# Homework Help: Constant Projections

1. Nov 29, 2006

### courtney1121

As a projectile moves along its parabolic trajectory, which of the following remain constant (ignoring air resistance, and defining the z-axis to point upward)? More than one answer may be correct!

a. Its speed.
b. Its velocity.
c. Its x-velocity and its y-velocity.
d. Its acceleration

I am thinking the only thing constant is acceleration. My book keeps saying that the horizontal velocity is constant but this is a parabolic trajectory so shouldn't that make a difference? Also velocity is going to change from positive to negative, and speed will change at different points of the trajectory.

2. Nov 29, 2006

### Xiju

Along the Y axis the acceleration is always constant. The speed is constant as long as the acceleration doesnt change. The velocity would be constant as well as its x velocity, however the Y velocity wouldnt be constant because at the highest peak of the projectile the y velocity would be 0.

3. Nov 29, 2006

### courtney1121

well i dont think velocity would be constant cause velocity is magnitude and direction and the direction changes so it would change, I think. I'm pretty sure I'm considering the projectile in the zx plane so y will always be 0.

4. Nov 29, 2006

### Xiju

Oh crap this is 3 dimensional. Scratch my post. I was talking about 2 D.