation of A and B on mixing)
Answer» Two solutions, A and B when taken separately in the same conductivity cell had a resistance of 40 and 60 respectively. If equal volumes of both the solutions are mixed, then the resistance of the mixture will be (Assume that there is no increase in the degree of dissociation of A and B on mixing)

Discussion

44.	If medium is present then what is the change in effect of force and what is K in the formula of coulomb's law when medium is present
Answer» If medium is present then what is the change in effect of force and what is K in the formula of coulomb's law when medium is present

Discussion

45.	A concave lens doesn't have a focus because
Answer» A concave lens doesn't have a focus because

Discussion

46.	21.When any body with mass m is moving with speed of light, it's mass increases. Why?
Answer» 21.When any body with mass m is moving with speed of light, it's mass increases. Why?

Discussion

47.	A liquid of mass m and specific heat c is heated to a temperature 2T. Another liquid of mass m2 and specific heat 2c is heated to a temperature T. If these two liquids are mixed, the resulting temperature of the mixture is
Answer» A liquid of mass $m$ and specific heat $c$ is heated to a temperature $2 T$ . Another liquid of mass $\frac{m}{2}$ and specific heat $2 c$ is heated to a temperature $T$ . If these two liquids are mixed, the resulting temperature of the mixture is

Discussion

48.	27.A 3^° C rise of temperature is observed in a conductor by passing a certain amount of current . When the current is doubled ,the rise of temperature will be-?
Answer» 27.A 3^° C rise of temperature is observed in a conductor by passing a certain amount of current . When the current is doubled ,the rise of temperature will be-?

Discussion

49.	79.A particle is moving in clockwise direction in a circular path as shown in figure.The instataneous velocity of particle at a certain instant is v=(3i+3j)m/s.Then in which quadrant does the particle lie at that instant?
Answer» 79.A particle is moving in clockwise direction in a circular path as shown in figure.The instataneous velocity of particle at a certain instant is v=(3i+3j)m/s.Then in which quadrant does the particle lie at that instant?

Discussion

50.	Figure shows a block A of mass 8m having a smooth semi circular groove of radius a placed on a smooth horizontal surface. A block B of mass m is released from a position in groove, where its radius is horizontal. The speed of block A when block B reaches its bottom is
Answer» Figure shows a block A of mass $8 m$ having a smooth semi circular groove of radius $a$ placed on a smooth horizontal surface. A block B of mass $m$ is released from a position in groove, where its radius is horizontal. The speed of block A when block B reaches its bottom is

Discussion

Explore topic-wise InterviewSolutions in Current Affairs.

Explain function of pseudopenis

A body is moving along a rough horizontal surface with an initial velocity 6 m/s. If the body comes to rest after travelling 9 m, then the coefficient of sliding firction will be

The load versus elongation graph for four wires made of the same material and having the same length is shown in the figure. The thickest wire is represented by the line

28 0.2F capacitor is charge to 600V by a battery,on removing the battery,it is connected with another parallel plate condenser of 1F.The potential decreases to

If f(x)=Ax2+Bx satisfies the conditions f′(1)=8 and 1∫0f(x)dx=83, then which of the following is/are correct ?

23.The speed of an electron having a wavelength of 10 to the power minus 10 m is.

The highest magnetic moment is observed for 1)Cu+ 2)Zn2+ 3)Cr3+ 4) Ti3+

is it correct to take the clockwise work done in the graph postive or negative?What does NTA follow

A capacitor is made of a flat plate of area A and the second plate having a stair- like structure as shown in figure. The width of each stair is a and the height is b. The equivalent capacitance of the combination is

12. How beta rays and cathode rays can be distinguished? Explain how ?

A truck travelling due north at 20m/s turns west and travels at same speed. The change in velocity be

The shear stress at a point in a liquid is found to be 0.03 N/m2. The velocity gradient for the fluid flow is 0.15 s−1. What will be its viscosity (in Poise) ?

37. The potential at a point x (measured in micrometre) due to some charges situated on the x-axis is given by V(x) = 20/(x^-4) V . The electric field E at x = 4 micrometre is given by

A particle of mass m is moving in a horizontal circle of radius r under a centripetal force equal to −Kr2 , where K is a constant. The total energy of the particle is

An air filled capacitor of capacity 10 μF is connected to a battery of 12 V. Now the space between the plates is filled with a liquid of dielectric constant 5. The additional charge that flows to the capacitor is

Pick out the statement which is incorrect.

A uniform long tubeis bent into a circle of radius R and it lies in a vertical plane. Two liquids of same volume but densities ρ and δ fill half the tube. the angle θ is.

A cube of ice floats partly in water and partly in kerosene oil. Specific gravity of kerosene oil and that of ice is 0.8 and 0.9 respectively. Ratio of the volume of ice immersed in water to that in kerosene oil is

A force of 600 newtons will compress a spring 0.5 metres. What is the spring constant of the spring?

The oscillation of body on a smooth horizontal surface is represented by the equation,x=Acos(ωt)Wherex= displacement at time tω= frequency of oscillationWhich one of the following graphs shows correctly the variation ′a′ with ′t′?

what is a reducing electrode ?

so4 oxidative state

A block of mass 10 Kg is moving in x direction with a cons†an t speed of 10 m/s it is subjected to a retarding force F=-0.1x joule/metre during its travel from x=20 metre to x=30 metre its final kinetic energy will be

Why rounding off 4.36514 gives 4.37 and not 4.36?

A galvanometer is shunted such that only 111th of the main current flows through it. If G=150Ω the shunt resistance is

What is the equivalent resistance between point A and B

The flow of liquid in which its layer slides over another without mixing, is called

A balloon contains 1500 m3 of helium gas at 27 ∘C and 4 atm . The volume of helium gas at −3 ∘C and 2 atm will be

19. The electric field at point P distant y upon the perpendicular bisector of long line charge(the upper portion has linear charge density - 2lambda and the lower portion has linear charge density lambda?

Aplane electromagnetic wave travels in vacuum along z-direction. Whatcan you say about the directions of its electric and magnetic fieldvectors? If the frequency of the wave is 30 MHz, what is itswavelength?

Why is charge always proportional to potential difference for any random conductor

what will happen if I place a dielectric in between 2 charges in air placed at some dis†an ce apar

What is a wave? Classify different types of waves.

The metre bridge is in balance position with P/Q=l1/l2 if yes now interchange the position of galvanometer and cell will the bridge work?if yes what will be the balanced condition

Find slope of the Straight line passing through the points (−3,8) and (1,4).

Two solutions, A and B when taken separately in the same conductivity cell had a resistance of 40 and 60 respectively. If equal volumes of both the solutions are mixed, then the resistance of the mixture will be (Assume that there is no increase in the degree of dissociation of A and B on mixing)

If medium is present then what is the change in effect of force and what is K in the formula of coulomb's law when medium is present

A concave lens doesn't have a focus because

21.When any body with mass m is moving with speed of light, it's mass increases. Why?

A liquid of mass m and specific heat c is heated to a temperature 2T. Another liquid of mass m2 and specific heat 2c is heated to a temperature T. If these two liquids are mixed, the resulting temperature of the mixture is

27.A 3^° C rise of temperature is observed in a conductor by passing a certain amount of current . When the current is doubled ,the rise of temperature will be-?

79.A particle is moving in clockwise direction in a circular path as shown in figure.The instataneous velocity of particle at a certain instant is v=(3i+3j)m/s.Then in which quadrant does the particle lie at that instant?

Figure shows a block A of mass 8m having a smooth semi circular groove of radius a placed on a smooth horizontal surface. A block B of mass m is released from a position in groove, where its radius is horizontal. The speed of block A when block B reaches its bottom is