How do you measure the initial velocity of a projectile from this experimental setup?

Question

Hello I'm given an assignment to build a homemade catapult which I already have, and I'm required to determine the initial velocity of a projectile.

To do this, I'm told to place my phone and ensure that it is facing the side of the catapult directly. I have to record the catapult in slow motion and determine its initial velocity from two consecutive images of the projectile position as it leaves the catapult. It's found by measuring the distance of the projectile and the time interval between the consecutive images but I'm still confused as to do.

Can someone please help me? What must I do, and what relevant formulas must I apply here?

Answer 1

V = dx/dt (change in distance divided by change in time). So in your pictures, you must have a way of determining both distance and time. If the background had some sort of grid that let you see (measure) how far the object traveled in the two consecutive pictures, you would have the distance. The time is a little tricky. You would either need to have a clock of some sort in the picture that displayed the time or you would need to dig into the features of your phone to see what "slo-mo" time is on that device.