# I got a question

1. Feb 7, 2005

### the_d

A plane landing on a small tropical island has
just 66 m of runway on which to stop.
If its intial speed is 58 m/s, what is the
maximum acceleration of the plane during
landing, assuming it to be constant? Answer
in units of m/s^2.

i know im supposed to use the equation v^2 = Vo^2 + 2a(x-Xo) but wht is V^2

2. Feb 7, 2005

### dextercioby

What formula would you have to use??

Daniel.

3. Feb 7, 2005

### the_d

v^2 = Vo^2 + 2a(x-Xo) but wht is V^2

4. Feb 7, 2005

### dextercioby

Well,the plane has to stop,so the final velocity should be...??

Daniel.

5. Feb 7, 2005

### the_d

ah ha, ok i understand

6. Feb 7, 2005

### the_d

How long does it take for the plane to stop
with this acceleration? Answer in units of s.

i used the equation X = Xo + Vot + 1/2at^2 is this correct?

7. Feb 7, 2005

### dextercioby

$$v_{fin}=v_{init}+at$$

??

The one u posted is correct as well,but te one suggested by me is more direct.

Daniel.

8. Feb 7, 2005

### the_d

i just got that

9. Feb 7, 2005

### the_d

one more question

A car starts from rest and travels for 6.4 s with
a uniform acceleration of 3.3 m/s^2. The driver
then applies the brakes, causing a uniform
acceleration of 1.5 m/s^2.
If the brakes are applied for 2.5 s, how fast
is the car going at the end of the braking
period? Answer in units of m/s.

this is what I did, i subtracted the final acceleration from the initial and the the time for braking from the total time the car was traveling. then i used the equation V = Vo + at is that what you would do?

10. Feb 7, 2005

### dextercioby

No,i would compute the velocity after 6.4 secs and then the final velocity.Do the same & it'll be okay.

Daniel.

11. Feb 7, 2005

### the_d

one final add on

same question, How far has it gone? Answer in units of m.

i used the formula X = Xo + Vot + 1/2at^2 for each time and acceleration then i subtracted the distance i got from the deceleration from the the distance i got for the acceleration

12. Feb 7, 2005

### dextercioby

Why did u subtract them...?It doesn't go backwards...

Daniel.

13. Feb 7, 2005

### the_d

because one answer was negative even though distance cannot be negative( the decleration distance) and the acceleration distance was positive. am i supposed to add them?

14. Feb 7, 2005

### dextercioby

Of course,it decelerates in the same direction as it was accelerating (the velocity vector keeps direction,just the modulus varies),so they have to be added.

Daniel.

15. Feb 7, 2005

### the_d

im getting 72.27meters, can u check that for me? i think its wrong

Last edited: Feb 7, 2005
16. Feb 7, 2005

### dextercioby

Nope,should be roughly 110m...

Daniel.

17. Feb 7, 2005

### here

plane on island

Vf^2 = Vi^2 + 2ad

Vf = 0. because the plane will be at stopped after it lands

rearrange formula

a = (-Vi^2)/(2*d)

a = (-58^2)/(2*66)

a = -25.5 m/s^2

aceleration is negative because the plane is slowing down while it lands.

18. Feb 7, 2005

### here

v2 is zero, once the plane lands it dosent have a velocity now does it