At what speed must I throw vertically an object.

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SUMMARY

The discussion focuses on calculating the initial speed required to throw an object vertically so that it loses 25% of its speed in the first second. The key equation used is V final = V initial + a * t, where acceleration (a) is -9.8 m/s² due to gravity. The correct relationship established is V final = 0.75 * V initial, leading to the conclusion that the initial speed must be 9.8 m/s to achieve the desired speed loss.

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  • Understanding of kinematic equations, specifically V final = V initial + a * t
  • Knowledge of gravitational acceleration, specifically -9.8 m/s²
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  • #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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