# Question on drag

1. Aug 10, 2006

### finchie_88

If the force of drag is proportional to the velocity, then:
$$F_d = -kv$$
However, the units of k are very strange. Does this mean that the model is completely wrong?

2. Aug 10, 2006

### ZapperZ

Staff Emeritus
What do you mean by "strange"? What did you expect the units of "k" supposed to be?

Note that just because something is a constant, doesn't mean it doesn't have any units, or that it must have only certain types of units.

Zz.

3. Aug 10, 2006

### finchie_88

By strange, I mean kg/s. Which seems strange in the idea of projectiles or falling objects (in rocket motion it would make much more sense).

4. Aug 10, 2006

### ZapperZ

Staff Emeritus
I still don't see why it is strange. Why does having the unit of kg/s for the constant seem strange? It is, after all, connecting a "force" (which has a mass dependence), to a velocity (which has a time rate of change dependence).

Sounds perfectly un-strange to me.

Zz.

5. Aug 10, 2006

### Hootenanny

Staff Emeritus
Would you consider it strange if we quoted the value of a force as 5 kg.m.s-2?