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Direction of friction

Direction of friction is opposite to the component of net external force parallel to the contact surface. It is the criteria to decide the direction of friction. There are, however, situations in which external force may not be obvious. In the illustration of the above section, there is no external force on the upper block "B". What would be the direction of friction on block "B"? As a matter of fact, it is the only force (we do not consider vertical normal force as it perpendicular to motion) on block acting parallel to contact surface. Clearly, we can not apply the criteria for determining direction in this case.

We actually analyze the forces on the underneath block "A". The net external force parallel to interface is acting towards right. It, then, follows that the friction on "A" is towards left. Taking cue from this revelation and using Newton’s third law of motion , we decide that the direction of friction on block "B" should be towards right as shown in the figure below.

Free body diagram

Upper block is accelerated by friction.

Alternatively, we can consider relative motion between bodies in contact. In the illustration, we see that block “A” has relative velocity (or tendency to have one) towards right with respect to block “B”. It means that the block “B” has relative velocity (or tendency to have one) towards left with respect to block “A”. Hence, friction force on block “B” is towards right i.e. opposite to relative velocity of “B” with respect to block “A”.

We can formulate a directional rule about the direction of friction as :

“The direction of friction on a body is opposite to the relative velocity of the body (or the tendency to have one) with respect to the body, which is applying friction on it.”

Contact forces revisited

There are two types of contact forces that we encounter when two bodies interact. The contact forces are normal and friction forces. The normal force is a reaction of a body against any attempt (force) to deform it. The ability of a body to resist deformation also has electromagnetic origin operating at the surface as in the case of friction.

The two contact forces, therefore, can be considered to be manifestation of same inter – atomic forces that apply at the contact interface. The resultant electromagnetic force acts in a direction inclined to the surface. Its component perpendicular to surface is the normal force and component parallel to the surface is friction.

Contact force

Forces on the block

The resultant or net electromagnetic contact force is the vector sum of the two components and is given by :

F C = ( F N 2 + F F 2 )

Where :

F C : Resultant contact force

F N : Normal force, also represented simply as "N"

F F : Friction force, which can be f s (static friction) , F s (maximum static friction) or F k (kinetic friction)

For the maximum static friction ( F s ) and a given normal force (N), the magnitude of resultant contact force is :

F C = ( F N 2 + F F 2 ) = ( N 2 + F s 2 ) F C = ( N 2 + μ s 2 N 2 ) F C = N ( 1 + μ s 2 )

This expression represents the maximum contact force between a pair of surfaces.

Questions & Answers

what is the definition of resolution of forces
Atinuke Reply
what is energy?
James Reply
can anyone tell who founded equations of motion !?
Ztechy Reply
n=a+b/T² find the linear express
Donsmart Reply
Sultan Reply
Moment of inertia of a bar in terms of perpendicular axis theorem
Sultan Reply
How should i know when to add/subtract the velocities and when to use the Pythagoras theorem?
Yara Reply
Centre of mass of two uniform rods of same length but made of different materials and kept at L-shape meeting point is origin of coordinate
Rama Reply
A balloon is released from the ground which rises vertically up with acceleration 1.4m/sec^2.a ball is released from the balloon 20 second after the balloon has left the ground. The maximum height reached by the ball from the ground is
Lucky Reply
work done by frictional force formula
Sudeer Reply
Misthu Reply
Why are we takingspherical surface area in case of solid sphere
Saswat Reply
In all situatuons, what can I generalize?
Cart Reply
the body travels the distance of d=( 14+- 0.2)m in t=( 4.0 +- 0.3) s calculate it's velocity with error limit find Percentage error
Clinton Reply
Explain it ?Fy=?sN?mg=0?N=mg?s
Admire Reply

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