330 + Interview Questions in FRICTION IN GENERAL AWARENESS Page 7 InterviewSolution

301.	Short note on Leonardo Davinci.
Answer» Leonardo Davinci (1452-1519): Leonardo Davinci was one of the first scholars to study friction systematically. He realized how important friction is for the working of machines. He focussed on all kinds of friction and drew a distinction between sliding friction and rolling friction.

301.

Short note on Leonardo Davinci.

Answer»

Leonardo Davinci (1452-1519):

Leonardo Davinci was one of the first scholars to study friction systematically.

He realized how important friction is for the working of machines.

He focussed on all kinds of friction and drew a distinction between sliding friction and rolling friction.

Discussion

302.	Observing the variation of friction. Push a heavy box which is kept on a floor. The box does not move. Now increase the applied force until the box moves.The heavy box starts movinga) Why does the box does not move at first?b) When does the box tends to move?
Answer» a) The applied force is less than frictional force (static friction). b) When the applied force is more than static friction.

302.

Observing the variation of friction. Push a heavy box which is kept on a floor. The box does not move. Now increase the applied force until the box moves.The heavy box starts movinga) Why does the box does not move at first?b) When does the box tends to move?

Answer»

a) The applied force is less than frictional force (static friction).

b) When the applied force is more than static friction.

Discussion

303.	Effect of roughness on frictional force. Set up an inclined plane on the horizontal floor. Use a wooden board as inclined plane. Put a mark at any point ‘A’. Now let a pencil cell or ball is move down from this point. Note down the distance travelled by the ball from the bottom of the inclined plane to point where it comes to a stop.Now, spread a cloth over the floor. Try again with ball and note down the distance.a) What are your observations from these experiments?b) In which case is the distance covered maximum?c) In which case is the distance covered minimum?d) Why is the distance covered by the pencil cell different on different surfaces?e) If the cloth is replaced with white marble surface or glass surface, what will be your prediction about the distance covered by the pencil cell?
Answer» a) The distance covered by ball in the two cases are different. b) When there is no cloth on the floor. c) When cloth is spread over the floor. d) Because, the friction offered by different surfaces are different. e) The pencil cell covers more distance.

303.

Effect of roughness on frictional force. Set up an inclined plane on the horizontal floor. Use a wooden board as inclined plane. Put a mark at any point ‘A’. Now let a pencil cell or ball is move down from this point. Note down the distance travelled by the ball from the bottom of the inclined plane to point where it comes to a stop.Now, spread a cloth over the floor. Try again with ball and note down the distance.a) What are your observations from these experiments?b) In which case is the distance covered maximum?c) In which case is the distance covered minimum?d) Why is the distance covered by the pencil cell different on different surfaces?e) If the cloth is replaced with white marble surface or glass surface, what will be your prediction about the distance covered by the pencil cell?

Answer»

a) The distance covered by ball in the two cases are different.

b) When there is no cloth on the floor.

c) When cloth is spread over the floor.

d) Because, the friction offered by different surfaces are different.

e) The pencil cell covers more distance.

Discussion

304.	What observations and experiments can you cite to show the existence of friction?
Answer» 1. Push a ball on a sandy surface and on a concrete surface and the observation is on a concrete surface it travels more distance, the reason is the concrete surface apply less frictional force on the ball. 2. Apply vehicle brakes during summer and rainy seasons and the observation is during summer season the brakes works more efficiently than rainy season the reason is friction between tyre and brakes is less because rainy water acts as lubricant.

304.

What observations and experiments can you cite to show the existence of friction?

Answer»

1. Push a ball on a sandy surface and on a concrete surface and the observation is on a concrete surface it travels more distance, the reason is the concrete surface apply less frictional force on the ball.

2. Apply vehicle brakes during summer and rainy seasons and the observation is during summer season the brakes works more efficiently than rainy season the reason is friction between tyre and brakes is less because rainy water acts as lubricant.

Discussion

305.	Does friction act on a table resting on the floor?
Answer» Yes, frictional force is acting on the table which is known as static friction.

Discussion

306.	When do we speak of sliding friction?
Answer» When applied force is more than the limit of static friction, the body starts to move then we speak about sliding friction.

Discussion

307.	Define sliding friction.
Answer» Sliding friction comes into play when an object is sliding over another.

Discussion

308.	Which of the following surfaces will offer the least friction?Vinyl floorPlywoodPlasticIce
Answer» Answer: (d) Ice

Discussion

309.	Least amount of friction is required in which of the following sports?Car RaceFootballIce SkatingTug of war
Answer» Answer: (c) Ice Skating

Discussion

310.	Relative to the surface, the direction of the friction is always A) In the direction of motion of the objectB) Opposite to direction of the motion of the object C) Perpendicular to direction of the motion of the object D) No relation between direction of the motion and friction
Answer» B) Opposite to direction of the motion of the object

Discussion

311.	A stationary body mass `m` is slowly lowered onto a rough massive plat from moving at a constant velocity `v_(o)=4m//s`. On smooth surface. The distance the body will slide with respect to the plat from `(mu=0.2)` is: A. `4m`B. `6m`C. `12m`D. `8m`
Answer» Correct Answer - A With respect to platform the initial velocity of the body of mass `m` is `4m//s` towards left and it starts retarding at the rate of `a=2m//s^(2)` Using `v^(2)=u^(2)+2as` we get: `0^(2)=4^(2)+2(-2)(s)rArrs=4"meter"`

Discussion

312.	A force of 750 N is applied to a block of mass 102 kg to prevent it from sliding on a plane with an inclination angle `30^@` with the horizontal. If the coefficients of static friction and kinetic friction between the block and the plane are 0.4 and 0.3 respectively, then the frictional force acting on the block isA. 750 NB. 500 NC. 345 ND. 250 N
Answer» Correct Answer - D

Discussion

313.	A force of 98 N is required to just start moving a body of mass 100 kg over ice. The coefficient of static friction isA. 0.6B. 0.4C. 0.2D. 0.1
Answer» Correct Answer - D

Discussion

314.	A car starts from rest on a `1600 m` long bridge. The coefficient of friction between the tyre and road is `0.8`. The minimum time taken by the car to cross the bridge isA. `5 s`B. `10 s`C. `15 s`D. `20 s`
Answer» Correct Answer - D The maximum acceleration possible on rough horizontal surface `= mu g` `s = (1)/(2) at^(2) rArr t = sqrt((2 s)/(a)) = sqrt((2 xx 1600)/(0.8 xx 10))` `t = 20 s`

Discussion

315.	When a car is speeding on a level track, it enters on a concrete road from a tar road. What happens to its speed when the acceleration is unchanged?
Answer» The friction between the tyres and the concrete road is less than that on the tar road. Therefore, the car accelerates more on the concrete road.

Discussion

316.	A small block of mass of `0.1 kg` lies on a fixed inclined plane `PQ` which makes an angle `theta` with the horizontal. A horizontal force of `1N` acts on the block through its centre of mass as shown in the figure. The block remains stationary if ( take `g=10 m//s^(2))` A. `theta=45^(@)`B. `theta gt 45^(@)` and frictional force acts on the block towards `P`.C. `theta gt 45^(@)` and frictional force acts on the block towards `Q`.D. ` theta lt 45^(@)` and frictional force acts on the block towards `Q`.
Answer» Correct Answer - A::C At `theta = 45^(@), mg sin theta = 1 xx cos theta` At `theta gt 45^(@), mg sin theta = 1 xx cos theta` (friction acts upward) At `theta lt 45^(@), mg sin theta = 1 xx cos theta` (Friction acts downward) `x a = g sin theta - mu g cos theta`

Discussion

317.	A wooden block is kept n a polished woden plank and the inclination of the plank is gradually increased. It is found that the block starts slipping when the pland makes an angle of `18^@` with the horizontal. However, once started the block can continue with uniform speed if the inclination is reduced to `15^@`. Find the coefficients of static and kinetic friction between the block and the plank.
Answer» The coefficient of static friction `mu_s=tan18^0` and the coefficient of kinetic friction is `mu_k=tan 15^0`

Discussion

318.	The maximum speed that can be achieved without skidding by a car on a circular unbanked road of radius R and coefficient of static friction `mu` , isA. `muRg`B. `Rgsqrt(mu)`C. `musqrt(Rg)`D. `sqrt(muRg)`
Answer» Correct Answer - D

Discussion

319.	The maximum static frictional force isA. Equal to twice the area of surface in contactB. Independent of the area of surface in contactC. Equal to the area of surface in contactD. None of the above
Answer» Correct Answer - B

Discussion

320.	A conveyor belt is moving at a constant speed of `2m//s` . A box is gently dropped on it. The coefficient of friction between them is `mu=0.5` . The distance that the box will move relative to belt before coming to rest on it taking `g=10ms^(-2)` is:A. zeroB. `0.4 m`C. `1.2 m`D. `0.6 m`
Answer» Correct Answer - B `v^(2) = u^(2) + 2 as = 0 + 2 mu gs` `(2)^(2) = 2 xx 5 xx 10 xx s rArr s = 0.4 m`

Discussion

321.	A block `A` of mass `m` is over a plank `B` of mass `2m`. Plank `B` si placed over a smooth horizontal surface. The coefficient of friction between `A` and `B` is `(1)/(2)`. Blocks `A` is given a velocity `V_(0)` towards right. Then A. Acceleration of `A` is `(g)/(2)`B. Acceleration of `A` is `g`C. Acceleration of `B` relative to `A` is `(3)/(4)g`D. Acceleration of `A` is zero
Answer» Correct Answer - A::C Force of friction between the two will be maximum i.e., `mu mg` Retardation of `A` is `a_(A) = (mu mg)/(m) = mu g` and acceleration of `B` is `a_(B) = (mu mg)/(2m) = (mu g)/(2)` `therefore` Acceleration of `B` relative to `A` is `a_(BA) = a_(A)+a_(B) = (3 mu g)/(2)` substituting `mu = (1)/(2)a_(BA) = (3g)/(4)`.

Discussion

322.	If a ladder weighting `250 N` is placed against a smooth vertical wall having coefficient of friction between it and floor 0.3, then what is the maximum force of friction available at the point of contact between the ladder and the floor?A. 75 NB. 50 NC. 35 ND. 25 N
Answer» Correct Answer - A

Discussion

323.	A block of mass 0.1 is held against a wall applying a horizontal force of 5N on block. If the coefficient of friction between the block and the wall is 0.5, the magnitude of the frictional force acting on the block is:A. 2.5 NB. 0.98 NC. 4.9 ND. 0.49 N
Answer» Correct Answer - B

Discussion

324.	Block `B` of mass `2 kg` is placed on smooth horizontal plane. Block `A` to mass `1 kg` is placed on block `B`. The coefficient of friction between `A` and `B` is `0.40`. The block `A` is imparted a velocity `16 m//s` "at" `t=0`. Find the time at which momentum of the twoblocks are equal ( in seconds). `(g=10m//s^(2))`
Answer» Correct Answer - 2 `P_(1)=m_(1)V_(1)=(1) (16-mu"gt")` till velocities are equal. `P_(2)=m_(2)V_(2)=(2)((mum_(1)g)/(m_(2)))t=mu(1)"gt"`till velocities are equal. For `P_(1)=P_(2), 16-mu"gt"=mu"gt"` `t=(16)/((2)(0.4)(10))=2sec`

Discussion

325.	A marble block of mass 2 kg lying on ice when given a velocity of `6m//s` is stopped by friction in 10s. Then the coefficient of friction isA. 0.01B. 0.02C. 0.03D. 0.06
Answer» Correct Answer - D

Discussion

326.	A block A kept on an inclined surface just begins to slide if the inclination is `30^0`. The block is replaced by another block B and it is found that it just begins to slide if the inclination is `40^0`A. mass of Agt mass of BB. mass of Altmass of BC. mass of A=mass of BD. all the three are possible.
Answer» Correct Answer - D

Discussion

327.	A block is lying on an inclined plane which makes `60^(@)` with the horizontal. If coefficient of friction between block and plane is 0.25 and `g=10m//s^2`, , then acceleration of the block when it moves along the plane will beA. `2.50m//s^(2)`B. `5.00m//s^(2)`C. `7.4m//s^(2)`D. `8.66m//s^(2)`
Answer» Correct Answer - C

Discussion

328.	When a body is lying on a rough inclined plane and does not move, the force of frictionA. is equal to `muR`B. is less than `muR`C. is greater than `muR`D. is equal to R
Answer» Correct Answer - B

Discussion

329.	A body of mass 10 kg is lying on a rough plane inclined at an angle of `30^(@)` to the horizontal and the coefficient of friction is 0.5. the minimum force required to pull the body up the plane isA. 914 NB. 91.4 NC. 9.14 ND. 0.914 N
Answer» Correct Answer - B

Discussion

330.	A ball is thrown vertically upward with speed `10 m//s` and it returns to the ground with speed `8 m//s`. A constant air resistance acts. The minimum height attained by the ball isA. `5 m`B. `4.5 m`C. `3.5 m`D. `4.1 m`
Answer» Correct Answer - D Upward motion : `0 = (10)^(2) - 2 (g + (R )/(m))h` (i) Downward motion : `(8)^(2) = 0 + 2(g + ( R )/(m))h` (ii) Solving , `4 gh = 100 + 64 rArr h = 4.1 m`

Discussion

Explore topic-wise InterviewSolutions in .

Short note on Leonardo Davinci.

Observing the variation of friction. Push a heavy box which is kept on a floor. The box does not move. Now increase the applied force until the box moves.The heavy box starts movinga) Why does the box does not move at first?b) When does the box tends to move?

What observations and experiments can you cite to show the existence of friction?

Does friction act on a table resting on the floor?

When do we speak of sliding friction?

Define sliding friction.

Which of the following surfaces will offer the least friction?Vinyl floorPlywoodPlasticIce

Least amount of friction is required in which of the following sports?Car RaceFootballIce SkatingTug of war

Relative to the surface, the direction of the friction is always A) In the direction of motion of the objectB) Opposite to direction of the motion of the object C) Perpendicular to direction of the motion of the object D) No relation between direction of the motion and friction

A stationary body mass `m` is slowly lowered onto a rough massive plat from moving at a constant velocity `v_(o)=4m//s`. On smooth surface. The distance the body will slide with respect to the plat from `(mu=0.2)` is: A. `4m`B. `6m`C. `12m`D. `8m`

A force of 98 N is required to just start moving a body of mass 100 kg over ice. The coefficient of static friction isA. 0.6B. 0.4C. 0.2D. 0.1

A car starts from rest on a `1600 m` long bridge. The coefficient of friction between the tyre and road is `0.8`. The minimum time taken by the car to cross the bridge isA. `5 s`B. `10 s`C. `15 s`D. `20 s`

When a car is speeding on a level track, it enters on a concrete road from a tar road. What happens to its speed when the acceleration is unchanged?

The maximum speed that can be achieved without skidding by a car on a circular unbanked road of radius R and coefficient of static friction `mu` , isA. `muRg`B. `Rgsqrt(mu)`C. `musqrt(Rg)`D. `sqrt(muRg)`

The maximum static frictional force isA. Equal to twice the area of surface in contactB. Independent of the area of surface in contactC. Equal to the area of surface in contactD. None of the above

A conveyor belt is moving at a constant speed of `2m//s` . A box is gently dropped on it. The coefficient of friction between them is `mu=0.5` . The distance that the box will move relative to belt before coming to rest on it taking `g=10ms^(-2)` is:A. zeroB. `0.4 m`C. `1.2 m`D. `0.6 m`

If a ladder weighting `250 N` is placed against a smooth vertical wall having coefficient of friction between it and floor 0.3, then what is the maximum force of friction available at the point of contact between the ladder and the floor?A. 75 NB. 50 NC. 35 ND. 25 N

A block of mass 0.1 is held against a wall applying a horizontal force of 5N on block. If the coefficient of friction between the block and the wall is 0.5, the magnitude of the frictional force acting on the block is:A. 2.5 NB. 0.98 NC. 4.9 ND. 0.49 N

A marble block of mass 2 kg lying on ice when given a velocity of `6m//s` is stopped by friction in 10s. Then the coefficient of friction isA. 0.01B. 0.02C. 0.03D. 0.06

A block is lying on an inclined plane which makes `60^(@)` with the horizontal. If coefficient of friction between block and plane is 0.25 and `g=10m//s^2`, , then acceleration of the block when it moves along the plane will beA. `2.50m//s^(2)`B. `5.00m//s^(2)`C. `7.4m//s^(2)`D. `8.66m//s^(2)`

When a body is lying on a rough inclined plane and does not move, the force of frictionA. is equal to `muR`B. is less than `muR`C. is greater than `muR`D. is equal to R

A body of mass 10 kg is lying on a rough plane inclined at an angle of `30^(@)` to the horizontal and the coefficient of friction is 0.5. the minimum force required to pull the body up the plane isA. 914 NB. 91.4 NC. 9.14 ND. 0.914 N

A ball is thrown vertically upward with speed `10 m//s` and it returns to the ground with speed `8 m//s`. A constant air resistance acts. The minimum height attained by the ball isA. `5 m`B. `4.5 m`C. `3.5 m`D. `4.1 m`