# Homework Help: Speed of projectile after t seconds

1. Sep 14, 2008

### Vanessa23

1. The problem statement, all variables and given/known data
A projectile is fired with an initial speed of 68 m/s at 37 degrees above horizontal on a flat firing range. c) Determine speed of the projectile 2.3 seconds after firing. d) Determine direction of projectile 2.3 s after firing.

2. Relevant equations
c) Vox=Vx=68m/s
Vy=Voy*sin(theta)-gt
V=sqrt(Vx^2+Vy^2)

d) tan^-1(Vy/Vx)= theta

3. The attempt at a solution

I got that Vx=68m/s
Vy=68*sin(37)-(9.8*2.3)
Vy=18.38

V=sqrt(68^2+18.4^2)
V=70.4
but the velocity is not 70.4 what am I doing wrong??
Thanks for any help!
The second part I get theta= 15.1 degrees but I don't want to submit it because if the first part is wrong then this is probably not right either.

2. Sep 14, 2008

### LowlyPion

You need to take the horizontal component of velocity in the RSS which is 68*Cos 37, not 68.

Then you should be fine.

You will need to get another calculation for theta.