Work Done by Friction

Q. Two point charges $q_1$ and $q_2$ are placed at a distance of $50\text{m}$ from each other in air, interact with a certain electrostatic force. If the medium is replaced with oil whose relative permittivity is $5$, then the interaction force between them is still the same. Now the separation between them is

1. $16.6\text{m}$
2. $22.3\text{m}$
3. $28.4\text{m}$
4. $25.0\text{m}$

Q.

The work done by kinetic friction on an object is always negative.

1. True

2. False

Q. A body of mass $2\text{kg}$ initially at rest moves under the action of an applied horizontal force of $7\text{N}$ on a table with coefficient of kinetic friction ${\mu }_k=0.1$.
Calculate the work done by kinetic friction in $10\text{sec}(g=9.8{\text{m/s}}^2)$

1. $246.96\text{J}$
2. $-246.96\text{J}$
3. $882\text{J}$
4. $-882\text{J}$

Q. A block of mass $1\text{kg}$ slides down on a rough inclined plane of inclination ${60}^{\circ }$ starting from its top. If the coefficient of kinetic friction is 0.5 and the length of the plane is $1\text{m}$, then work done against friction is (Take $g=9.8{\text{m/s}}^2$)

1. $9.82\text{J}$
2. $4.94\text{J}$
3. $2.45\text{J}$
4. $1.96\text{J}$

Q.

In a children's park, there is a slide which has a total length of 10 m and a height of St m (figure 8-E3). Vertical ladder are provided to reach the top. A boy weighing 200 N climbs up the ladder to the top of the slide and slides down to the ground. The average friction offered by the slide is three tenth of his weight. (a) the work done by the boy on the ladder as he go: up, (b) the work done by the slide on the boy as he cornea down, (c) the work done by the ladder on the boy as he goes up. Neglect any work done by forces inside the body of the boy.

Q.

A block of mass m is kept over another block of mass M and the system rests on a horizontal surface (figure 8-E1). A constant horizontal force F acting on the lower block produces an acceleration $\frac{F}{2(m+M)}$ in the system, the two blocks always move together. (a) Find the coefficient of kinetic friction between the bigger block and the horizontal surface. (b) Find the frictional force acting on the force of friction on the smaller block by the bigger block during a displacement d of the system.

Q.

A point particle of mass m, moves along the uniformly rough track PQR as shown in the figure. The coefficient of friction, between the particle and the rough track equals $\mu$. The particle is released from rest, form the point P and it comes to rest a point R. The energies lost by the ball, over the parts PQ and QR of the track, are equal to each other, and no energy is lost when particle changes direction from PQ to QR. The values of the coefficient of friction $\mu$ and the distance x (= QR), are respectively close to

1. 0.2 and 3.5 m

2. 0.29 and 3.5 m

3. 0.2 and 6.5 m

4. 0.29 and 6.5 m

Q.

Does friction act when an object is at rest?

Q. A block of mass $10\text{kg}$ is put gently on a belt-conveyor system of infinite length at $t=0$, which is moving with constant speed $20\text{m/s}$ towards right at all time. A constant force of magnitude $15\text{N}$ is applied on the block continuously during its motion. Work done by the kinetic friction on the block of mass $10\text{kg}$ is

1. $1250\text{J}$
2. $-1250\text{J}$
3. $2500\text{J}$
4. $-2500\text{J}$

Q. A $10\text{kg}$ block placed on a rough horizontal floor is being pulled by a constant force $F=100\text{N}$ acting at angle ${37}^{\circ }$. The coefficient of kinetic friction between the block and the floor is $0.4$. Find the total work done due to force $F$ acting on the block over the displacement of $5\text{m}$.

1. $320\text{J}$
2. $400\text{J}$
3. $480\text{J}$
4. $360\text{J}$

Q. A block of mass $5\text{kg}$ slides down on a rough inclined plane of coefficient of friction $0.75$ having angle of inclination ${37}^{\circ }$ as shown in the figure. Initially if the block is at rest on the top of inclined plane of length $2\text{m}$, then the work done against the friction in moving the block to the bottom of inclined plane is (Take $g=10{\text{m/s}}^2$).

1. $0\text{J}$
2. $10\text{J}$
3. $-5\text{J}$
4. $5\text{J}$

Q.

A car weighing 1400 kg is moving at a speed of 54 km/h up a hill when the motor stops. If it is just able to reach the destination which is at a height of 10 m above the point, calculate the work done against friction (negative of the work done by the friction).

Q.

A block of mass 250 g slides down an incline of inclination ${37}^{\circ }$ with a uniform speed. Find the work done against the friction as the block slides through 1.0 m.

Q. A block of mass $m$ is placed on an inclined plane which is moving with constant velocity $v$ in horizontal direction as shown in figure. Then find out work done in ground frame by the friction in time $t$, if the block is at rest with respect to the inclined plane.

1. $-mgvt\sin \theta \cos \theta$
2. $-mgvt{\cos }^2\theta$
3. $mgvt\sin \theta \cos \theta$
4. $-mgvt\sin \theta$

Q. A block of mass $1.5\text{kg}$ rests on a rough horizontal surface. A horizontal force applied to the block increases uniformly from $0$ to $15N$ in $5$ seconds. The velocity (in $m/s$) of the block after $5$ seconds is (given ${\mu }_s=0.6,{\mu }_k=0.5andg=10{m/s}^2$

Q. Block $A$ weighs $4kg$ and block $B$ weighs $8kg$. Coefficient of kinetic friction between all surfaces in contact is $0.2$. The force $F$ necessary to drag the block $B$ at constant speed if $A$ is held at rest as shown in the figure is
(Take $g=10m/s^2$)

1. $12\text{N}$
2. $32\text{N}$
3. $16\text{N}$
4. $24\text{N}$

Q. A block of mass $50kg$ moves on a rough horizontal surface with a coefficient of kinetic friction µ = 0.5. If the distance traveled is $20m$, how much work is done by the friction force? (take g = 10 $m/s^{\mathrm{2)}}$

1. $5000J$
2. $-5000J$
3. $-1000J$
4. $1000J$

Q. Consider the situation as shown in figure. A constant force acting on the lower blocks produces an acceleration of $4{\text{m/s}}^2.$ The two blocks always move together. Find the work done by static friction on the upper block during displacement $2\text{m}$ of the system.

1. $8\text{J}$
2. Zero
3. $16\text{J}$
4. $10\text{J}$

Q.

A large slab of mass 5 kg lies on a smooth horizontal surface, with a block of mass 4 kg lying on top of it. The coefficient of friction between the block and the slab is 0.25. If the block is pulled horizontally by a force of F = 6 N, the work done by the force of friction on the slab, between the instants t=2 s and t=3 s, is (g = 10 m$s^{-2}$)

1. 10 J

2. 2.4 J

3. 5.55 J

4. 4.44 J

Q.

A block of mass 5.0 kg slides down from the top of an inclined plane of length 3 m. The first 1 m of the plane is smooth and the next 2 m is rough. The block is released from rest and again comes to rest at the bottom of the plane. If the plane is inclined at ${30}^{\circ }$ with the horizontal, find the coefficient of friction on the rough portion.

1. 

2. 

3. 

4. 

Q.

Two masses $M_1$ and $M_2$ are connected by a light rod and the system is slipping down a rough incline of angle $\theta$ with the horizontal. The friction coefficient at both the contacts is $\mu .$ Find the acceleration of teh system and the force by the rod on one of the blocks.

Q. In a figure shown below, the work done by force of friction on the block is

1. Zero
2. Positive
3. Negative
4. cannot be determined

Q. A block of mass $m$ is moving down with constant velocity along an inclined plane of inclination $\theta$. What is the work done by an external force (applied parallel to the inclined plane) in pulling the block up the inclined plane through a height ${}^{\prime }{h}^{\prime }$ with constant velocity?

1. $3mgh$
2. $2mgh$
3. $mgh$
4. $mgh\sin \theta$

Q. A block of mass $3\text{kg}$ is kept on a $6\text{kg}$ block and is kept on a smooth surface. The coefficient of static friction between them is $0.3$. A force $F$ is applied on the upper $3\text{kg}$ block and it is on the verge of slipping when the system moves together for a distance of $1\text{m}$. Calculate the work done by static friction on $3\text{kg}$ and $6\text{kg}$ respectively. (Take $g=10{\text{m/s}}^2)$

1. $0\text{J};0\text{J}$
2. $18\text{J};-18\text{J}$
3. $-9\text{J};9\text{J}$
4. None

Q.

The force which needs to be overcome at the instant an object starts moving from rest is?

Q. In the arrangement shown in figure $m_A=4.0\text{kg}$ and $m_B=1.0\text{kg}.$ The system is released from rest and block $B$ is found to have a speed $0.3\text{m/s}$ after it has descended through a distance of $1\text{m}$. Find the coefficient of friction between the block $A$ and the table. Neglect friction elsewhere. (Take $g=10m/s^2$)

1. $0.231$
2. $0.501$
3. $0.110$
4. $0.23$

Q. A block of mass $3\text{kg}$ is kept on a $6\text{kg}$ block and is kept on a smooth surface. The coefficient of static friction between them is $0.3$. A force $F$ is applied on the upper $3\text{kg}$ block and it is on the verge of slipping when the system moves together for a distance of $1\text{m}$. Calculate the work done by static friction on $3\text{kg}$ and $6\text{kg}$ respectively. (Take $g=10{\text{m/s}}^2)$

1. $-9\text{J};9\text{J}$
2. $0\text{J};0\text{J}$
3. $18\text{J};-18\text{J}$
4. None

Q. A block of mass $10\text{kg}$ lies on a rough plane as shown in the figure. A constant force $F$ is applied for $10$ seconds, find out the value of work done by the force $F$.

1. $10000\text{J}$
2. $-6000\text{J}$
3. $0\text{J}$ (because friction is present)
4. $12000\text{J}$

Q. A block of mass $10\text{kg}$ lies on a rough plane as shown in the figure. A constant force $F$ is applied for $10$ seconds, find out the value of work done by the force $F$.

1. $12000\text{J}$
2. $10000\text{J}$
3. $-6000\text{J}$
4. $0\text{J}$ (because friction is present)

Q.

Which of the following statements is/are true

1. Work done by friction is always negative

2. Static friction always does negative work

3. If you change the reference frame, then friction can do positive, negative, or zero work depending upon what reference frame you chose

4. Static friction can never do work