Two balls are thrown vertically upward, one with an initial speed twic

[cmkc109]

Homework Statement

Two balls are thrown vertically upward, one with an initial speed twice that of the other. The ball with the greater initial speed will reach a height

- twice that of the other
- 4 times that of the other

Homework Equations

dy = vt + 1/2 at^2
[if i use this equation, i will get twice of the other]

vf^2 = v1^2 + 2ad
-v1^2 = 2ad
[if i use the equation, i will get 4 times of the other]

The Attempt at a Solution

I get both 2 times and 4 times..

 

[voko]

Homework Equations

dy = v + 1/2 at^2
[if i use this equation, i will get twice of the other]

This is incorrect. You might want to show here how you get this result.

 

[cmkc109]

This is incorrect. You might want to show here how you get this result.

i made a typo
it is dy = vi t + 1/2 at^2

this is one of the kinematic equation

 

[lep11]

It seems to me that projectile motion is difficult for many because every second post is about it but that's ok.

 

[voko]

i made a typo
it is dy = vi t + 1/2 at^2

this is one of the kinematic equation

It is indeed a valid equation, but your result is still incorrect.

 

[voko]

It seems to me that projectile motion is difficult for many because every second post is about it but that's ok.

Galileo created the whole science called "physics" by considering projectile motion, so it should not be surprising that it is a hot topic in teaching of this science. That's why it is so frequent here, not because it is difficult for many. Well, some problems may be surprisingly difficult.

 

[lep11]

Galileo created the whole science called "physics" by considering projectile motion, so it should not be surprising that it is a hot topic in teaching of this science. That's why it is so frequent here, not because it is difficult for many. Well, some problems may be surprisingly difficult.

Yes, but in general projectile motion problems are easy because if you understand the concept you can solve the problem using only a couple of kinematic equations.

 

[cmkc109]

It is indeed a valid equation, but your result is still incorrect.

why is it incorrect? can u explain why?

 

[voko]

why is it incorrect? can u explain why?

The correct result is 4 times. I cannot explain why you are not getting the correct result using this method, you have not shown your working.

 

[cmkc109]

The correct result is 4 times. I cannot explain why you are not getting the correct result using this method, you have not shown your working.

i did not do any work. it's simply because dy - at^2 = vi t
so there is no square sign

 

[voko]

What about t? Is it the same in both cases?

 

[cmkc109]

What about t? Is it the same in both cases?

no.. the ball with higher velocity has shorter time..o...i get it loll
using vf^2 = vi^2 + 2ad , a is constant for both cases
but if u use the other equation, t varies so the ans is not correct !
thhxx

 

[voko]

It might be helpful for you to compute the time it takes to reach highest point in both cases.