# Simple kinematics

1. Jan 20, 2010

### songoku

1. The problem statement, all variables and given/known data
At the airport, persons A and B walk at an average speed of 0.90 ms-1 and 0.80 m s-1 respectively. Person B now walks on a elevator but person A does not. The elevator moves at the speed of 0.50 m s-1. How much faster will person B take to travel 50 m compared to person A ?
A. 125 s
B. 44.4 s
C. 17.1 s
D. 26.8 s
E. 77.7 s

2. Relevant equations
s = vt

3. The attempt at a solution
On the manual, it is written : New speed of person B = 0.80 + 0.50 = 1.3 m s-1

I find it strange because B walks on elevator. Maybe it means travelator.
But if it is really elevator, the speed of person B should be 0.5 ms-1 because he can't walk in elevator.

For both cases, there are the answers on the choices. So what is the best interpretation for the question?

*I feel so stupid asking this question...

2. Jan 20, 2010

### aim1732

Maybe they do mean a travelator.
A person can walk in an elevator. In that case his velocity will be a vector resultant.
However the vector resultant speed comes out to be 0.9433 m/s which does not match any of the choices.

3. Jan 20, 2010

### songoku

Hi aim1732

Yeah actually B can walk in elevator, although it is not common case to be considered. And if B really walks in elevator, I think we still consider the elevator's speed because it is in the same direction as the distance traveled.

Thanks

4. Jan 20, 2010

### aim1732

Can't say.
The question says distance of 50m, so why talk about direction in the first place?

5. Jan 20, 2010

### songoku

Hi aim

Hm...because I think B can't move in the direction of the resultant velocity. B can only move in the direction of 0.80 ms-1 or the elevator. But I just realized your point of view. You only consider the resultant velocity and the distance that needed to be taken.

Thanks