# Homework Help: Who I can find the displacement in this Q ?

1. May 29, 2010

### r-soy

Hi all

who I can find the displacement in this Q ?

A jetliner lands with a speed of 90 m/s on runway . It runs with constant speed for 2 s , before the brakes are applied . The brakes produce a deceleration of 20 m/s2 . What is its displacement on the runway .

I try to solve

V0 = 90 m/s

t1 = 2s

x = V0 X t1 = 180 m

a = -20 m/s2

then I don't know who I complte it ?

2. May 29, 2010

### tiny-tim

Hi r-soy!

(btw, it's how (rhyming with now and Mao) not who (rhyming with new and you) )
This is how

The total displacement is the sum of the two displacements.

For the second displacement, use one of the standard constant acceleration equations, with distance = s, acceleration = a = -20, u = 90, and v = 0.

3. May 30, 2010

### r-soy

see

Here I must find two displacement .
firist one I got it
now the secound now

I have Dx1 = 180 and a = -20, u = 90, and

by equat5ion constant I use v^2 = v0^2 = 2aDx
fainal velocity is 0 beacuse it will stop after using brake
now :
0 = 90^2 + 2(-20) Dx2
Dx2 = 202.5

Now collect two dispkacement : will get : 362.5 m

4. May 30, 2010

### tiny-tim

(try using the X2 tag just above the Reply box )

Yes, that's fine

(except 180 + 202.5 = 382.5 )

5. May 30, 2010

### TwigLaser

Hmm not sure what you guys are talking about..but il get it later.