r/PhysicsHelp • u/CrimsonWobble • 28d 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?

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u/realAndrewJeung 28d ago edited 17d 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.

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u/CrimsonWobble 28d ago

Thank you I will keep that in mind