Rocket Stop Time & Distance: 3.156kg, 28.72 m/s

[Max CR]

A rocket is moving upwards at 28.72 meters per second. Then, its engines are turned off. How much time will it take for it to come to a stop if it is traveling upwards exactly straight? Also, how far would the rocket have had traveled after its engines were shut off? The weight of the rocket is 3.156 kilograms.

Thanks. This is a personal project that I am working on and is not a homework question.

 

[mgb_phys]

Ignoring air resistance it's quite easy - including air resistance is a little trickier but possible if you don't mind some programming or a spreadsheet.

Any falling object accelerates at the same rate 9.8m/s^2 and any rising object (once the motor has stopped) slows by 9.8m/s^2

Then it's simply v=u+at since v=0 when it stops you simply have t = 0-28.72/-9.8 = 2.9s

 

[Max CR]

Ok great. That told me how long it will take. Now i just need to know how far it will travel in those 2.9 seconds. Thanks

 

[Max CR]

Any ideas?

 

[Max CR]

Ok. I believe i found a formula but I am not sure. The formula is

d = v^2/2ug = (28.72 m/s)^2/2(9.8 m/s) = 42.08 meters

Is this correct?

 

[mgb_phys]

Yes the equation is normally written as, V^2=U^2 + 2 g s
Again v=0 and u=28.72m/s g=9.8m/s/s
There is a sticky in the intro physics with all these equations

 

[Max CR]

Ok. I have the following

0 = (28.72 m/s)^2 + 2(9.8 m/s)(2.9 seconds)

Now how do I find how far the rocket traveled? I am trying to figuer out how far the rocket will travel in the 2.9 seconds that it takes for it to stop and come back to the ground.

Thanks

 

[mgb_phys]

Sorry 's' is displacement ie. distance (for slightly complicated reasons)
0 = (28.72 m/s)^2 + 2(9.8 m/s/s) * distance
so rearranged
distance = 0-(28.72 m/s)^2 / 2 * (9.8 m/s/s) = 42.08m

This is the distance up (ie until speed is zero) the total time and total distance is twice this.

 

[russ_watters]

You can also take the average speed, since the acceleration is constant: (28.72)/2*2.9

 

[Max CR]

Perfect! Thank you!

 

[Phrak]

OK. But you can't neglect drag.

 

[Max CR]

Ok now how do I calculate drag?

 

[maverick_starstrider]

OK. But you can't neglect drag.

The guy doesn't even know basic kinematics and you're suggesting he calculate drag?

Don't worry about drag, it's a much, much more complicated creature and in most cases makes little to no difference.

 

[Max CR]

Oh good. I am glad to hear that I don't have to redo all of the calculations regarding the rockets altitude. Haha. Thanks.

I will learn how to calculate drag later in my college years. Thanks though.

 

[Phrak]

The guy doesn't even know basic kinematics and you're suggesting he calculate drag?

Don't worry about drag, it's a much, much more complicated creature and in most cases makes little to no difference.

It's a rocket not a rock.


...and why would you think I'm asked the OP to answer his own question?