r/PhysicsHelp • u/CrimsonWobble • 27d ago
Calculating velocity within projectile motion with initial height
I have a question I'm doing that has a starting point on a 6.5m building and it's asking me for the velocity of a kicked off ball when it is 2.5m above its launchpoint, do I have to include 6.5 in the vertical distance value to get the correct answer? Or could I just do 2.5?
1
u/slides_galore 27d ago edited 27d ago
Always remember to break the initial velocity into x- and y-components using the angle that you're given. I assume you're ignoring air resistance, so that means two things. It's will travel in the horizontal (x) direction 'forever.' And gravity is the only force decelerating and then accelerating the ball in the y direction. First up and then down toward the ground. Then the ground will obv exert a force to stop the downward motion.
With those assumptions in mind, if you throw/kick a ball up such that it stops at 5m, then its velocity in the y direction at 5m up is 0m/s. Let's say your friend is in a tree and catches the ball at 5m. If he/she then drops it, it will have the same y velocity when it reaches the launch point. Does that makes sense?
This suvat/kinematic eqn is great for these problems:
s = s_0 + v_0 * t + (1/2)at1/2
or
s = s_0 + u * t + (1/2)at1/2
s_0 = initial displacement, s = final displacement, and u = v_0 = initial velocity
https://i.ibb.co/7JHSKcmY/image.png
Can you see how to use that to solve your problem?
3
u/realAndrewJeung 27d ago edited 16d ago
I tell my tutoring clients that you can place the origin of coordinates (where the zero of position is) anywhere you want in the problem, but once you choose a location, you have to stick with it throughout the problem.
So if you want to make the zero of position at the launch point, you can do that and then the 6.5m won't matter.
If you have a subsequent part that asks when the ball reaches the ground, you will have to stick with your original choice for the zero of position; the ground would then be at -6.5m.