# What equation should I use?

1. Sep 15, 2007

### frasifrasi

Ok, for our lab on projectile (2-d) motion,

We know :

Angle of launcher(little canon) with respect to surface
height of launcher abover surface
average horizontal distance ball traveled at that angle/height
-9.8 gravity force

and are required to find the launch speed. What equation should I use to accomplish that?

Thank you.

Last edited: Sep 15, 2007
2. Sep 15, 2007

### Timo

The equation of motion.

3. Sep 15, 2007

### frasifrasi

which is...?

4. Sep 16, 2007

### Timo

The equation of motion for constant acceleration a:
$$\vec x (t) = \vec x (0) + \vec v(0) t + \frac{1}{2} \vec a t^2$$

5. Sep 16, 2007

### arildno

frasifrasi:

What equation is ALWAYS the basic equation of motion in classical mechanics?
What is that law called?

6. Sep 16, 2007

### frasifrasi

I don't think I have learned that yet. Don't have a known time in our measurements so that equation wouldn't work. Am I supposed to use the trajectory equation? But would it would it work in this case since the launcher was launching the projectile from a table onto the floor?

7. Sep 16, 2007

### Timo

I hadn't told you to use it if it wasn't an equation that helps you solve your problem. Just count the degrees of freedom: How many real-valued parameters are unknown? How many real-valued equations did I give you?

8. Sep 16, 2007

### frasifrasi

yeah, those riddles are helping a lot.
Can anyone comment or suggest an equation?