Kinematics: Throwing ball up the building with unknown initial velocity

AI Thread Summary
To solve the problem of a ball thrown upward from a 65m high building, the initial velocity can be determined using kinematic equations. The total time of flight is 4 seconds, which can be divided into the ascent and descent phases. The motion is symmetric, meaning the velocity when the ball returns to the thrower's height is equal in magnitude but opposite in direction to the initial velocity. By applying the equations of motion, particularly considering the acceleration due to gravity, the initial velocity and maximum height can be calculated. Understanding the relationship between the upward and downward motion is crucial for finding the solution.
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Homework Statement



At the top of the building that's 65m high, a guy throws a ball upward.
The ball comes back down and hits the ground in 4 seconds.

A)What is the initial velocity when the guy threw the ball?

B)What is the highest height of the ball?

C)At what time does the ball comes back to him?I'm just stumped how the hell I can start off even...

There was a homework problem very similar to this but it gave initial velocity..this problem doesn't and I don't know how to start...

Any advice would be fantastic..

(This was my first physics quiz...is it even possible to solve this?!?)

Please help...

Homework Equations



all the kinematic equations..

https://www.physicsforums.com/showpost.php?p=905663&postcount=2

The Attempt at a Solution



I can't...can't even start with A
 
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It should be possible. Think about splitting this problem up into a symmetric projectile motion part and an asymmetric projectile motion part (i.e. up and down are not equal). What do you know about the velocity of the ball as it passes the guy on the way down? (assuming no air resistance)
 
gamer_x_ said:
It should be possible. Think about splitting this problem up into a symmetric projectile motion part and an asymmetric projectile motion part (i.e. up and down are not equal). What do you know about the velocity of the ball as it passes the guy on the way down? (assuming no air resistance)

I tried exactly that...but no avail...

Well you can say when the ball reaches the top...and starts to fall you can do the following..

Initial V= 0 m/s
A= -10m/s/s
but that's it..

Still t and delta x are unknown...
 
you can say that, but you can also say something when the ball reaches the side of the building going downward. remember that if it's allowed to travel the same distance up as it was down, the velocity going down will be directly related to the initial velocity going up.

Perhaps you should set up your equations for a parabolic arc for a ball with an initial velocity v_i going up and coming down to the same height. What is final velocity before it hits the ground (at that height)?
 
gamer_x_ said:
you can say that, but you can also say something when the ball reaches the side of the building going downward. remember that if it's allowed to travel the same distance up as it was down, the velocity going down will be directly related to the initial velocity going up.

Perhaps you should set up your equations for a parabolic arc for a ball with an initial velocity v_i going up and coming down to the same height. What is final velocity before it hits the ground (at that height)?


So how do I go about solving the initial v?

Any step by step advice?
 
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