# An upside down rocket

1. Jul 5, 2013

### smarthimandrew

If I were to take a model rocket and secure it upside down to a kg scale, what would I be reading on the scale's display? I would assume it would be the thrust of the rocket but are there other factors at play here? If it is the thrust of the rocket, how would I convert the said value to newtons?

2. Jul 5, 2013

### bigfooted

You can draw a free body diagram to analyze the forces on the rocket.

The trust vector is pointing down, as well as gravity: F + m*g.

EDIT: assuming you measure this on earth and not in space

3. Jul 6, 2013

### Staff: Mentor

Model rocket engine thrust is published - you can just look it up. And a typical scale measures force and converts to mass by assuming you are on the surface of the earth and applying f=ma. So it is easy enough to convert back.

4. Jul 6, 2013

### CWatters

Put rocket on scale pointing downwards.
The display will read the weight of the rocket in Kg.
Press the reset button on the scales so they read zero.
Light the rocket and the scales will read the thrust in Kg.
To convert to Newtons multiply by 9.8.

Edit:

I'm not an expert but model rocket motors are classified by the impulse they produce. The impulse is the force * duration.

So for example a Class B that is rated 5.00 N·s might produce..

5N for 1 Second
2.5N for 2 Seconds
1N for 5 Seconds
etc

They would all be Class B I believe.

I've yet to find a model shop that will sell me a 6Z class motor.....

http://en.wikipedia.org/wiki/Model_rocket_motor_classification

Last edited: Jul 6, 2013