Friction Physics Notes

Friction Physics Notes

Friction:
If we slide or try to slide a body over a surface, the motion is resisted by a bonding between the body and the surface. This resistance is represented by a single force and is called friction force.

The force of friction is parallel to the surface and opposite to the direction of intended motion.
Look at this diagram, at first, the wooden block is at rest. A push causes the block to slide across the desk. The force of the push (big F) keeps the block moving. To stop a moving object, a force must act in the opposite direction to the direction of motion. As the block slides across the desk, it slows down and stops moving. The force that opposes the motion of an object is called friction.

• The force of friction is parallel to the surface and opposite to the direction of intended motion.
• Frictional force refers to the force generated by two surfaces that contact and slides against each other. These forces are mainly affected by the surface texture and amount of force impelling them together. The angle and position of the object affect the amount of frictional force.
• If an object is placed against an object, then the frictional force will be equal to the weight of the object.
• If an object is pushed against the surface, then the frictional force will increase and becomes more than the weight of the object.
• The maximum amount of friction force that a surface can apply upon an object can be easily calculated with the use of the given formula.
  Friction force = µ normal
  f = µN
• According to the old view, the roughness of surfaces is the cause of friction. A surface that appears very smooth to the naked eye is found to have irregularities (roughness). When two bodies are in contact with each other, the irregularities in the surface of one body get interlocked in the irregularities of. the other surface.
• The modern view is that it añses on account of strong atomic or molecular forces of attraction between the two surfaces at the point of actual contact.

Types of Friction:
Frictional forces are of three types:

1. Static frictional force
2. Kinetic frictional force
3. Rolling frictional force

 

1. Static Frictional Force: Static friction is the friction that exists between a stationary object and the surface on which it resting. Static friction keeps an object at rest. It must is overcome to start moving the object.

Imagine trying to push an object across the floor. You push on it with a small force, but it does not move, this is because it is not accelerating. However, according to Newton’s second law, the object must move with an acceleration.
a = \(\frac{F}{m}\)

Now, as the body remains at rest, it implies that an opposing force equal to the applied force must have come into play resulting in zero net force on the object. This force is called static friction. It is denoted by Fs.

Thus, static friction is the opposing force that comes into play when one body tends to move over the surface of another, but the actual motion has not started.

1. The static friction depends upon the nature of the surfaces of the two bodies in contact.
2. The static friction does not exist by itself. When there is no applied force, there is no static friction. It comes into play only when the applied force tends to move the body.
3. The static friction is a self-adjusting force.

Limiting Friction: If the applied force is increased, the force of static friction also increases. If the applied force exceeds a certain (maximum) value, the body starts moving. This maximum value of static friction up to which the body does not move is called limiting friction.

1. The magnitude of limiting- friction between any two bodies in contact is directly proportional to the normal reaction between them.
(f_s)_max ∝ N
(f_s)_max = µ_sN ……..(1)
Where the constant of proportionality µs is called the coefficient of static friction. Its value depends upon the nature of the surfaces of the two bodies in contact that means whether dry or wet; rough or smooth; polished or non-polished. For example, when two polished metal surfaces are in contact, µ_s = 0.2, when these surfaces are lubricated, µ_s = 0.1

The value of µs lies between 0 and 1 or 0 < µ_s < 1
∵ N = Mg
∵ (F_s)max = µ_smg ……….(2)

2. Direction of the force of limiting friction is always opposite to the direction in which one body is on the verge of moving over the other.

3. Coefficient of Static Friction: µs is called the coefficient of static friction and is defined as the ratio of the force of limiting friction and normal reaction
From(1), µ_s = \(\frac{\left(f_{s}\right)_{\max }}{N}\)
Dimension: [M⁰L⁰T⁰]
Unit: It has no unit.
The value of µs does not depend upon the apparent area of contact.

2. Kinetic Friction: We know that when the applied force on a body is small, it may not move but as the applied force becomes greater than the force of limiting friction, the body is set into motion. The force of friction acting between the two surfaces in contact which are moving relatively, so as to oppose their motion, is known as kinetic frictional force.

1. Kinetic friction is directly proportional to the normal reaction i. e.,
f_k ∝ N
or f_k = µ_kN………(1)
Where µk, is called the coefficient of kinetic friction.
∵ N = mg
∴ f_k = µ_kmg ……(2)

2. Value of µ_k depends upon the nature of the surface in contact.

3. Kinetic friction is always lesser than the limiting friction
f_k < (f_s)_max
∴ µ_k < µ_s
i. e., the Coefficient of kinetic friction is always less than the coefficient of static friction. Thus we require more force to start a motion than to maintain it against friction. This is because once the motion starts actually; the inertia of rest has been overcome. Also when motion has actually started, irregularities of one surface have little time to get locked again into the irregularities of the other surface.

4. Kinetic friction does not depend upon the velocity of the body.
The graph between applied force and force of friction. If we plot a graph between the applied force and the force of friction, we get the curve of the type shown in the figure. The part OA represents static friction, fs which goes on the increase with the applied force. The body remains at rest till the applied force does not exceed OK and at A, the static friction is maximum. This represents the limiting friction (F_s)_max (= AK or OM). Once the body starts moving, the force of friction drops to a value CL or ON, slightly less than limiting static friction. Thus CL represents the kinetic friction (F_k)(= ON).

As the portion BC of the curve is parallel to the X-axis, therefore, kinetic friction does not change with the applied force. It remains constant, whatever be the applied force.

Further, dynamic friction of friction may be of two types:

1. Sliding friction
2. Rolling friction

1. Sliding Friction: The opposing force that comes into play when one body is actually sliding over the surface of the other body, is called sliding friction. For example, a flat block is moving over a horizontal table.

2. Rolling Friction: Frictional force which opposes the rolling of bodies (like a cylinder, sphere, ring, disc) over any surface is called rolling frictional force.
Rolling friction is directly proportional to the normal reaction (N) and inversely proportional to the radius (r) of the rolling cylinder or wheel.
∴ Frolling = µ_r \(\frac{N}{r}\)
Here μ_r is called the coefficient of rolling friction.

Rolling friction is often quite small as compared to sliding friction. That is why heavy loads are transported by placing them on carts with wheels.

Cause of Rolling Friction: When anybody rolls over any surface it causes a little depression and a small hump is created just ahead of it. The hump offers resistance to the motion of the rolling body, this resistance is rolling frictional force. Due to this reason only, hard surfaces like cemented floors offer less resistance as compared to the soft sandy floors because the hump created on a hard floor is much smaller as compared to the soft floor.

Friction is a necessary evil: Although frictional force is a non-conservative force which opposes the motion. It always causes lots of wastage of energy in the form of heat yet it is very useful to us in many ways. That is why it is considered as a necessary evil.

Physics Notes