# Constant variables in projectiles

1. Sep 15, 2006

### Joules23

At each moment during its flight, a projectile has a velocity v and an acceleration a. The components of these variables are:
vx = x component of the velocity
vy = y component of the velocity
ax = x component of the acceleration
ay = y component of the acceleration
Which of these components remain constant throughout the motion? Assume that air resistance is absent.

None of the variables is constant.

Only vx and ay are constant.

vx, ax and ay are constant.

vy, ax and ay are constant.

Only ax and ay are constant.

My answer:
vy, ax and ay are constant.

is this correct? i know for sure ax is constant at 0m/s and ay is constant at +-9.8m/s,.... but i think Vy instead of Vx would be constant because of gravity (ay) ... am i right?

2. Sep 15, 2006

### radou

Only $$v_{x}$$ and $$a_{y}$$ are constant.

3. Sep 15, 2006

### Joules23

wouldnt $$a_{x}$$ also be constant at 0m/s

4. Sep 15, 2006

### radou

What does at 0 m/s mean? $$a_{x}$$ equals 0 all the time, so it does not exist. Hence, it's not constant. Speaking strictly math, I guess it could be constant.. But in this context, it's not constant.

5. Sep 15, 2006

### HallsofIvy

Staff Emeritus
What? 0 does not exist? Let's not speak non-sense! Even in physics, 0 exists and is a constant.

Joules23, acceleration is the rate of change of velocity. If ax= 0, then vx does has 0 change and so is a constant. On the other hand, precisely because ay= -9.8 m/s2 which is not 0, vy does change and is not a constant.

6. Sep 15, 2006

### radou

Yeah, what I said was nonsence. But I just wanted to say that ax = 0 is a 'in-this-context-non-relevant' constant, if this makes any sense.

Anyway, ax, ay and vx are constants, Jules is right.

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