Question

What is the projectile motion equation?

Answer 1

Basically, any of the kinematic equations works, if you know when to use which equation.

For a projectile shot at an angle, to find time, first consider the first half of the motion. You can set up a table to organize what you have and what you need to figure out which kinematic equation to use.
For example: A child kicks a ball with initial velocity of `15 m/s` at an angle of `30^o` with the horizontal. How long is the ball in the air?

You can start with the table of givens. For time you're going to need the y-component of the velocity.
`v_i rarr 15*sin(30) = 7.5 m/s`
`v_f rarr 0 m/s`
`a rarr -9.8 m/s^2`
`t rarr FIND`
`Delta x rarr unknown`

You can use the kinematic equation `v_f = v_i + at`. Substitute:
`0 = 7.5 +(-9.8)t`
`t = 0.77 s`
REMEMBER that this is only for the first half of the motion, so multiply the calculated time by `2` to find the total time. In this case `2*0.77 = 1.54 s`

So, the moral is, if you can identify what the question is asking for, and you can find the givens, organize them into a table similar to the one I made, and chose the appropriate kinematic equation, you should be fine. Sorry if this was a bit long.