# Density and Projectile Motion

1. Dec 29, 2004

### apiche

I'm still new to the field of physics and have undertaken a project in furthering my understanding of projectile motion. Can density affect the parabolic shape of a projectile especially concerning the tail end of its motion?

2. Dec 29, 2004

### FulhamFan3

Density of the medium it's travelling in or density of the object? Density of the medium would definetly affect it. This is called drag and it's why skydivers reach a terminal velocity. The center of gravity of an object follows normal projectile motion no matter what spin the object has. I have no idea what you mean by the tail end of it's motion.

3. Dec 29, 2004

### apiche

Let me clarify. When launching bottle rockets, I've noticed that they typically start off strong and then have the tendency to "fizzle out" and almost float down to the ground instead of following a more parabolic motion. Does the density of the actual rocket affect that motion?

4. Dec 29, 2004

### krab

You are quite right. A projectile follows a curve that is sort of like the shape of a female breast. (I forget the technical name.) The amount of departure from parabolic depends upon the ratio of air density to projectile density. It also depends upon drag coefficient.

5. Dec 29, 2004

### FulhamFan3

As the bottle rocket is in it's burn phase it's accelerating. As soon as the burn is over it slows down due to gravity. The reason why the bottle rocket doesn't follow a parabolic path on the way down is because it's pretty much an empty carboard tube and has major drag. If you were on the moon it would follow a parabolic path after the burn phase. I'm willing to bet if you fix some weights on the rocket it would follow that path as well. It was probably desiged like that anyway so rockets don't come back as deadly projectiles.

edit: forgot to say that the drag is directly proportional to the velocity and has mothing to do with the mass. so objects with more mass are unaffected by the miniscule drag where as less massive objects are greatly affected by it.

Last edited: Dec 29, 2004
6. Dec 29, 2004

### Gokul43201

Staff Emeritus
A bottle rocket is not a projectile.