# who I can find the displacement in this Q ?

#### 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 ?

#### may

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
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.

#### Illusion3

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

#### r-soy

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
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?

#### Illusion3

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)