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Ask iit jee aieee pet cbse icse state board experts Expert Question: solve this question on frrrriction
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ASHAMZ.K.VINCENT (8)

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The friction coefficient between the athlete?s shoes and the ground is 0.9. Suppose a superman wears these shoes and races for 50 m. There is no limit for his capacity of running at high speeds. (a) Find the minimum time that he will take in completing the 50 m starting from rest. (b)Suppose he takes exactly this minimum time will he take to stop?
    
edison (4435)

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Frictional force = mg = 0.9mg
 
where m = mass of the man
 
Now to complete the distance of 50 m from rest he need to accelerate by a (say)
 
This acceleration 'a' is such that the force by man should not  exceed firctional force otherwise he will slip, thus
 
F = ma = 0.9mg
 
or a = 0.9g = 9m/s2, is the maximum acceleration with which he can run
 
use
 
s = (1/2) a t2   (as the man starts from rest)
 
or t = (2s/a)1/2 , s= 50m, subtitute and get the result.
 
Yes while stopping also this is the minimum time required to stop.
 
 

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ASHAMZ.K.VINCENT (8)

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Dear sir, why the "superman" business in the question?
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