Friction Questions and Answers Part-1

1. A uniform rope of length l lies on a table. If the coefficient of friction is \[\mu\], then the maximum length \[l_{1}\] of the part of this rope which can overhang from the edge of the table without sliding down is
a) \[\frac{l}{\mu}\]
b) \[\frac{l}{\mu+1}\]
c) \[\frac{\mu l}{1+\mu}\]
d) \[\frac{\mu l}{1-\mu}\]

Answer: c

2. Which of the following statements is not true
a) The coefficient of friction between two surfaces increases as the surface in contact are made rough
b) The force of friction acts in a direction opposite to the applied force
c) Rolling friction is greater than sliding friction
d) The coefficient of friction between wood and wood is less than 1

Answer: c
Explanation: Sliding friction is greater than rolling friction

3. A block of 1 kg is stopped against a wall by applying a force F perpendicular to the wall. If \[\mu =0.2\]  then minimum value of F will be
a) 980 N
b) 49 N
c) 98 N
d) 490 N

Answer: b

4. A heavy uniform chain lies on a horizontal table-top. If the coefficient of friction between the chain and table surface is 0.25, then the maximum fraction of length of the chain, that can hang over one edge of the table is
a) 20%
b) 25%
c) 35%
d) 15%

Answer: a

5. The blocks A and B are arranged as shown in the figure. The pulley is frictionless. The mass of A is 10 kg. The coefficient of friction of A with the horizontal surface is 0.20. The minimum mass of B to start the motion will be
a) 2 kg
b) 0.2 kg
c) 5 kg
d) 10 kg

Answer: a

6. Work done by a frictional force is
a) Negative
b) Positive
c) Zero
d) All of the above

Answer: d
Explanation: Work done by friction can be positive, negative and zero depending upon the situation

7. A uniform chain of length L changes partly from a table which is kept in equilibrium by friction. The maximum length that can withstand without slipping is l, then coefficient of friction between the table and the chain is
a) \[\frac{l}{L}\]
b) \[\frac{l}{L+1}\]
c) \[\frac{l}{L-1}\]
d) \[\frac{L}{L+1}\]

Answer: c

8. When two surfaces are coated with a lubricant, then they
a) Stick to each other
b) Slide upon each other
c) Roll upon each other
d) None of these

Answer: b
Explanation: Surfaces always slide over each other

9. A 20 kg block is initially at rest on a rough horizontal surface. A horizontal force of 75 N is required to set the block in motion. After it is in motion, a horizontal force of 60 N is required to keep the block moving with constant speed. The coefficient of static friction is
a) 0.38
b) 0.44
c) 0.52
d) 0.60

Answer: a

10. A block A with mass 100 kg is resting on another block B of mass 200 kg. As shown in figure a horizontal rope tied to a wall holds it. The coefficient of friction between A and B is 0.2 while coefficient of friction between B and the ground is 0.3. The minimum required force F to start moving B will be
a) 900 N
b) 100 N
c) 1100 N
d) 1200 N

Answer: c