Question

Two girls, each of weight 400 N climb up a rope through a height of 8 m. We name one of the girls A and the other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the power expended by each girl?

NCERT Class IX
Sciences - Main Course Book

Chapter 11. Work and Energy

Answer 1

There are two girls, both take respective time to climb up a rope through a height of 8 m, this shows they completed the particular task but in variable time period. They share same weight which is 400N.

Power is defined as rate of doing some work or at some rate transfer energy.  

Power = W/T or E/T

Where W expresses work done by girls – A and B, T denotes time has been taken by girls to climb up a rope. It is calculated in watt(W).

Answer 2

$\tt{Question:}$

Two girls, each of weight 400 N climb up a rope through a height of 8 m. We name one of the girls A and the other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the power expended by each girl?

$\tt{Solution:}$

$\sf{Girl A :- }$

Weight = 400 N

Height = 8 m

Energy = Work = Force × Displacement

Here, Force = 400 N

And, Displacement = 8 m

Energy =》 400 × 8

Power = $\frac{E}{T}$

Time = 20 s

Power = $\frac{3200}{20}$

=》 $\tt{\boxed{P_{1} = 160 W}}$

$\sf{Girl B :-}$

Weight = 400 N

Height = 8 m

Energy = Work = Force × Displacement

Here, Force = 400 N

And, Displacement = 8 m

Energy =》 400 × 8

Power = $\frac{E}{T}$

Time = 50 s

Power = $\frac{3200}{50}$

=》 $\tt{\boxed{P_{2} = 62 W}}$

Thus, by all this, we found that :-

Power expended by Girl A = 160 W

Power expended by Girl B = 64 W