FRICTION QUESTIONS
Question
a. List the five laws of friction [5]
b. Give any three characteristics of a friction force [3]
c. State any three applications of friction in a workshop [3]
d. List any three disadvantages of friction [3]
e. List any three method of reducing friction [3]
f. Give any the factors that affect the value of coefficient of friction [3]
g. With the aid of diagrams, explain angle of friction and angle of repose [6]
Question
It requires a horizontal force of 200N to drag a body of mass 100 kg. If the mass is increased by 10 kg,
what will be the horizontal force needed to keep the body moving steadily. [5]
[Answer: P = 220 N]
Question
A pull of 25N at 30º to the horizontal is necessary to move a block of wood on a horizontal table. If
the coefficient of friction between the bodies in contact is 0.2, what is the weight of the block? [5]
[W= 120.75 N]
Question
A body is pulled along a horizontal floor by a rope at 45° to and above the floor. The force in the rope
is 1.5kN. Calculate the coefficient of friction between the body and the floor if the mass of the body is
250kg. [5]
[Answer: µ = 0.762]
Question
A stores person pushes a 30kg trolley up a 10 degree incline with a force of 85N. Calculate the
coefficient of friction if the trolley is pushed at a constant speed up the slope. [5]
[Answer: µ = 0.117]
Question
Calculate the magnitude of an upward force P parallel to the inclined plane that is required to pull a
body of 250kg mass inclined on a plane at 20° to the horizontal. Take µ as 0.26. [5]
[Answer: P = 1 438 N]
Question
A body of mass 25kg is subjected to a force P acting parallel to and down the plane. If µ = 0.3, determine
the value of P. [5]
[Answer: P = 7.6 N]
Question
A casting of mass 10kg rests on a sloping surface making an angle of 13° with the horizontal. The
coefficient of friction is 0.5. Calculate the force parallel to the slope which has to be applied to move
the casting:
a. Upwards against the slope [8]
b. Downwards against the slope [7]
[Answers: i) P = 69.86 N ii) P = 25.73 N]
Question
An 80 kg mass rests on a plane inclined at 24º to the plane above the plane. If the coefficient of friction
between the plane and the mass is 0.65, calculate:
(i) Force required to pull the mass up the plane [6]
(ii) The force required to pull the mass down the plane [6]
[Answers: i) P = 785.225 N ii) P = 146.811 N]
Question
A 340 N force parallel to an inclined plane is required to just move a body of mass 50kg up the plane
which is inclined at 15° above the horizontal. Calculate
i. The coefficient of friction [7]
ii. The force parallel to the plane required to drag a body down the plane at a constant speed [5]
[Answers: i) µ = 0.45 ii) P = 86.254 N]
Question
A force of 650N parallel to a plane inclined at 30° to the horizontal is sufficient to pull up a body at
uniform speed. If the body’s mass 100kg, calculate the following:
i. The coefficient of friction between the body and plane [6]
ii. The force required to prevent it from moving down the plane [6]
[Answers: i) µ = 0.19 ii) P = 331 N]
Question
The force of 540N acting parallel to a plane inclined at 20º to the horizontal is required to just move a
body of mass 61.3 kg up the plane. Find
a. The coefficient of friction between the surfaces [5]
b. The force parallel to the plane required to drag the body down the plane at a steady speed. [6]
c. If the surface of the plane can be lubricated to alter the value of the coefficient of friction,
what should this value be for the body to be just on the point of moving down under its own
weight? [4]
[Answers: i) µ = 0.5917 ii) P = 129N iii) µ = 0.364]
Question
A carriage of mass one tonne is to be pulled up a track inclined at 30º to the horizontal by a force P
inclined at 20º to and above the ground. Calculate the value of P if the effective coefficient of friction
is 0.15. [12]
[Answer: P = 6.236 KN]
Question
A body of 4 kg rests on an inclined plane and it is found that in order to move the body up the incline,
a force of 15 N acting parallel to the plane is required. To move the body down the plane, a force of 5
N acting parallel to the plane is required. Calculate the inclination of the plane. [15]
[Answer: θ = 7.3°]
Question
Determine a horizontal force that is required to prevent the body from sliding down the plane due to its
own weight when it has a mass of 250 kgs. µ=0.26 [10]
[Answer: P = 232.6N]
Question
A body of weight 200 N is at rest on an inclined plane as shown in the figure below. A force P is applied
horizontally as shown. If the coefficient of friction is 0.5, find the range of values of P over which the
body will remain at rest. [15]
200N
30°
[Answer: P values range between 12N and 302.93N]
Question
A body of weight 50N is at rest on an inclined plane as shown in the figure. A force P is applied
horizontally as shown. If the coefficient of friction is 0.4, find the range of values of P over which the
body will remain at rest. [15]
50N
30°
[Answer: P values range between 7.205N and 63.544N]
Question
It requires a horizontal force of 375N to drag a truck of mass 300kgs at a uniform speed along a level
track. Calculate the force required to move the track if the force is inclined at 40º to the horizontal and
is:
a. A push in the track
b. A pull away from the track [15]
[Answers: a) 548.17 N b) 442.27 N]
Question
A body resting on a rough horizontal plane required a pull of 18N inclined at 30º to the plane just to
move it. It was found that a push of 22N inclined at 30º to the plane just moved the body. Determine
the weight of the body and the coefficient of friction. [15]
[Answer: w = 99 N and µ = 0.1732]
Question
A body of mass m on a rough plane inclined at 20° to the horizontal is moved steadily up the plane by
a force of 200N applied upwards and parallel to the plane. When the force is reduced to 75N, the body
slides steadily downwards. Find the values of mass and coefficient of friction. [15]
[Answers: m = 41kg and µ = 0.17]
Question
A body which weighs 20N, is placed upon a plane inclined at 30° to the horizontal. Find the normal and
frictional force exerted by the plane to maintain equilibrium if the following forces are applied
separately in turn.
i. A force of 15N directly up the plane [2]
ii. A horizontal force of 10N tending to produce motion up the plane [2]
iii. A force of 20N at 30° to the vertical tending to produce motion up the plane [2]
iv. A similar force as in (iii) tending to produce motion down the plane [2]
v. A force of 10N normal to the plane tending to lift the body off the plane [2]
Question
A load of 51 kg is pulled along a level floor by a force inclined at an angle α to the floor. The
coefficient of friction between load and floor is 0.3. If the pull is to be 300N, find angle α which
the force must make with the horizontal. If the pull is to be the least possible, find its value.
[Answers: α = 78° above and 44° below, P = 144 N]
