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Friction is the force which opposes motion.
It acts in the opposite direction to that of the force causing motion.
Types of friction
These are:
a) Static friction b) Sliding or dynamic friction c) Viscosity
a) Static Friction:
Is friction which opposes motion between two surfaces in contact at rest.
It prevents motion.
Experiment for measurement of static friction
i) Adding weight on pan
Know weights are added to the scale pan until the block moves with a uniform velocity.
ii) The above will occur when weight of scale pan + weights on the pan (loads) = maximum friction, F. This maximum friction is called limiting friction.
iii) Repeating
The procedure is repeated by increasing weight of the block “W” and obtaining the corresponding “F”
iv) Plotting a graph
A graph of F against W (= normal reaction “R”) is plotted .and its slope = coefficient of static friction 𝛍..
Thus Coefficient of static friction μ is the ratio of limiting frictional force F to the normal reaction R. i.e μ = F/R
Note: Coefficient of friction μ has no units.
Example
Calculate the static friction when a body of mass 6kg rests on a surface; given that coefficient of friction of surface is 0.5.
Given: m= 6kg; μ=0.5; g = 10ms-2
From; Frictional force,F=μR But R=W=mg =6(10) R =60N
Then From; F=μR
F=0.5(60)
F= 30N
b) Sliding or kinetic or dynamic friction:
Is friction which opposes relative motion between two surfaces in contact and in motion.
It is the force which opposes the motion of one surface moving with uniform velocity over another surface.
It slows down motion.
Experiment for measurement of static friction
i) Adding weight on pan
Known weights are added to the scale pan and a small push is given each time until the block move with a uniform velocity.
ii) The above will occur when weight of scale pan + weights on the pan (loads) = maximum friction, F. This maximum friction is called limiting friction.
iii) Repeating
The procedure is repeated by increasing weight of the block “W” and obtaining the corresponding frictional force F’.
iv) Plotting a graph
A graph of “F” against R is plotted and its slope = coefficient of sliding friction 𝛍..
c) Viscosity:
Is the opposition to the relative motion between layers of a fluid.
Advantages of friction
i) It enables bodies to come to rest
ii) It enables bodies to move without sliding
iii) It helps in writing
iv) It helps in making fire
Disadvantages of friction
i) causes unnecessary heat
ii) reduces the efficiency of machines
iii) Causes things like tyres, soles of shoes to wear out.
iv) Causes parts of machines to break.
v) Causes un necessary noise
Reduction of friction
i) Lubrication using oil or grease
ii) Using ball bearings or rollers
Laws of friction
-The limiting does not depend on the surface area in contact but depends on the nature of the face contact.
-Frictional force is not affected by velocity with which a body moves for sliding friction.
-Friction always opposes motion
-Limiting friction is proportional to normal reaction which is equal and opposite to the weight of the body.
-Friction always increases from zero to maximum value called limiting friction.