# Projectile Motion rock throw Help

1. Jan 23, 2014

### evan1997ere

If a rock is thrown horizontally from a cliff at 25 m/s, what will its velocity be (magnitude and direction) after 3.0 seconds. The answer is 38.6 m/s, and 49.6 degrees down, but I cannot figure out how to get to it.

Not sure which equations to use, I just started and I am learning on my own.

I assumed that to find the velocity, you should average the horizontal and vertical velocities together. So since it has been 3 seconds, the vertical would be 29.4 m/s, because the gravitational constant is 9.8 m/s/s, and the horizontal would remain 25 m/s. Averaged, it would be 27.2 m/s. Obviously that was incorrect. As far as direction goes, I have no idea.

2. Jan 23, 2014

### Staff: Mentor

The horizontal and vertical velocities are the components of the total velocity. You must add them like vectors, not average them. A bit of trig will allow you to find to the magnitude and direction of that vector.

3. Jan 23, 2014

### evan1997ere

Oh okay, I'll look that up and see if I can get it. Thanks