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Static Friction - Definition, Laws, FAQs

Static Friction - Definition, Laws, FAQs

Edited By Vishal kumar | Updated on Sep 24, 2024 04:13 PM IST

Static friction is the force that resists the initiation of motion between two surfaces in contact. Unlike kinetic friction, which acts when objects are already moving, static friction is what keeps an object at rest despite the presence of an external force. It plays a crucial role in everyday life, from preventing your car from sliding on a hill to ensuring that a book stays put on a slanted desk. Understanding static friction is essential in fields like engineering and physics, where the ability to predict and control motion is key to designing safe and efficient systems. For instance, when you push a heavy piece of furniture, the resistance you initially feel is due to static friction, and only once this force is overcome does the object begin to move.


Static friction is the opposing force that is set up between the surfaces of contact of the two bodies when one body tends to slide over the surface of another body.

Static friction

  1. It occurs when there is a tendency of relative motion, i.e., the body is still at rest and is just about to move.

  2. When two bodies do not slip over each other, then the force of friction is called static friction.

  3. It is a variable force or self-adjusting force as it changes itself according to the applied force.

  4. It is denoted by fs and the static friction is in between:- 0<fs<fl

where fl is limiting friction.

5. Limiting friction is the maximum static friction that a body can exert on the other body in contact with

It is given by

FlαR or fl=μsR

fl= limiting friction μs= coefficient of friction R= reaction force

6. Generally,

fK<Fl∴μK<μs

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Solved Example Based on Static Friction

Example 1: Given in the figure are two blocks A and B of weight 20 N and 100 N, respectively. These are being pressed against a wall by a force F as shown. If the coefficient of friction between the blocks is 0.1 and between block B and the wall is 0.15, the frictional force (in N) applied by the wall on block B is :

1) 120

2) 80

3) 100

4) 150

Solution :

Given :
Weights of blocs A=20 N
Weight of block B=100 N
As the blocks are at rest, both blocks must be in equilibrium.
Let the Friction force between the blocks be f1 and between block B and wall be f2
F.B.D of the blocks :

From F.B.D

F=Nf1=20f2=f1+100⇒f2=20+100=120N
Example 2. A block rests on a rough inclined plane making an angle of 30o with the horizontal. The coefficient of static friction between the block and the plane is 0.8. If the frictional force on the block is 10 N, the mass of the block (in kg) is (take g = 10 m/s2 )


1) 2.0

2) 4.0

3) 1.6

4) 2.5

Solution

Static Friction

Self-adjusting force because it changes itself according to the applied force.

wherein

It is always equal to a net external force. Static friction (F = P) using diag.

For equilibrium of block,

f=mgsin⁡Θ∴10=m×10×sin⁡30∘ or m=2 kg

Hence, the correct option is 1.

Qu 3. An automobile, travelling at 40 km/h, can be stopped at a distance of 40 m by applying brakes. If the same automobile is travelling at 80 km/h, the minimum stopping distance in metres is (assume no skidding) :

1) 160

2) 100

3) 150

4) 75

Solution

Limiting Friction

The magnitude of limiting friction between any two bodies in contact is directly proportional to the normal reaction between them.

FlαR or fl=μsRfl= limiting friction μs= coefficient of friction R= reaction force

The maximum value of static friction is limiting friction.

a=μgs=u22a=u22μgsαu2s2s1=(u2u1)2=(8040)2=4s2=160m(∵s1=40m)
Hence, the answer is 160.

Example 4. A disc rotates about its axis of symmetry in a horizontal plane at a steady rate of 3.5 revolutions per second. A coin placed at a distance of 1.25 cm from the axis of rotation remains at rest on the disc. The coefficient of friction between the coin and the disc is: (g=10 m/s2)

1) 0.6

2) 0.3

3) 0.7

4) 0.5

Solution

Limiting Friction

The magnitude of limiting friction between any two bodies in contact is directly proportional to the normal reaction between them.

FlαR or fl=μsRfl= limiting friction μs= coefficient of friction R= reaction force

wherein

* The maximum value of static friction is limiting friction.

* Direction is always opposite to relative motion.

In equilibrium,

mω2r=μmgμ=ω2rg(ν=3.5rev/sec)ω=2Πv=7Πrad/s=7∗227=22rad/sr=1.25 cm;g=10μ=222∗1.25∗10−210=0.6

Qu 5. A block of mass 4 kg, kept on a rough surface is being pulled by applying a horizontal force of 5N, as shown in the figure. The coefficient of static friction between the block and the surface is 0.2. The friction force acting on the block is-

1) 8N

2) 5N

3) 2N

4) 10N

Solution:

Given
mass of the block, m=4 kg,
Coefficient of the static friction, μs=0.2
Driving force on the block, F=5 N
Let the friction force on the block be f as shown in the diagram

From F.B.D

N=mg
Limiting friction-

fl=μsNfl=μsmgfl=0.2×4×10=8N⇒F<fl
As the applied driving force on the block is less than the limiting friction the block will remain at rest. In such cases static friction acts of magnitude equal to the applied force.

⇒f=F=5N

Conclusion

The maximum value of the force of friction which comes into play before a body just begins to slide over the surface of another body is called the limiting value of static friction.

Frequently Asked Questions (FAQs)

1. Define Static friction

Static friction is friction between two or more constituent objects that are not moving respect to one other. For example, static friction can prohibit an object from tumbling down an inclined surface. A force that holds an object at rest is known as static friction. The friction encountered when persons attempt to move a stationary object on a surface without actually causing any relative motion between the body and the surface it is on is known as static friction.

2. What are the laws of static friction?

According to the law of static friction, the force of friction that resists a body's motion when it begins to descend down a surface is proportional to the normal which is perpendicular force that the body exerts on the surface. The following are the empirical rules of friction, Friction is independent of the contact surface.

  • The frictional force always acts opposite to the tendency of the motion on the body that is resting over the surface of a fixed body in the direction tangential to the surface in contact under the action of external force.

  • Friction always acts in the direction tangential to the common surfaces of contact between the two bodies.

  • The frictional force prevents the motion of the body when the applied force is less than the limiting frictional force.

  • At the start of the motion, the net externally applied force in the direction of motion is equal to the maximum frictional force, which is called limiting force.

  • The normal reaction is proportional to the magnitude of the limiting friction.

  • Limiting friction is independent of the contact area for the case when the normal reaction remains constant.

  • Limiting friction depends on materials of the two surfaces of the bodies in contact and roughness between the surfaces.

3. What are the difference between static friction, limiting friction and dynamic friction?

Difference between static friction, limiting friction and kinetic friction:

  • Static friction is friction that occurs when a body is immovable. 

  • The greatest value of static friction is known as limiting friction. It is the amount of force required for the body to begin moving. 

  • The frictional force that acts on a moving body is known as kinetic friction.

4. Define Static force.

The frictional force opposes the force given to an object in static friction, and the object remains at rest until the static friction force is overcome. The contact surface between the bridge and the car is subjected to significant force and pressure when a car is parked on it. The car or the bridge do not change their status, location, or shape as a result of this force. As a result, the force that exists between the bridge and the car is referred to as static force.

5. Define Static motion.

Static motion:

Each of these two contradictory phrases ceases to exist or be understood without its antithesis; each of these two conflicting terms fails to exist or be understood without its antithesis. Static Motion, as the title suggests, examines the complicated and cryptic nature of art-making via the lens of two artists whose works are created using quite different methods.

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