Projectile motion on an inclined plane and linear equations

[krequi]

So we did a lab in class using an inclined air table, two pucks and a spark timer. Only one puck was used and we "launched" it in a way that when the spark timer was activated, it traced a parabola on the sheet of paper over the air table.
What I don't understand is: why is horizontal acceleration = 0? Apparently that's why acceleration is constant and therefore the initial velocity is 0 too. But if there were no horizontal acceleration, then the puck shouldn't have advanced in the first place because there are no forces acting on it...

Also, we have to figure out the time for each dot on the paper (there are about 100). The frequency of the spark timer was 50 Hz. We did not time how long it took for the puck to do the parabola. So I'm thinking that maybe every dot is one "cycle", and therefore each should be like 1/50th of a second? Does that make sense, if every 50 dots is one second?

Any help is welcome :)

 

[lewando]

What I don't understand is: why is horizontal acceleration = 0? Apparently that's why acceleration is constant and therefore the initial velocity is 0 too. But if there were no horizontal acceleration, then the puck shouldn't have advanced in the first place because there are no forces acting on it...

During the "launching" process, you accelerated the puck briefly in both the horizontal and vertical directions. This got the puck moving. After it was launched, the acceleration was constant in both directions--some fraction of g in the vertical, and zero (a constant) in the horizontal.

So I'm thinking that maybe every dot is one "cycle", and therefore each should be like 1/50th of a second? Does that make sense, if every 50 dots is one second?

Yes.