At what speed must I throw vertically an object.

[astrololo]

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.

 

[Doc Al]

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.

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.

*I have a feeling that you're not going to be happy.

Not yet.

Hint: Is v_final ten times bigger than v_initial? Would you write: v_final = 10v_initial?

How would you write their relative size?

 

[astrololo]

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.

Hint: Is v_final ten times bigger than v_initial? Would you write: v_final = 10v_initial?

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.

 

[Doc Al]

No, we actually know that the object is going to lose 25 % of its speed after 1 second.

Exactly. Now express that mathematically: v_final = ?v_initial.

 

[astrololo]

Exactly. Now express that mathematically: v_final = ?v_initial.

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 %

 

[Doc Al]

vfinal = 0.75 * vinitial

Exactly!

Now combine that with your other equation to eliminate v_final. Then you can solve for v_initial.

 

[astrololo]

Exactly!

Now combine that with your other equation to eliminate v_final. Then you can solve for v_initial.

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 !