# 8.5 Other force types  (Page 2/3)

 Page 2 / 3

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.

$N=mg$

## 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.

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.

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

${F}_{f}=\mu N$

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 :

${F}_{C}=\sqrt{{N}^{2}+{F}_{f}^{2}}$

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

$\mathrm{tan}\theta =\frac{N}{{F}_{f}}$

## 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.

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.

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.

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