Friction Understanding at Molecular Level

Q. Two blocks $A$ and $B$ of mass $2\text{kg}$ and $4\text{kg}$ respectively are placed one over the other. A horizontal force $F=2t$, which varies with time is applied on the upper block. If coefficiet of friction between blocks $A$ and $B$ is $0.5$ and horizontal surface is smooth. Then assuming $t$ is in seconds, the total time upto which both blocks will move together without slipping over each other is

1. $10\text{sec}$
2. $12.5\text{sec}$
3. $5\text{sec}$
4. $7.5\text{sec}$

Q. Two blocks $A$ and $B$ of masses $m_A$ and $m_B$ have velocity $v$ and $2v$, respectively, at a a given instant. A horizontal force $F$ acts on the block $A$. There is no friction between ground and block $B$ and coefficient of friction between $A$ and $B$ is $\mu$. The friction

1. on $A,$ supports its motion.
2. on $B,$ opposes its motion relative to $A$.
3. on $B,$ opposes its motion.
4. opposes the motion of both.

Q. If two surfaces are polished highly smooth, the friction between them increases.

1. True
2. False

Q. 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 $0.5$. The distance that the box will move relative to belt before coming to rest on it (taking $g=10m{s}^{-2}$), is:

1. $1.2m$
2. $0.6m$
3. zero
4. $0.4m$

Q. Two blocks of masses $M_1=4$kg and $M_2=6$kg are connected by a string of negligible mass passing over a frictionless pulley as shown in the figure. The coefficient of friction between the block $M_1$ and the horizontal surface is $0.4$. When the system is released, the masses $M_1$ and $M_2$ start accelerating. What additional mass m should be placed over $M_1$ so that the masses $(M_1+m)$ slide with a uniform speed?

1. $10$ kg
2. $12$ kg
3. $1$kg
4. $8$kg

Q. A block of mass $1kg$ lies on a horizontal surface in a truck. The coefficient of friction between the block and the surface is $0.6$. If the acceleration of the truck is $5m{s}^{-2}$, the frictional force acting on the block is

1. $10N$
2. $5N$
3. $20N$
4. $15N$

Q. A body of mass $10kg$ is moving on a horizontal surface by applying a force of $10N$ in forward direction. If body moves with constant velocity, the work done by force of friction for a displacement of $2m$ is :

1. $-20J$
2. $10J$
3. $20J$
4. $-5J$

Q. A bullet of mass $0.01kg$ is fired horizontally into a $4kg$ wooden block at rest on a horizontal surface. The coefficient of kinetic friction between the block and surface is $0.25$. The bullet remains embedded in the block and combination moves $20m$ before coming to rest. Find the speed of the bullet when it strike the block.

Q. A $5g$ bullet was fired horizontally into a $1.2kg$ wooden block resting on a wooden surface. The coefficient of kinetic friction between the block and surface is $0.2$. The bullet remained embedded in block. The block was found to slide $0.23m$ along the surface before stopping. Find the initial speed of the bullet.

1. 241 ms${}^{-1}$
2. 229 ms${}^{-1}$
3. 221 ms${}^{-1}$
4. 202 ms${}^{-1}$

Q. A block of mass $M$ is placed on a rough floor of a lift. The coefficient of friction between the block and the floor is $\mu$. When the lift falls freely, the block is pulled horizontally on the floor. What will be the force of friction?

1. $\mu Mg/2$
2. $2\mu Mg$
3. None of the above
4. $\mu Mg$

Q. A body is stuck to the front part of the truck. The coefficient of friction between the body and truck is $\mu$. The minimum acceleration with which the truck should travel, so that the body does not slide down is:

1. $\mu g$
2. $\frac{\mu }{g}$
3. ${\mu }^{2}g$
4. $\frac{g}{\mu }$

Q. Two blocks $A$ and $B$ of masses $m_A$ and $m_B$ have velocity $v$ and $2v$, respectively, at a a given instant. A horizontal force $F$ acts on the block $A$. There is no friction between ground and block $B$ and coefficient of friction between $A$ and $B$ is $\mu$. The friction

1. on $A,$ supports its motion.
2. on $B,$ opposes its motion relative to $A$.
3. on $B,$ opposes its motion.
4. opposes the motion of both.

Q. A chain of length $L$ starts sliding down from the horizontal surface of a table. If the hanging length is $l$, the coefficient of friction between the table and chain is:

1. $\frac{L-l}{l}$
2. $\frac{L}{l}$
3. $\frac{l}{L}$
4. $\frac{l}{L-l}$

Q. An observer and a vehicle, both start moving together from rest (towards right) with acceleration 5 $m/s^2$ and 2 $m/s^2$, respectively. There is a $1kg$ block on the floor of the vehicle and coefficient of friction is $\mu =0.3$ between their surface. Friction force observed by the observer is $2N$. What is the magnitude of the work done (in J) by frictional force on the $1kg$ block observed by the running observer, during first second of the motion.

Q. A person pushes a desk across the horizontal floor with a constant force of $20.0N$. The desk moves at a constant $2.0m/s$. If the only other acting on the desk is the force of friction, how does the amount of frictional force compare to the amount of force the person applies?

1. The force of friction must be greater than the force the person applies
2. The force of friction must be the same as the force the person applies
3. The force of friction must be zero
4. The force a friction must be less than the force the person applies
5. Not enough information is given to answer this question

Q. A body rolled on a surface with speed of$10{ms}^{-1}$ comes to rest after covering a distance of $50m$. Find the coefficient of friction. Take $g=10{ms}^{-2}$.

Q. Work done by the friction force on $2kg$ block upto $5$ sec with respect to $3kg$ block is:

1. $100J$
2. $400J$
3. $160J$
4. zero

Q. In the given figure, the force is applied on block B. The co-efficient of friction for different contact surface is as shown in figure. Choose the correct option (s)

1. if $F=6N$, there is no motion at any part of the system
2. if $F>12N$, there is motion between A and B
3. If $F>11N$, there is motion between B and C
4. if $F<6N$, there is motion between A and B

Q. The blocks $A$ and $B$ are arranged as shown in the figure. The pulley is friction less. The mass of $A$ is $10Kg$. The coefficient of friction of $A$ with the horizontal surface is $0.20$ The minimum mass of $B$ to start the motion will be

1. $2Kg$
2. $0.2Kg$
3. $5Kg$
4. $10Kg$

Q. A book of weight $20$ $N$ is pressed between two hands and each hand exerts a force of $40$ $N$. If the book just starts to slide down, then the coefficient of friction is

1. $0.25$
2. $0.2$
3. $0.5$
4. $0.1$

Q. Two masses $m$ and $M$ are lying on a surface moving with acceleration $a$. Only the given supporting and moving surface has coefficient of friction as $\mu$. The frictional force for $\mu >a/g$ is :

1. $ma$
2. $\mu Ma$
3. $\mu Mg$
4. $Ma$

Q. A yo-yo, arranged as shown, rests on a frictionless surface. When a force $F$ is applied to the string, the yo-yo:

1. moves to the left and rotates clockwise.
2. moves to the right and rotates counterclockwise.
3. moves to the right and rotates clockwise.
4. moves to the left and rotates counterclockwise.

Q. A block of mass m starts from rest and slides down a frictionless semi-circular track from a height h. When it reaches the lowest point of the track, it collides with a stationary piece of putty also having mass m. If the block and the putty stick together and continue to slide, the maximum height that the block-putty system could reach is:

1. h/4
2. h/2
3. h
4. independent of h

Q. An external force is applied to a box that is free to move on a rough surface. Which of the following statements could be correct?

1. The frictional force is larger than the applied force
2. The frictional force is smaller than the applied force
3. All of these statements could be correct
4. The weight of the object is larger than the applied force
5. The weight of the object is smaller than the applied force

Q. A block is placed on a plank which is placed on a horizontal plane. A massless but elastic string deviates by an angle $\theta$ with vertical when a force $F$ is applied to the plank to shift it to the right making the block slide over it. If $F_P$ and $F_B$ are frictional forced between plank and plane and between block and plane respectively then work done by applied force is given by

1. work done against friction acting on plank + energy in the elastic string - work done by friction acting on block.
2. work done against frictions acting on plank and block + elastic energy
3. difference of work done against friction acting on plank and block.
4. elastic energy of string.

Q. If two surfaces are polished highly smooth, the friction between them increases.

1. True
2. False

Q. A $4g$ bullet is fired horizontally with a speed of $300m/s$ into $0.8kg$ block of wood, which is at rest on a table. If the coefficient of friction between the block and the table is $0.3$, how far will the block slide approximately?

1. 0.375 m
2. 0.20 m
3. 0.565 m
4. 0.750 m

Q. A force $\overrightarrow{F}=\hat{i}+4\hat{j}$ acts on block shown. Determine the force of friction acting on the block is :

1. $-\hat{i}$
2. $-1.8\hat{i}$
3. $-2.4\hat{i}$
4. $-3\hat{i}$

Q. Two bodies having the same mass, 21kg each have different surface areas 50 m${}^2$ and 100 m${}^2$ in contact with a horizontal plane. If the coefficient of friction is 0.2, the forces of friction that come into play when they are in motion will be in the ratio.

1. 1:1
2. 1:2
3. 2:1
4. 1:4

Q. The work done by friction when force acting is 2N and displacement of body 100m is::

1. $-200J$
2. $200J$
3. $600J$
4. $-600J$