Introduction to Dry Friction

Q. A block R of mass 100 kg is placed on a block S of mass 150 kg as shown in the figure. Block R is tied to the wall by a massless and inextensible string PQ. If the coefficient of static friction for all surfaces is 0.4, the minimum of force F(in kN) needed to move the block S is

1. 0.69
2. 0.88
3. 0.98
4. 1.37

Q. Which of the following statements regarding laws governing the friction between dry surfaces are correct?

1. The friction force is dependent on the velocity sliding.
2. The friction force is directly proportional to the normal force.
3. The friction force is dependent on the materials of the contact surfaces.
4.The friction force is independent of the area of contact

Select the correct answer using the codes given below

1. 2, 3 and 4
2. l and 3
3. 2 and 4
4. 1 , 2, 3 and 4

Q. A block of mass M is released from point P on a rough inclined plane with inclination angle $\theta$, shown in the figure below. The coefficient of friction is $\mu$. if $\mu <\tan \theta ,$ then the time taken by the block to reach another point Q on the inclined plane, where PQ= s, is

1. $\sqrt{\frac{2s}{g\cos \theta (\tan \theta -\mu )}}$
2. $\sqrt{\frac{2s}{g\cos \theta (\tan \theta +\mu )}}$
3. $\sqrt{\frac{2s}{g\sin \theta (\tan \theta -\mu }}$
4. $\sqrt{\frac{2s}{g\sin \theta (\tan \theta +\mu }}$