Question

Ello!❤

A boy of mass 50kg runs up a staircase of 45 steps in 9 s. If the height of each step is 15cm, find his power. Take g=10ms^-2.
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Answer 1

During the motion of the boy, he is under the force of gravity, which is exerting downwards.

• boy works against the gravity to move up the stairs.
• work done by him = potential energy when it reachs height.
• potential energy = amount of work done by earth during that height.

Solution:

total height = 45 × (15) cm

(as each step is 15, so height of total steps will ×45)

• h = 45 × 15 = 675cm
• h = 6.75 m

Work = U = mgh

• W = 50 × 10 × 6.75
• W = 3375 J

Power = rate of work done = work/time

• P = W/t = 3375/9
• P = 375 W

Answer 2

Explanation:

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During the motion of the boy, he is under the force of gravity, which is exerting downwards.

boy works against the gravity to move up the stairs.

work done by him = potential energy when it reachs height.

potential energy = amount of work done by earth during that height.

Solution:

total height = 45 × (15) cm

(as each step is 15, so height of total steps will ×45)

h = 45 × 15 = 675cm

h = 6.75 m

Work = U = mgh

W = 50 × 10 × 6.75

W = 3375 J

Power = rate of work done = work/time

P = W/t = 3375/9

P = 375 W

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