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A force of 20 N is applied on upper bloc...

A force of `20 N` is applied on upper block as shown in the figure. Te total work done by frictional during the time interval in which the upper block has a displacement of `15 m` with respect to the ground is (take `g = 10 m//s^(2)` )

A

`50 J`

B

`-50 J`

C

`75 J`

D

`- 75 J`

Text Solution

Verified by Experts

The correct Answer is:
B
Block and plank both slide
so, `a_("Block") = (20 -5)/(2) = (15)/(2)`
`a_("plank") = (5)/(2)`
`S = ut + (1)/(2)at^(2)`
`15 = 0 + (1)/(2) (15)/(2)t^(2)`
`rArr t = 2 sec`
Desplacement of plane `= 0 + (1)/(2) (5)/(2)2^(2)`
So, displacement of block with respect to plank `= 15 - 5 = 10m`
`W_(f) = - 10 xx 20 xx 1//4 = -50J`
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