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.

astrololo
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Homework Statement


At what speed must I throw vertically an object so that it will lose 25 % of its speed during the first second.

Homework Equations


V final = v initial + a * t

The Attempt at a Solution



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

That's pretty much what I figured out only ...
 
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astrololo said:
only ...
"Only,..." what?
 
Bystander said:
"Only,..." what?
Is there a problem ?
 
You haven't answered the original question, what's the original vertical speed?
 
Bystander said:
You haven't answered the original question, what's the original vertical speed?
Well that's the thing I have no idea of how to obtain the answwer...
 
You're interested in the rate at which the object loses speed, and specifically asked for a 25% loss in one second. What's "vfinal" for 100% loss?
 
Bystander said:
You're interested in the rate at which the object loses speed, and specifically asked for a 25% loss in one second. What's "vfinal" for 100% loss?
Well it's 0.
 
astrololo said:
V final = v initial + a * t
Substitute that value, and rearrange. What's "v(t=1s)" relative to "vinitial?"
 
Bystander said:
Substitute that value, and rearrange. What's "v(t=1s)" relative to "vinitial?"
Vi=9,8 m/s
 
  • #10
What's "a?"
 
  • #11
Bystander said:
What's "a?"
-9.8 m/s^2
 
  • #12
9.8 is what percentage of 9.8?
 
  • #13
Bystander said:
9.8 is what percentage of 9.8?
100%
 
  • #14
In one second, the speed changes by how much?
 
  • #15
Bystander said:
In one second, the speed changes by how much?
it goes from 9.8 to 0.
 
  • #16
astrololo said:
it goes from 9.8 to 0.
No. It changes by 9.8.
 
  • #17
Bystander said:
No. It changes by 9.8.
Yes, that's what I said. ok, it changes by 9.8.
 
  • #18
astrololo said:
v final * 0.75 = v initial + -9.8 m/s^2 * 1 second
Fix the left hand side of that equation. Then you should be able to solve for vinitial.
 
  • #19
Doc Al said:
Fix the left hand side of that equation. Then you should be able to solve for vinitial.
I put it that way so that the speed would be 75%... it's not correct?
 
  • #20
astrololo said:
I put it that way so that the speed would be 75%... it's not correct?
No, it's not correct.

This is the formula you are trying to use:
astrololo said:
V final = v initial + a * t

Hint: express Vfinal in terms of Vinitial.
 
  • #21
Doc Al said:
No, it's not correct.

This is the formula you are trying to use:Hint: express Vfinal in terms of Vinitial.
v initial = Vfinal/(a * t)

Vinitial = V final / (-9.8* 1)

Vinitial = V final / -9.8

This is what you want ?
 
  • #22
astrololo said:
This is what you want ?
No. I want you to fix the left hand side of your equation, just like I asked.

First step: Can you tell me what's wrong with it?
 
  • #23
Doc Al said:
No. I want you to fix the left hand side of your equation, just like I asked.

First step: Can you tell me what's wrong with it?
Well I multiplied the left side by 75%
So we have v final = v initial + a* t
 
  • #24
astrololo said:
Well I multiplied the left side by 75%
So we have v final = v initial + a* t
But the left side of your equation was 0.75vfinal, not vfinal. Fix that!
 
  • #25
Doc Al said:
But the left side of your equation was 0.75vfinal, not vfinal. Fix that!
Ok, so I had v final * 0.75 = v initial + -9.8 m/s^2 * 1 second

and I change it to v final = v initial + -9.8 m/s^2 * 1 second
 
  • #26
astrololo said:
Ok, so I had v final * 0.75 = v initial + -9.8 m/s^2 * 1 second

and I change it to v final = v initial + -9.8 m/s^2 * 1 second
Good. Now express vfinal in terms of vinitial. (That's where the 0.75 comes in.) That way you can solve for vinitial.
 
  • #27
Doc Al said:
Good. Now express vfinal in terms of vinitial. (That's where the 0.75 comes in.) That way you can solve for vinitial.
So we have Vf + 9.8 = Vi

I will multiply Vf by 0.75 ?
 
  • #28
astrololo said:
So we have Vf + 9.8 = Vi

I will multiply Vf by 0.75 ?
No. First things first. Forget about that equation for a moment.

Express vfinal in terms of vinitial. Once you get that correct, then you can go back to the equation.
 
  • #29
Doc Al said:
No. First things first. Forget about that equation for a moment.

Express vfinal in terms of vinitial. Once you get that correct, then you can go back to the equation.
Sorry, english isn't my first language. You mean that you want V final in function of Vinitial, right ? Like what we had with v final = v initial + -9.8 m/s^2 * 1 second
 
  • #30
astrololo said:
Sorry, english isn't my first language. You mean that you want V final in function of Vinitial, right ? Like what we had with v final = v initial + -9.8 m/s^2 * 1 second
Forget about that equation for the moment. What I want you to do is express this fact:
astrololo said:
that it will lose 25 % of its speed during the first second.

Then you can combine that with the other equation.
 

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