Ap physics b question (high school)

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Homework Help Overview

The problem involves two stones: one dropped from a height of 75 meters and another thrown downward with an initial velocity. The goal is to determine the initial velocity of the second stone such that both stones hit the ground simultaneously. The context is kinematics, specifically focusing on motion under gravity.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the use of kinematic equations to analyze the motion of both stones. There are attempts to calculate the time it takes for the first stone to fall and the initial velocity required for the second stone. Questions arise regarding the final velocity of the first stone and the interpretation of motion parameters.

Discussion Status

The discussion is ongoing, with participants providing insights and calculations. Some participants express confusion about the problem setup and the application of kinematic equations. There is a mix of attempts to clarify assumptions and explore different interpretations of the problem.

Contextual Notes

Participants note confusion regarding the initial and final velocities of the stones, particularly whether the final velocity of the first stone is zero upon reaching the ground. There are also references to specific values obtained during calculations, which differ from expected results.

zhanglshi
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a stone is dropped from a 75m building. when this stone has dropped 15m, a second stone is thrown downaward with an initial velocity such that the two stones hit the ground at the same time. what was the initial velocity of the second stone?

please use kinematic equations. that is all i know on this subject. thanks in advance.
 
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Why don't you give it a try? Use kinematic equations, of course.

Start by figuring out how long it takes for the first stone to reach the ground.
 
well the first stones velocity is 38.340m/s and the time is 3.912s. but idk where to go from there..
 
zhanglshi said:
well the first stones velocity is 38.340m/s and the time is 3.912s. but idk where to go from there..
OK. How long does it take for the first stone to reach 15 m? Then figure out how much time the second stone has to make it to the ground.
 
i am confused on something. i didntt understand the problem well enough so i restarted my work. for the 1st stone, is the final velocity 0? if so, then when i used the v=v(initial)-at, my equation was 0=0+(-9.8)(t) and i got t=0. that is obviously false but idk what i did wrong.. i am terribly confused ):
 
zhanglshi said:
i am confused on something. i didntt understand the problem well enough so i restarted my work. for the 1st stone, is the final velocity 0?
No, but its initial velocity is zero, since it is just dropped, not thrown.
 
but when the stone is at 0m, isn't its velocity 0?
 
zhanglshi said:
but when the stone is at 0m, isn't its velocity 0?
No. It just fell 75 m, so why would you think its velocity would be zero? (We're talking about just before it hits the ground.)
 
oh right. so i got t=2.161 for the2nd stone to hit the ground. i used thex=x(not)=v(not)^2+(1/2)at^2 and got v(not)=-28.654. but according to my answers sheet its supposed to be -24.34. i have no idea what i did wrong..
 
  • #10
zhanglshi said:
oh right. so i got t=2.161 for the2nd stone to hit the ground.
OK.
i used thex=x(not)=v(not)^2+(1/2)at^2
That should be:
x = x_0 + v_0 t + (1/2) a t^2
 

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