Friction is a kind of force that opposes the motion of two objects towards each other. It is a contact force; when two objects are in contact, they experience friction.
Static friction, as the name suggests, means at rest, so static friction acts on objects between two surfaces that are at rest relative to each other. It is a form of dry friction occurring between solid surfaces that aren't moving in relation to one another. The coefficient of static friction, denoted as μs, is a scalar quantity and describes the strength of friction when the object is not moving.
Formula of Coefficient of Static Friction
The coefficient of static friction can be calculated with the formula: -
μs = F /N
Where,
F = Static force of friction
μs = coefficient of static friction
N = Normal force
Laws of Static Friction
- In static friction, the maximum force is independent of the area of contact.
- The normal force is comparative to the maximum force of static friction it means that, if the normal force increases, the maximum external force that the object can possess without moving, also increases.
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Solved Examples of Static Friction
A force of 400 N is exerted on a box of 10 kg still on the floor. If the coefficient of friction is 0.3, what is the value of static friction?
Given,
Force, F = 400 N,
Coefficient of friction, μs= 0.3
Static friction is given by Fs = μs Fn
= 0.3 × 400 N
Fs = 120 N.
State the laws of Static friction.
- In static friction, the maximum force is independent on the area of contact.
- The normal force is comparative to the maximum force of static friction it means that, if the normal force increases, the maximum external force that the object can possess without moving, also increases.
A box kept on the floor experience a force of 90N with a coefficient of static friction of 0.4. Find the friction force.
Given,
Force applied or normal force N = 90 N
Coefficient of friction = 0.4
Static friction can be calculated as: F = μs × N.
F = 0.4 × 90
= 36 N
A box of 40Kg kept on the floor experience a force of 60N horizontally and its coefficient of static friction of 0.2. Find the frictional force. Will the box move from its position.
Given,
Force applied = 60 N
Coefficient of friction = 0.2
Normal force Fn = mg = 40 × 10 = 400 N.
Static friction can be calculated as: F = μs × N.
F = 0.2 × 400
= 80 N
We can see that static frictional force i.e. 80N, is greater than applied force 60 N which means that box will remain in its position.
A Force of 50 N is exerted on the box kept on the floor with the coefficient of static friction of 0.2. Find the friction force.
Given,
Force applied or normal force N = 50 N
Coefficient of friction = 0.2
Static friction can be calculated as: F = μs × N.
F = 0.2 × 50
= 10 N
Force of 30N is exerted horizontally on the box of 20Kg kept on the floor with the coefficient of static friction of 0.3. Find the friction force. Determine the will box move or not?
Given,
Force applied = 150 N
Coefficient of friction = 0.3
Normal force Fn = mg = 30 × 10 = 300 N.
Static friction can be calculated as:F = μs × N.
F = 0.3 × 300
= 90 N
We can see that static frictional force i.e. 90N, is greater than applied force 30 N which means that box will not move from its position.
A box having a mass of 20 kg is placed on a smooth surface. Static friction between these two surfaces is given as the 30 N. Find the coefficient of static friction?
Given,
Mass of box, m = 20 kg
Friction between them, F = 30 N
Coefficient of static friction μs =?
We know that,
Normal force, N = mg
So, N = 20 × 9.81 = 196.2 N (g = 9.81)
For coefficient of static friction is,
μs = F/N
μs = 30/196.2
μs = 0.153