Department of Mechanical Engineering, MUST-FET Name:Engineering Mechanics (ME-105) Tutorial. Batch:Spring Semester 2013-2014 Roll No.:Engineering Mechanics (ME 105)

TUTORIAL SHEET-71. Define the term friction.

2. Difference between wet and dry friction

3. Differentiate between Coefficient of static friction and Coefficient of Kinetic friction

4. State the laws of dry friction.

5. Explain the following terms.

a) Coefficient of friction b) angle of friction

c) angle of repose d) cone of friction

6. How tensions on two sides are related? (Proof not required)

7. What do you mean by the term wedge?

8. Determine the minimum horizontal force P required to hold the crate from sliding down the plane and up the plane. The crate has a mass of 50 kg and the coefficient of static friction between the crate and the plane is s = 0.25.Ans: P = 140 N, From sliding down P = 474 N, From sliding up

9. Crates A and B weigh 200 N and 150 N, respectively. They are connected together with a cable and placed on the inclined plane. If the angle is gradually increased, determine when the crates begin to slide. The coefficients of static friction between the crates and the plane are A = 0.25 and B = 0.35.Ans : = 16.3, FCD = 8.23 N

10. The forces acting on the block are shown in figure. Determine whether the block is in equilibrium and find out the magnitude and direction of friction force

S = 0.35 and K = 0.25Ans: F = 33 N, The block is in equilibrium

11. Determine the distance s to which the 90 kg painter can climb without causing the 4 m ladder to slip at its lower end A. The top of the 15 kg ladder has a small roller and at the ground the coefficient of static friction is 0.25. The mass center of the painter is directly above his feet.S = 2.55 m

12. Block A weighing 1500 N is to be raised by means of 15o block B as shown in figure. Assume the coefficient of friction between all contacts surfaces be 0.2. Weight of the block B is 500 N. Determine the minimum force required to move the block A upwards.

Ans: P = 1247.2 N

13. Draw the free body diagram of crate A and wedge B hence determine the smallest horizontal force P required to lift the 200-kg crate. The coefficient of static friction at all contacting surfaces is 0.3. Neglect the mass of the wedge.P = 1.98 kN

14. What is the minimum coefficient of friction between the rope and the fixed shaft which will prevent the unbalanced cylinders shown in figure from movingAns: s = 0.221

15. Determine the force P required to (i) raise and (ii) lower the 75 kg cylinder shown in figure. The coefficient of friction between the cord and its supporting surface is 0.225

Ans: P to raise the cylinder = 1639.25 N

P to lower the cylinder = 330.23 N

16. Determine the smallest force P required to move the block B shown in figure if (i) block A is restrained by cable CD (ii) cable CD is removed

s = 0.3, K = 0.25Ans: (i) P = 1545 N (ii) P = 1103.63 N

17. Determine the horizontal force P required to cause impending motion

Ans: P = 135.37 N