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Normal force

The surface applies a normal force on the body.

The rigid bodies counteract any deformation in equal measure. A rigid table, therefore, applies a force, which is equal in magnitude and opposite in direction. As there is no relative motion at the interface, this contact force has no component along the interface and as such it is normal to the interface.

N = m g

Friction force

Force of friction comes into play whenever two bodies in contact either tends to move or actually moves. The surfaces in contact are not a plane surface as they appear to be. Their microscopic view reveals that they are actually uneven with small hills and valleys. The bodies are not in contact at all points, but limited to elevated points. The atoms/molecules constituting the surfaces attract each other with electrostatic force at the contact points and oppose any lateral displacement between the surfaces.

Friction force

The surfaces are uneven.

For this reason, we need to apply certain external force to initiate motion. As we increase force to push an object on horizontal surface, the force of friction also grows to counteract the push that tries to initiate motion. But, beyond a point when applied force exceeds maximum friction force, the body starts moving.

Friction force

The friction force acts tangential to the interface between two surfaces.

Incidentally, the maximum force of friction, also known as limiting friction, is related to the normal force at the surface.

F f = μ N

where "μ" is the coefficient of friction between two particular surfaces in question. Its value is dependent on the nature of surfaces in contact and the state of motion. When the body starts moving, then also force of friction applies in opposite direction to the direction of relative motion between two surfaces. In general, we use term “smooth” to refer to a frictionless interface and the term “rough” to refer interface with certain friction. We shall study more about friction with detail in a separate module.

The two contact forces i.e. normal and friction forces are perpendicular to each other. The magnitude of net contact force, therefore, is given by the magnitude of vector sum of two contact forces :

Contact force

The contact force is equal to the vector sum of normal and friction forces.

F C = N 2 + F f 2

The tangent of the angle formed by the net contact force to the interface is given as :

tan θ = N F f

String tension

String is an efficient medium to transfer force. We pull objects with the help of string from a convenient position. The string in taut condition transfers force as tension.

Tension in string

Let us consider a block hanging from the ceiling with the help of a string. In order to understand the transmission of force through the string, we consider a cross section at a point A as shown in the figure. The molecules across "A" attract each other to hold the string as a single piece.

Tension in string

If we consider that the mass of the string is negligible, then the total downward pull is equal to the weight of the block (mg). The electromagnetic force at “A”, therefore, should be equal to the weight acting in downward direction. This is the situation at all points in the string and thus, the weight of the block is transmitted through out the length of the string without any change in magnitude.

Questions & Answers

What are the system of units
Jonah Reply
A stone propelled from a catapult with a speed of 50ms-1 attains a height of 100m. Calculate the time of flight, calculate the angle of projection, calculate the range attained
Samson Reply
58asagravitasnal firce
Amar
water boil at 100 and why
isaac Reply
what is upper limit of speed
Riya Reply
what temperature is 0 k
Riya
0k is the lower limit of the themordynamic scale which is equalt to -273 In celcius scale
Mustapha
How MKS system is the subset of SI system?
Clash Reply
which colour has the shortest wavelength in the white light spectrum
Mustapha Reply
how do we add
Jennifer Reply
if x=a-b, a=5.8cm b=3.22 cm find percentage error in x
Abhyanshu Reply
x=5.8-3.22 x=2.58
sajjad
what is the definition of resolution of forces
Atinuke Reply
what is energy?
James Reply
Ability of doing work is called energy energy neither be create nor destryoed but change in one form to an other form
Abdul
motion
Mustapha
highlights of atomic physics
Benjamin
can anyone tell who founded equations of motion !?
Ztechy Reply
n=a+b/T² find the linear express
Donsmart Reply
أوك
عباس
Quiklyyy
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

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Source:  OpenStax, Physics for k-12. OpenStax CNX. Sep 07, 2009 Download for free at http://cnx.org/content/col10322/1.175
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