# If a rocket is moving upwards at 28.72 meters per second, how long until it stops?

1. Jul 14, 2009

### 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.

2. Jul 14, 2009

### mgb_phys

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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

3. Jul 14, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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

4. Jul 14, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

Any ideas?

5. Jul 14, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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?

6. Jul 14, 2009

### mgb_phys

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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

7. Jul 14, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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

8. Jul 14, 2009

### mgb_phys

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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.

9. Jul 14, 2009

### Staff: Mentor

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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

10. Jul 14, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

Perfect! Thank you!

11. Jul 14, 2009

### Phrak

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

OK. But you can't neglect drag.

12. Jul 14, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

Ok now how do I calculate drag?

13. Jul 15, 2009

### maverick_starstrider

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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.

14. Jul 15, 2009

### Max CR

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

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.

15. Jul 15, 2009

### Phrak

Re: If a rocket is moving upwards at 28.72 meters per second, how long until it stops

It's a rocket not a rock.

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

Last edited: Jul 15, 2009