Mechanical Engineering Principles

180 MECHANICAL ENGINEERING PRINCIPLES

5. The coefficient of friction should have a
   .........value for materials concerned
   with braking systems.


6. The coefficient of dynamic or sliding
   friction is given by············


7. The coefficient of static or limiting fric-
   tion is given by············when.........
   is just about to take place.


8. Lubricating surfaces in contact result in
   a.........of the coefficient of friction.


9. Briefly discuss the factors affecting the
   size and direction of frictional forces.


10. Name three practical applications where
    a low value of coefficient of friction is
    desirable and state briefly how this is
    achieved in each case.


11. Name three practical applications where
    a high value of coefficient of friction
    is required when transmitting forces and
    discuss how this is achieved.


12. For an object on a surface, two different
    values of coefficient of friction are pos-
    sible. Give the names of these two coef-
    ficients of friction and state how their
    values may be obtained.


13. State the formula for the angle of repose.


14. What theory can be used for calculating
    the efficiency of a screw jack.

Exercise 79 Multi-choice questions on fric-

tion (Answers on page 285)

1. A block of metal requires a frictional
   forceFto keep it moving with constant
   velocity across a surface. If the coeffi-
   cient of friction isμ, then the normal
   forceNis given by:

(a)

μ

F

(b)μF

(c)

F

μ

(d)F

2. The unit of the linear coefficient of fric-
   tion is:

(a) newtons (b) radians

(c) dimensionless (d) newtons/metre

Questions 3 to 7 refer to the state-

ments given below. Select the statement

required from each group given.

(a) The coefficient of friction depends

on the type of surfaces in contact.

(b) The coefficient of friction depends

on the force acting at right angles

to the surfaces in contact.

(c) The coefficient of friction depends

on the area of the surfaces in con-

tact.

(d) Frictional force acts in the opposite

direction to the direction of motion.

(e) Frictional force acts in the direction

of motion.

(f) A low value of coefficient of fric-

tion is required between the belt and

the wheel in a belt drive system.

(g) A low value of coefficient of fric-

tion is required for the materials of

a bearing.

(h) The dynamic coefficient of friction

is given by (normal force)/(frictional

force) at constant speed.

(i) The coefficient of static friction is

given by (applied force) ÷ (fric-

tional force) as sliding is just about

to start.

(j) Lubrication results in a reduction in

the coefficient of friction.

3. Which statement is false from (a), (b),
   (f) and (i)?


4. Which statement is false from (b), (e),
   (g) and (j)?


5. Which statement is true from (c), (f), (h)
   and (i)?


6. Which statement is false from (b), (c),
   (e) and (j)?


7. Which statement is false from (a), (d),
   (g) and (h)?


8. The normal force between two surfaces
   is 100 N and the dynamic coefficient
   of friction is 0.4. The force required