At what speed must I throw vertically an object.

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Homework Help Overview

The discussion revolves around determining the initial speed required to throw an object vertically so that it loses 25% of its speed within the first second of its ascent. The problem involves concepts from kinematics, specifically the equations of motion under constant acceleration due to gravity.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the relationship between initial and final speeds, questioning how to express the final speed in terms of the initial speed. There are attempts to clarify the equations used and the implications of a 25% speed loss.

Discussion Status

The discussion is active with participants providing guidance on how to express the final speed mathematically. There is recognition of misunderstandings regarding the equations, and participants are encouraged to clarify their expressions and assumptions.

Contextual Notes

There is a noted language barrier for one participant, which may affect their understanding of the mathematical expressions and terminology used in the discussion.

  • #31
Doc Al said:
Forget about that equation for the moment. What I want you to do is express this fact:Then you can combine that with the other equation.
Ok, so you want this

v final = v initial + -9.8 m/s^2 * 1 second
AND
v final * 0.75 = v initial + -9.8 m/s^2 * 1 second

*I have a feeling that you're not going to be happy.
 
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  • #32
astrololo said:
Ok, so you want this

v final = v initial + -9.8 m/s^2 * 1 second
This one's good. Put it aside for now.

astrololo said:
AND
v final * 0.75 = v initial + -9.8 m/s^2 * 1 second
This one's plain wrong. Realize that this one contradicts the previous one. Toss it out.

astrololo said:
*I have a feeling that you're not going to be happy.
Not yet. :wink:

Hint: Is vfinal ten times bigger than vinitial? Would you write: vfinal = 10vinitial?

How would you write their relative size?
 
  • #33
Doc Al said:
This one's good. Put it aside for now.This one's plain wrong. Realize that this one contradicts the previous one. Toss it out.Not yet. :wink:

Hint: Is vfinal ten times bigger than vinitial? Would you write: vfinal = 10vinitial?

How would you write their relative size?
No, we actually know that the object is going to lose 25 % of its speed after 1 second.
 
  • #34
astrololo said:
No, we actually know that the object is going to lose 25 % of its speed after 1 second.
Exactly. Now express that mathematically: vfinal = ?vinitial.
 
  • #35
Doc Al said:
Exactly. Now express that mathematically: vfinal = ?vinitial.
vfinal = 0.75 * vinitial

In this case, we are saying that the initial speed is going to lose 25 %, which makes us say that the final speed is going to be equal to 0.75 * vinitial because the final speed is the moment where we lost 25 %
 
  • #36
astrololo said:
vfinal = 0.75 * vinitial
Exactly!

Now combine that with your other equation to eliminate vfinal. Then you can solve for vinitial.
 
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  • #37
Doc Al said:
Exactly!

Now combine that with your other equation to eliminate vfinal. Then you can solve for vinitial.
WOW. How could I have been such an idiot. We know that our final is going to be 25 less than the initial speed. What smart idea do I bring ? Take 75 % of 75% of V initial. I have been been taking 75 % TWO times. Anyway, thank you for your help and patience !
 

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