Mon August 22, 2011 By:

i hav a doubt. an ideal string is cinsidered to be massless.therefore whatever be its acceleration the external force on it is 0 becausef=ma and m=0.but this violates newtons first law.explain.

Expert Reply
Fri August 26, 2011

An ideal string is assumed as massless to simplify a given problem.

Consider a block of mass m which is suspended from a fixed beam by means of a string, as shown in fig.

The string is assumed to be massles. The string is clearly being stretched, since it is being pulled at both ends by the block and the beam. By Newton's third law, the string exerts oppositely directed forces of equal magnitude (i.e. tension) on both the block and the beam.

If the mass of the string is also taken into account, then the tension at each point of the string would not be constant but would vary with respect to the mass of the string. This would complicate the situation. Thus, to make the tension constant along the string so as to simplify the problem, we consider the string to be massless.

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