At what speed must I throw vertically an object.

[astrololo]

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 ...

 

[Bystander]

only ...

"Only,..." what?

 

[astrololo]

"Only,..." what?

Is there a problem ?

 

[Bystander]

You haven't answered the original question, what's the original vertical speed?

 

[astrololo]

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...

 

[Bystander]

You're interested in the rate at which the object loses speed, and specifically asked for a 25% loss in one second. What's "v_final" for 100% loss?

 

[astrololo]

You're interested in the rate at which the object loses speed, and specifically asked for a 25% loss in one second. What's "v_final" for 100% loss?

Well it's 0.

 

[Bystander]

V final = v initial + a * t

Substitute that value, and rearrange. What's "v(t=1s)" relative to "v_initial?"

 

[astrololo]

Substitute that value, and rearrange. What's "v(t=1s)" relative to "v_initial?"

Vi=9,8 m/s

 

[Bystander]

What's "a?"

 

[astrololo]

What's "a?"

-9.8 m/s^2

 

[Bystander]

9.8 is what percentage of 9.8?

 

[astrololo]

9.8 is what percentage of 9.8?

100%

 

[Bystander]

In one second, the speed changes by how much?

 

[astrololo]

In one second, the speed changes by how much?

it goes from 9.8 to 0.

 

[Bystander]

it goes from 9.8 to 0.

No. It changes by 9.8.

 

[astrololo]

No. It changes by 9.8.

Yes, that's what I said. ok, it changes by 9.8.

 

[Doc Al]

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 v_initial.

 

[astrololo]

Fix the left hand side of that equation. Then you should be able to solve for v_initial.

I put it that way so that the speed would be 75%... it's not correct?

 

[Doc Al]

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:

V final = v initial + a * t

Hint: express Vfinal in terms of Vinitial.

 

[astrololo]

No, it's not correct.

This is the formula you are trying to useint: 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 ?

 

[Doc Al]

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?

 

[astrololo]

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

 

[Doc Al]

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 0.75v_final, not v_final. Fix that!

 

[astrololo]

But the left side of your equation was 0.75v_final, not v_final. 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

 

[Doc Al]

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 v_final in terms of v_initial. (That's where the 0.75 comes in.) That way you can solve for v_initial.

 

[astrololo]

Good. Now express v_final in terms of v_initial. (That's where the 0.75 comes in.) That way you can solve for v_initial.

So we have Vf + 9.8 = Vi

I will multiply Vf by 0.75 ?

 

[Doc Al]

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 v_final in terms of v_initial. Once you get that correct, then you can go back to the equation.

 

[astrololo]

No. First things first. Forget about that equation for a moment.

Express v_final in terms of v_initial. 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

 

[Doc Al]

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:

that it will lose 25 % of its speed during the first second.

Then you can combine that with the other equation.