# FRICTION

## DEFINITION OF FRICTION

Friction (Fr) is defined as a force which acts at the surface of separation between two objects or two bodies in contact and tend to oppose the motion of one over the other. It is simply force of opposition. We have two types of friction:

(a) Static friction, Fs

(b) Dynamic friction, Fd. Fs is greater than Fd

LAWS OF SOLID FRICTION

1. Friction opposes the relative motion of two surfaces in contact.
2. It is independent of the area of the surface of contact.
3. It depends on the nature of the surface.
4. It is proportional to normal reaction (R).
5. It is independent of relative velocity between the surfaces

Fr α R

Fr = μR………………….1.

where Fr-frictional force   μ-coefficient of friction  &  R-normal reaction

The weight ( W) of an object is acting vertically downward.. the normal reaction (R ) is always acting perpendicular to the plane.. the normal reaction is equal to the weight.

Case one: if the force  P is applied, and the object  is stationary.

P – Fr = ma

Since no motion a = 0

P – F Fr = O

P = Fr   …………………….. 3

Case two:  when the force P is applied and the body moves.

P – Fr = ma

P = Fr + ma

But Fr = μmg

P =  μmg + ma

P =  m [  μg + a ]    …………………………….. 4

For an object on a smooth inclined plane

Case one:  if the body moves upward, a > 0

P – mgsinø = ma

P = mgsinø  + ma   …………………………. 5

Case two:  if the body is stationary  a = 0

P – mgsinø = ma

P – mgsinø = 0

P  =  mgsinø   ……………………………… 6

Case three: if the body slides down the plane, a >0

mgsinø – P = ma

P = ma + mgsinø ………………………… 7

For a body on a rough inclined plane.

P – mgsinø – Fr = ma

But Fr =  μmg

P –  mgsinø – μmg = ma  ………………………. 8

Also, R = mg cosø

P –  mgsinø – μ mg cosø = ma  …………………………… 9

If the body moves upward the incline plane

μ = tanӨ………………………………….. 10

EVALUATION

1. Differentiate between static and dynamic friction.
2. State the laws governing solid friction.

(1) It makes walking and running possible.

(2) It enables gripping of belt in machines possible.

(3) It enables nails to stay in the wall when driven.

(4) It stops tires from slipping.

(5) Enable cars to stop when breaks are applied.

(6) Enables human to use mouse in surfing web.

(1) Causes wear and tear.

(2) Causes the efficiency of the machines.

(3) Causes a lot of energy to be consumed by the machine.

(4) Causes loss of resources.

## REDUCING FRICTION

1. Lubricating surfaces with grease, oil etc.
2. Using ball or roller on wheels.
3. Smoothing or polishing the surface.
4. By streamlining.

EVALUATION

2. State three (3) ways of reducing friction.

## WEEKEND ASSIGNMENT

1. Friction depends on the area of surface in contact (a) true (b) false (c) true & false (d) none of the above
2. A metal block of mass 8kg lies on a rough horizontal platform. If the horizontal resistive force is 10N, find the coefficient of static friction (g=10m/s2) (a) 0.25 (b) 0.125 (c) 0.8 (d) 0.124
3. Which of the statement is correct (a) static friction is less than dynamic friction (b) static friction equals dynamic friction (c) static friction is greater than dynamic friction (d) none of the above
4. A metal block of mass 0.5kg lies on a rough horizontal plane, what is the normal reaction (g=10m/s2)(a) 50N (b) 0.05N (c) 500N (d) 5N
5. If the angle between the incline length and the horizontal platform of an incline plane is 300 calculate the coefficient of friction (a) 0.542 (b) 0.577 (c) 0.467 (d) 0.866

## THEORY

1. Define friction and state the laws governing solid friction.
2. A body of weight 6N rest on a plane inclined at an angle of 300 to the horizontal (a) what force keeps it sliding down the plane? (b) what is the coefficient of friction?
3. State two

(i) Laws of friction

(iii) Methods of reducing friction (WAEC,2006)

• A force, 10N drags a mass 10kg on a horizontal table with an acceleration of 0.2ms-2. If the acceleration due to gravity is 10ms-2. Calculate the coefficient of friction between the moving mass and the table. (UME,1998)

MOTION IN NATURE

MEASUREMENT OF MASS

INTRODUCTION TO PHYSICS

MACHINES

SIMPLE HARMONIC MOTION