Friction is defined as the force that opposes the motion of a solid object over another. There are mainly four types of friction: static friction, sliding friction, rolling friction, and fluid friction. Friction and normal force are directly proportional to the contacting surfaces and it doesn’t depend on the hardness of the contacting surface. With the increase in relative speeds, the sliding friction reduces whereas fluid friction increases with the increase in the relative speed, also fluid friction is dependent on the viscosity of the fluid.
In sliding motion, each point on the body has only translational or linear motion. But in case of rolling motion, different points have a combination of linear as well as rotational motion. Look at the following figure:
If we consider only the rotational motion then point B tends to move backward suppose with a velocity VR. Similarly, we consider only linear motion then point B moves with a velocity VL in the forward direction. Now the resultant motion of the point B will be a superposition of both the above velocities. Depending upon the values of VR and VL the rolling friction can act in the following directions:
If VR > VL then the point B tends to move backward so as we know friction opposes relative motion between the surfaces in contact hence it will act in the forward direction.
Similarly, if VR < VL then the point B tends to move forward and hence the friction acts in the backward direction.
Also, if VR = VL although the ball moves in the forward direction the frictional force acting on the ball will be zero. This is unlike sliding friction where the friction force will definitely act if the body is moving.
Types Of Friction
Following are the friction types which depend on the types of motion:
- Static Friction
- Sliding Friction
- Rolling Friction
- Fluid Friction
What Is Static Friction?
Static friction is defined as
the frictional force that acts between the surfaces when they are at rest with respect to each other.
The magnitude of the static force is equal in the opposite direction when a small amount of force is applied. When the force increases, at some point maximum static friction is reached.
Static Friction Examples
Following are the examples of static friction:
- Skiing against the snow
- Creating heat by rubbing both the hands together
- Table lamp resting on the table
Coefficient Of Static Friction
The coefficient of static friction is denoted as µs. The maximum force of static friction is given as the product of the coefficient of static friction and normal force and force of static friction is less than or equal to the product of the coefficient of static friction and normal force. It is given as:
|Fs max = µs η and Fs ≤ µs η|
- Fs is the force of static friction
- µs is the coefficient of static friction
- η is the normal force
- Fs max is the maximum force of static friction
What Is Sliding Friction?
Sliding friction is defined as
the resistance that is created between any two objects when they are sliding against each other.
Examples Of Sliding Friction
Following are the examples of sliding friction:
- Sliding of the block across the floor
- Two cards sliding against each other in a deck
Coefficient Of Sliding Friction
The coefficient of sliding friction is denoted as µs. The force of sliding friction is defined as the product of the coefficient of sliding friction and the normal force. It is given as:
|Fs = µs. Fn|
What Is Rolling Friction?
Rolling friction is defined as
the force which resists the motion of a ball or wheel and is the weakest types of friction.
Examples Of Rolling Friction
Following are the examples of rolling friction:
- Rolling of the log on the ground
- Wheels of the moving vehicles
Coefficient Of Rolling Friction
The coefficient of rolling friction is denoted as µr. The force of rolling friction is defined as the product of the coefficient of rolling friction and the normal force. It is given as:
|Fs = µr. Fn|
What Is Fluid Friction?
Fluid friction is defined as
the friction that exists between the layers of the fluid when they are moving relative to each other.
Examples Of Fluid Friction
Following are the examples of fluid friction:
- The flow of ink in pens
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