Solving Kinematics Problems: Understanding Initial and Final Velocity in Physics

[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.

 

[M98Ranger]

Hello there! I'm glad you found this forum for help with your physics class. Kinematics can definitely be a challenging topic, but with some practice and understanding of initial and final velocities, you'll be able to solve those problems with ease.

To answer your question, in this scenario, the initial velocity (Vi) would indeed be 0 and the final velocity (Vf) would be 9 m/s. This is because the ball starts at rest in someone's hand before it is thrown, so its initial velocity is 0. The final velocity is 9 m/s because that is the velocity at which the ball is traveling just before it hits the ground.

When something is thrown up into the air, its acceleration due to gravity would be negative. This is because gravity is pulling the object down towards the ground, so its velocity is decreasing. This can be represented by a negative sign in equations.

I'm glad you find physics interesting and challenging! It's a subject that requires critical thinking and problem-solving skills, but it can also be very rewarding when you finally understand a concept or solve a difficult problem. Keep up the good work and don't be afraid to ask for help when needed. Best of luck in your studies!