Friction

 

Friction
• Definition, types of friction and laws of friction.
• Advantages and disadvantages etc.

At the end of this lesson, the students should be able to:
• Identify a force resisting the motion between two surfaces in contact and moving relative to each other.
• Define friction
• State the advantages and disadvantages of friction.
• Reduce friction in a given situation

WEEK 5

TOPIC: FRICTION

SUB – TOPIC:

Definition, Types of friction and laws of friction.

Advantages and disadvantages etc.

FRICTION

This is the force which acts to oppose motion at the surface separating two objects in contact. Friction always acts to oppose motion i.e. direction opposite to the motion.

TYPES OF FRICTION 

1. Static or limiting friction: is the maximum force that must be overcome before a body can just start to move over another. 

2. Kinetic or dynamic friction: is the force that must be overcome so that a body can move with uniform speed over another.

LAWS OF SOLID FRICTION

1. Friction opposes the relative motion between two surfaces in contact.

2. Frictional force increases to the same extent as the force which tends to start the motion.

3. Frictional force is independent of the areas of the surfaces in contact.

4. Solid friction depends on the nature of the surfaces in contact.

5. Coefficient of static friction μs is always greater than the coefficient of kinetic friction, μk.

6. Solid friction depends on the normal reaction (i.e. the normal force pressing the surfaces together) i.e. F∝R so that F=μR

Where μ is known as the coefficient of friction, therefore

Coefficient of friction μ= (frictional force, F)/(normal reaction between the two surfaces in contact, R) where

Coefficient of static friction, μs is greater than the coefficient of dynamic or kinetic friction μ_k

CLASSWORK

I. Define coefficient of static friction.

II. A rectangular block rest on a horizontal surface. Explain why a minimum force is required to move the block along the surfaces. State whether this force will be larger, smaller or unchanged when

(a) The block is turned on to another side such that a bigger area is in contact with the surface;

(b) A film of oil is placed between the block and the surface.

ASSIGNMENT

State the laws of solid friction.

Describe an experiment to determine the coefficient of static friction between two surfaces.

FRICTION 

CALCULATION EXAMPLES

1. A block of wood weighing 4N is placed on a horizontal table. It is then pulled by means of a spring balance attached to one of its ends. The block just begins to move when the spring balance records the horizontal force of 2.5N.

(i) What is the coefficient of static friction?

(ii) What is the frictional force when a horizontal force of 2.0N is recorded by the spring balance?

SOLUTION 

(i) Limiting frictional force F = 2.5 N

Normal reaction R = 4 N

μ = F/R = 2.5N/4N = 0.625

(ii) When the applied force is 2.0 N the block will not move because 2.0 N is less than the limiting friction of 2.5 N. the applied force less than limiting frictional force is always balanced by opposing frictional force.

Hence frictional force = 2.0 N

CLASSWORK

1. A metal block of mass 5kg lies on a rough horizontal platform. If a horizontal force of 8N applied to the block through its centre of mass just slides the block on the platform, then the coefficient of limiting friction between the block and the platform is:

A. 0.16  B. 0.63 C. 0.80 D. 1.6 E. 2.00

ASSIGNMENT

A body of mass 25kg, moving at 3ms^-1 on a rough horizontal floor is brought to rest after sliding through a distance of 2.5m on the floor. Calculate the coefficient of sliding friction (g=10.0ms^-2)

ADVANTAGES AND DISADVANTAGES OF FRICTION

Friction has both desirable and undesirable effects. 

Advantages of friction and its Uses

Friction is desirable for a number of reasons:

1. Friction allows for our locomotion. Without friction walking is impossible because we would slip and fall without it.

2. Friction allows for a firm grip of automobile tyres on the roadway.

3. Friction is utilized in fan belt used over wheels and pulleys in machinery.

4. Friction enables a screw or a nail to remain in place after being screwed into position.

5. Friction enables the brakes to stop the car and locomotive to pull the train.

6. Friction is used in the grindstone to sharpen knives and chisels

7. Friction enables us to ball a ladder against the wall without slipping

Disadvantages of friction

1. Friction leads to much wear and tear on moving parts of machines.

2. Friction causes loss of energy (in the form of heat and noise) of machines, and thus reduces their efficiency.

3. Friction causes heating of engines.

CLASSWORK

State 

1. Two advantages of friction
2. Two disadvantages of friction
3. Two methods of reducing friction

METHODS OF REDUCING FRICTION 

Three chief ways of reducing friction are:

By lubrication: The use of lubricants like oil, grease, air, and graphite

The use of a ball and roller bearings, and 

The streamlining the body shapes of moving objects.

Reduction of angle of Banking of curves on roads

CLASSWORK

1. What type of force is frictional force? Explain.

2. Differentiate between centripetal and centrifugal force.

ASSIGNMENT

Explain how banking of curves on roads help to reduce the chances of skidding off and thus derive the equation for calculating the angle of banking.