who I can find the displacement in this Q ?

Oct 2009
290
0
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 ?
 

may

May 2010
13
0
maybe u should use this formula
s=1/2(v+u)t

s is displacement
v is initial velocity
u is final velocity
t is time

s=1/2(90+70)2
 

mr fantastic

MHF Hall of Fame
Dec 2007
16,948
6,768
Zeitgeist
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 ?
180 + x where x is calculated below:

initial velocity = 90 m/s.
final velocity = 0 m/s.
a = 20 m/s^2.
x = ?

Now use the usual formula.
 
Feb 2010
5
1
Hi,

You could try drawing a rough graph, then calculate the area under the graph for the total distance

It starts at 90m/s, so in a speed time graph there is a horizontal line at 90 for 2 sec. Then the line slopes downwards, losing 20m's every second, so in 4.5 seconds it has a speed of 0.

When you calculate the area under the graph:

2x90 + (6.5-2)x90x0.5 = 382.5m
 
Oct 2009
290
0
Hi,

You could try drawing a rough graph, then calculate the area under the graph for the total distance

It starts at 90m/s, so in a speed time graph there is a horizontal line at 90 for 2 sec. Then the line slopes downwards, losing 20m's every second, so in 4.5 seconds it has a speed of 0.

When you calculate the area under the graph:

2x90 + (6.5-2)x90x0.5 = 382.5m
hI

which formula you used here ??

and can help me in the gragh why you say in 4.5 seconds it has a speed of 0.
 

mr fantastic

MHF Hall of Fame
Dec 2007
16,948
6,768
Zeitgeist
hI

which formula you used here ??

and can help me in the gragh why you say in 4.5 seconds it has a speed of 0.
Reply #3 has given you a very simple approach. Why are you trying to do it this way?
 
Feb 2010
5
1
hI

which formula you used here ??

and can help me in the gragh why you say in 4.5 seconds it has a speed of 0.
There is a general rule that the area under a speed time graph is the distance traveled. So off course you can just calculate the area under it just like calculating the area of a square and triangle.

Because the deceleration is 20 meters per second per second, this means that a speed of 20meters per second is lost every second. So it takes 4.5 seconds for a speed of 90m/s to reach 0m/s (90/20 = 4.5)