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jyoti

· started a discussion

· 1 Months ago

109 (Question: 298784.0 )

GIVE ME DETAILED SOLUTION OF THIS

Question:

With what speed should a body be thrown upwards so that the distance traversed in 7th second and 8th second are equal:

Options:
A)

98 m/s

B)

73.5 m/s

C)

80 m/s

D)

10 m/s

Solution:

Ans: (b) 73.5 m/s



Let u be the initial velocity of the ball. Distance travelled in the nth sec is given by,

Sn = u + 1/2  x a x (2n - 1)

Distance covered in 5th second: (put a = -9.8 and n = 7) 

S7= u - 63.7

Distance covered in the 8th second: (put n = 8, a=-9.8 in equation):

S8 = u - 83.3

as the body is projected upwards, the distance travelled in two different time intervals will be same only when at one time it is moving upwards and the other time it is coming downwards (as there is symmetry in time and height). So, during 7th second, the object was moving upwards, and during the 8th second, the object is coming downwards. Therefore, the distance S8 will be negative.

S8 = -(u - 83.3)

Since they are equal, equating S7 and S8:

u - 63.7= -u +83.3

2u = 147

u = 73.5 m/s

Knowledge Expert

· commented

· 1 Months ago

Dear Student,

We fixed its explanation.

Keep learning with us!
Best wishes
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