# Homework Help: What value of the velocity should I insert in the equation?

1. Jun 9, 2009

### antigona

Hi,

My problem is very simple... but yet it rises some doubt. I am calculating how long a projectile goes taking into account air drag. For the displacement I have the following formula:

x=(v0+Vterminal)/k*(1-e^(-kt)-Vterminal*t+x0

where k is the air drag coefficient

I know that this formula already includes both components of the velocity, thus it should give the correct displacement.
The equation states that my initial velocity is: 31.1m/s
The X component of the velocity is: 15.55m/s.

Which value of the velocity should i insert on the equation?

I have done the calculations with both and the results differed by very little, hence my confusion.

Appreciate the help, thanks.

R.

2. Jun 9, 2009

### Cyosis

Which equation of motion did you solve to get that formula for the displacement?

3. Jun 9, 2009

dv/dt=-kv+g

4. Jun 9, 2009

### Cyosis

You're missing an m in there I think and +g should be -g. Since you have a g in that particular equation of motion the v and x you calculate will be in the y-direction. Therefore you need to calculate the y-component of the initial velocity.