Engineering Mechanics – Assignment 4

1. Friction is a force that exists between two bodies in contact. It is tangent to the contact sur

face between thebodies and opposes the sliding motion between them. True or False?

a. True

b. False

c. Cannot be determined

Ans: a

2. A body of weight 50N is stationary on the ground even after the application of a horizont

al force of 10N. The coefficient of friction is 0.2. True or False?

a. True

b. False

c. Cannot be determined

Ans: c

3. A frictional force is proportional to the normal force between contacting bodies and to the

area of the contact between the bodies. True or False?

a. True

b. False

c. Cannot be determined

Ans: b

4. The coefficient of static friction is the ratio of the static frictional force to the normal forc

e for bodies in contact.True or False?

a. True

b. False

c. Cannot be determined

Ans: b

5. The coefficient of kinetic friction is the ratio of the frictional force to the normal force for

bodies in contact and sliding relative to one another. True or False?

a. True

b. False

c. Cannot be determined

Ans: a

6. The kinetic frictional force is always larger than the maximum static frictional force. True

or False?

a. True

b. False

c. Cannot be determined

Ans: b

7. Two blocks are connected by a light, flexible cable that passes a frictionless pulley. Block

A weighs 500N and block B weighs 200N. The coefficient of static friction between A

and the ramp is 0.30. Which of the following is correct?

a. Two blocks are in equilibrium

b. A is moving upwards

c. A is moving downwards

Ans: a

8. What is the tension in the cable?

Ans: 200 N

9. If the cable is now cut, check all that apply.

a. A would side downwards

b. A would not slide

c. The frictional force would be 120 N

d. The frictional force cannot be determined with the data provided

Ans: a, d

10. A wedge A and a block B are subject to a load Q (1000 N) and a force P. Ignore the

weights of A and B. The coefficients of static friction for surfaces 1, 2 and 3 are 0.1, 0.2

and 0.3 respectively. The angle of the wedge A is 30 degrees. What is the maximum

value of P so that B doesn’t move? (answer will be deemed correct if it lies within some

range)

Ans: 1329 N

11. What is the minimum value?

Ans: 216.3 N

12. The boxes A and B are connected by a flexible cord that passes over a frictionless pulley.

A pull T is exerted on box B. Box A weighs 600 N and box B weighs 900 N. The

coefficients of static and kinetic friction between the bodies and the planes are 0.5 and 0.2

respectively. What is the value of T (in Newton) when B just starts slipping? (answer will

be deemed correct if it lies within some range)

Ans : 576N

13. If B is sliding downwards at a constant speed, what is the value of T?

Ans: 215 N

14. Three flat blocks are positioned on an incline at 30 degrees as shown. A force P is applied

to the middle block. The upper block is prevented from moving by a wire. The

coefficients of static friction between the surfaces are as shown in the figure. What is the

maximum value of P (in Newton) such that no slipping occurs? Take value of g to be 9.81

m/s^2.

Ans : 93.8 N

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