Help with simple motion equations problem

[RyanH42]

Homework Statement

The picture of the question is given

Homework Equations

simple motion equations

The Attempt at a Solution

[/B] I am so confused so I can't solve the question

 

[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?

 

[RyanH42]

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

 

[RyanH42]

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

 

[RyanH42]

I found the b) its 9

 

[RyanH42]

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

 

[haruspex]

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?

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.

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

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.

 

[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
already at the door.

 

[haruspex]

N
Since the third person would travel (3 * 1.75 - 2 * .25) because there was a .50 layer of people
already at the door.

How far from the door is the third person at time 0?