A $50 k g$ boy stands on a plateform spring scale in a lift that is going down with a constant speed $3 m / s$ . If the lift is brought to rest by a constant deceleration in a distance of $9 m$ what does the scale read during this period ? $(g = 9.8 m / s^{2})$

500 N

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465 N

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515 N

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Solution

The correct option is D $515 N$
Given that,

Mass $m = 50 k g$

Initial speed $u = 3 m / s$

Distance covered $s = 9 m$

The lift brought to rest by constant acceleration in distance of 9m.

So, final speed $v = 0$

Now, from equation of motion

$v^{2} = u^{2} + 2 a s$

$0 = 9 + 2 a \times 9$

$a = - 0.5 m / s^{2}$

Negative sign shows that acceleration is downward

Now, the net acceleration is

$a = - (a + g)$

$a = - (0.5 + 9.8)$

$a = - 10.3 m / s^{2}$

Now, the force is

$F = m a$

$F = 50 \times 10.3$

$F = 515 N$

Hence, the force is $515 N$

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A 50kg boy stands on a plateform spring scale in a lift that is going down with a constant speed 3m/s . If the lift is brought to rest by a constant deceleration in a distance of 9m what does the scale read during this period ?(g=9.8m/s2)

A $50 k g$ boy stands on a plateform spring scale in a lift that is going down with a constant speed $3 m / s$ . If the lift is brought to rest by a constant deceleration in a distance of $9 m$ what does the scale read during this period ? $(g = 9.8 m / s^{2})$