Question Comprehension

[Dil]

Hello, I've just found this forums while looking for help with my Physics class.

I am having trouble on certain questions, not the actual Math part, but figuring out which velocity is initial or final (this is for kinematics). For example, if the question is "Someone standing on a 20 meter cliff throws a ball into the air at 9 m/s, how long does it take to reach the ground"

Would Vi(initial velocity) be 0 and Vf(final velocity) be 9? Because it started in someone's hand, or would it be 9 and the final velocity be unknown.

Also, when something is thrown up into the air(gaining altitude), would it's acceleration due to gravity (9.81) be negative or positive?

Physics is pretty interesting, and a good challenge, although it seems like sometimes it's too much of a challenge Thanks

 

[HallsofIvy]

You appear to be having difficulty more with English than with either math or physics!

"Initial" means "at the start". "Initial velocity" is the speed with which the stone is INITIALLY thrown into the air. No, it is not 0- it doesn't matter that someone was holding it before the problem even began. "Initial" means "at the beginning of the problem". In this problem the initial speed is 9 m/s (and the initial velocity is +9 m/s assuming you are taking + to be upward).

"Final" means "at the end". Here, the end of the problem is when the stone hits the ground. You are correct that you do not know the final velocity in this problem.

Whether the acceleration due to gravity depends upon how you set up your "coordinate system". I would recommend that you always choose to set up your measurements ("coordinate system") so that positive is upward (as I assumed above). That's most natural to most people. In that case, since the acceleration due to gravity is downward, it will be negative.

Another thing you will want to think about is where to put your "zero point". If you take h=0 to be the point from which the stone is thrown, then the initial height of the stone is 0 and, since the cliff is 20 meters high, the stone will "hit the ground" when h= -20.

You might also see what happens if you choose the "zero point" at the base of the cliff. In that case, the stone will "hit the ground" when h= 0 but the stone's initial height will be h= +20. You should get slightly different equations but exactly the same value for the time.