# Obtaining Speed From Thrust

1. Jun 26, 2014

### qalz

Hi guys, I would like to count sth., but to do that i need a speed of thing. The promblem is that i've got mass ~150 gram (0,33 pound), enginee thrust 1700 gram (3,75 pound) which work 4 seconds (it moves up) and I don't know how to count this. Do you know, it is possible to count speed based on that data?
Sorry for the low level of English, I'm from Poland (there is no forum where somebody helped me). :/

Last edited: Jun 26, 2014
2. Jun 26, 2014

Engine thrust cannot be in grams.

3. Jun 26, 2014

### dean barry

Find the engine thrust in Newtons, subtract from it ( 0.15 * 9.81 ), leaving the net force, calculate the acceleration rate, accelerate for 4 seconds, use newtons rules of motion for the solution ( final velocity).

4. Jun 26, 2014

### qalz

Sorry, this is power of engine. I measured it putting engine upside-down on weight :)

5. Jun 26, 2014

### CWatters

Did you zero the scales before firing the rocket? I assume you did so..

1.7kg equates to 1.7*9.81 = 16.7N of thrust.

The total force on your rocket is

16.7N - (0.15 * 9.81) = 14.4N

The acceleration is given by F=ma or a=F/m

a = 14.4/0.15 = 96m/s/s

That's very fast. I suspect if you use that figure to calculate the velocity after 4 seconds you will get a figure that is too fast. eg You can't ignore air resistance.

6. Jun 26, 2014

### Staff: Mentor

If this is a rocket, the problem is further complicated by the fact that the mass changes during the four-second period as the propellant is burned. Try googling for "Tsiolkovsky rocket equation"; you'll find what you need there.

7. Jun 26, 2014

### qalz

Yes, that's a rocket. I'll google it, thanks for help :)