# Homework Help: The confusing motion equation

1. Oct 10, 2015

### RyanH42

1. The problem statement, all variables and given/known data
The picture of the question is given

2. Relevant equations
simple motion equations

3. The attempt at a solution
I am so confused so I cant solve the question

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2. Oct 10, 2015

### J Hann

Let t1 = 1.75 / 3.5 the time for the first person to reach the door
t2 = (2 * 1.75 - .25) / 3.5 time for second to reach first person
t3 = (3 * 1.75 - 2 * .25) / 3.5 time for third person to reach second person
Can you arrange this into a geometric series?

3. Oct 10, 2015

### RyanH42

well I didnt see why theres a relationship between water level and the distance to person each other or simply L.

4. Oct 10, 2015

### RyanH42

$t_n=n.1.75-(n-1).0.25)3.5$

5. Oct 10, 2015

### RyanH42

I found the b) its 9

6. Oct 10, 2015

### RyanH42

but stilll I didnt understand where that equation come from and a option

7. Oct 11, 2015

### haruspex

The 1.75 is the gap between people, so you don't need to subtract .25 from it.
And I don't think you mean a geometric series.
Water level?
"Depth" here means in a horizontal direction. If there are three people pressed one against the other at the door then the depth is three people, or 3x.25m=0.75m.

8. Oct 11, 2015

### J Hann

No, I don't think you need a series (arithmetic not geometric).
If everyone starts moving at the same time then you should only need to calculate the
time required for the n'th person to reach layer of people at the locked door.
Incidentally, I subtracted the .25 to account for the increase in depth of people at the door.
Since the third person would travel (3 * 1.75 - 2 * .25) because there was a .50 layer of people