Physics

Physi Video Notes

cs Video Notes

Friction

Class 11ᵗʰ
 Didn’t understand? Watch

Friction the video (Click Here)

Friction is a resistive force that opposes the relative motion or the tendency of
such motion between two surfaces in contact.

It acts tangentially to the surfaces and depends on the nature of the surfaces and
the normal force pressing them together.

It is a self-adjusting force, it can adjust its magnitude to any

value between zero and the limiting (maximum) value
i.e 0 ≤ f ≤ fmax

Types Of Friction

Static Friction Kinetic Friction Rolling Friction

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Static Friction the video (Click Here)

Acts when there is no relative motion between the two surfaces.

Prevents the initiation of motion.

It has a maximum value given by: fₛ ≤ μₛN​

Where, μₛ is the coefficient of static friction, and N is the normal force.

Kinetic Friction
Acts when the surfaces are in relative motion (the body is sliding over a
surface).
 It is always less than static friction.

Magnitude is given by: fₖ = μₖN​

Where, μₖ is the coefficient of kinetic friction, and N is the normal force.

Direction of Friction Didn’t understand? Watch

the video (Click Here)

Opposes Motion: The direction of friction always opposes the relative motion (or
the intended motion) between the surfaces in contact.
If a body moves or tends to move to the right, friction acts to the left, and vice
versa.

Acts Tangentially: Friction acts along the tangential direction of the contact
surface. It does not act perpendicular to the surfaces.

On Inclined Planes: Friction acts upward along the plane, opposing the component
of gravitational force that causes the object to slide downward.

Didn’t understand? Watch

Magnitude of Friction the video (Click Here)

The magnitude of friction mainly depends on the "normal force," which surfaces
exert on the objects sitting on them, as well as the characteristics of the specific
surface you're considering.

For most purposes, the formula is: f = μ​N

where:
μ = coefficient of friction
N = normal reaction force

The Magnitude of static friction is given by: fₛ ≤ μₛN​

Where, μₛ is the coefficient of static friction, and N is the normal force.

The Magnitude of kinetic friction is given by: fₖ = μₖN​

Where, μₖ is the coefficient of kinetic friction, and N is the normal force.

 Didn’t understand? Watch
Properties of Coefficient of Friction the video (Click Here)

Dimensionless Quantity: The coefficient of friction (μ) has no unit or dimension

because it is a ratio of two forces (frictional force and normal reaction).

Independent of Area of Contact: The coefficient of friction is independent of the

apparent area of contact as long as the normal force remains constant.

Two Types: Static(μₛ)

: Kinetic(μₖ). Generally, μₛ>μₖ

Independent of Velocity: For moderate speeds, the coefficient of kinetic friction

remains constant and does not depend on the velocity of the moving object.

Didn’t understand? Watch

Graph the video (Click Here)

A plot of Frictional force (F) vs. external force (Fₑₓₜ​).

The graph shows:

Static friction increasing linearly with Fₑₓₜ until it reaches Fₘₐₓ.

Transition to kinetic friction (Fₖ) at a constant value once the block moves.
 Static friction is self-adjusting and can resist motion up to its maximum value
(Fmax).
Once motion begins, static friction is replaced by kinetic friction, which is
constant and lower than Fmax​.
Acceleration depends on the net force after overcoming friction, calculated
using Newton's second law: F = ma.

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Angle of Friction the video (Click Here)

The angle of friction (θ) is defined as the angle made by the resultant of the
normal reaction force (N) and the limiting frictional force (Fₘₐₓ​) with the normal
to the surface.
It is mathematically related to the coefficient of static friction (μₛ):
tanθ = μₛ​

Angle of Repose
The angle of repose(Φ) is the minimum angle the inclined plane makes with the
horizontal when the body on it begins to slide downwards.
It is mathematically related to the coefficient of static friction (μₛ):
tanΦ = μₛ

Application of Friction Didn’t understand? Watch

the video (Click Here)
Friction plays a crucial role in our daily lives and has many applications in both
natural and technological processes.

Walking and Running

When you try to walk, you will apply a downward force F onto the ground, a
resultant force F′ will be produced which is equal and opposite to the applied
according to Newton's third law of motion. So we can write,
F = − F′
If we resolve the force F, that we apply to the ground to different components, we
will get a component called the weight (W) acting on the foot acting downwards
and the effective horizontal force (Fₕ) acting on the foot which is directed to the
right side of the foot in this case.

If we try to resolve the resultant force F′ into

components, we will get a component called the normal
component (N) which is acting upwards (opposite to the
weight) and horizontal component of friction (Fᵣ) which
is directed to the left side of the foot in this
case(opposite to the effective horizontal force).

The horizontal frictional force acting towards the left,

which acts opposite to the effective horizontal force
applied by our foot prevents us from slipping while
walking.

Rolling Friction Didn’t understand? Watch

the video (Click Here)

When an object rolls over a surface, the force resisting its motion due to
deformation of the surface and the rolling object is called rolling friction.

The rolling friction force is given by:

fᵣ = μᵣ. N
Where:
fᵣ​= rolling friction force,
μᵣ​= coefficient of rolling friction (much smaller than μs​or μk​,
N = normal reaction force.
 Didn’t understand? Watch
Pushing - Pulling the video (Click Here)

Effect of Pushing:

When you push an object at an angle θ downward:

A part of the applied force (Fsinθ) adds to the normal force (N).
This increases the effective N, leading to greater frictional force.

Net normal force:

N = mg + Fsin⁡θ
Frictional force:
f = μ(mg + Fsin⁡θ)

Effect of Pulling:

When you pull an object at an angle θ upward:

A part of the applied force (Fsin⁡θ) reduces the normal force (N).
This decreases the effective N, leading to less frictional force.

Net normal force:

N = mg − Fsin⁡θ
Frictional force:
f = μ(mg − Fsin⁡θ)

Thus, pulling is easier than pushing when moving an object on a

horizontal surface.