Physics questions (urgent)

👨🏻‍🏫 學術台其他 - 科學

[匿名]

2014-12-18 2:24 am

1.A stationary man uses a escalator and it takes 40seconds to reach first floor. A man uses 60seconds to walk from a stationary escalator to first floor. If the escalator and the man move at the same time, how many seconds does it take to reach first floor?
Please help me! thanks :D

更新1:

2.a box of water is put on balance A and reading is 6N. a ball is connected to balance B by a rope and reading is 2N. If the ball is immersed into the box of water, the upper balance(spring balance)'s reading is 1.5N, what's the reading of balance A?

回答 (2)

天同

2014-12-18 3:48 am

✔ 最佳答案

1. The man's walking speed = d/60
where d is the length of the escalator
The speed of the escalator in bringing the man up = d/40

Hence, speed of man relative to ground when he stands on a moving escalator
= d/60 + d/40 = d/24
Time taken = d/(d/24) s = 24 s

2. After the ball is immersed in water, its apparent weight, as measured by spring balance B, becomes 1.5 N
Hence, upthrust acts on the ball by water = (2 - 1.5) N = 0.5 N

By Newton's 3rd Law (action and reaction), the same reaction acts on the water and which is recorded by balance A.
Therefore, reading on balance A = (6 + 0.5) N = 6.5 N

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heidicykwan

2014-12-18 7:09 am

1. Time spent = 1/[ (1/40) + (1/60)] = 24 (s)

2. Its reading = 6 + 2 - 1.5 = 6.5 (N)

2014-12-17 23:17:28 補充：
In Q1, it is assumed that the man walks in the same direction as the escalator. However, if he walks in the opposite direction, the time spent will be 1 / [ (1/40) - (1/60) ] = 120 (s) instead of 24 (s).

參考: myself :)

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