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serenitymantra · Sep 11, 2008, 09:54 PM

From a bridge 50m above water, one person throws a rock down at 20 m/s and at exactly the same time a second person throws a rock up with the same speed. Ignore air resistance. How much time elapses between the first splash and the second splash.

ChihuahuaMomma · Sep 11, 2008, 10:18 PM

The same amount of time.

Unknown008 · Sep 12, 2008, 04:01 AM

Careful CM! The second one is thrown up! I'm not sure about how to get the answer, but first find the time for the first stone to hit water. Then calculate the time for the second stone to go up ans down, and finally subtract your answers.

To find the time that the second stone takes, apply the force of gravity, which is 10m/s^2 (this is 9.8m/^2 rounded off). The speed of the upgoing stone will decrease by 10m/s each time. To make matters easier, make the speed of the second stone negative and add 10m/s each time. After 2 seconds, the ball reaches its max height (20m above bridge)and starts to fall. Find this and complete your work.

PS, I'm not so sure, but perhaps, someone will be able to correct me if ever I made a mistake somewhere.

ChihuahuaMomma · Sep 12, 2008, 04:08 AM

Anyone guess why I never graduated high school and got my GED? I'm horrible at math!

Capuchin · Sep 12, 2008, 05:03 AM

use $d_f = d_i + v_it + \frac{1}{2}at^2$ for each case

In each case $d_f = 0$ , $d_i = 50$ , and $a = -9.8$

$v_i$ will be your initial velocity (either + or - 20, here we are taking upwards as positive)

Then just solve for t in each case and work out the difference.

serenitymantra · Sep 12, 2008, 05:35 AM

Originally Posted by Capuchin

use $d_f = d_i + v_it + \frac{1}{2}at^2$ for each case

In each case $d_f = 0$ , $d_i = 50$ , and $a = -9.8$

$v_i$ will be your initial velocity (either + or - 20, here we are taking upwards as positive)

Then just solve for t in each case and work out the difference.

how do you isolate t to solve for it? This is my 1st physics class ever and I am so lost... professor is not much help clarifying everything.. just goes straight into equations... :confused: any help would be appreciated... thanks!

Capuchin · Sep 12, 2008, 05:36 AM

Do you not know how to solve a quadratic equation from math class?

serenitymantra · Sep 12, 2008, 05:40 AM

Originally Posted by Capuchin

Do you not know how to solve a quadratic equation from math class?

very very vaguely... I have not dealt with equations in years... the last math class I had was years ago.. :(

Capuchin · Sep 12, 2008, 05:44 AM

For $ax^2+bx+c=0$ :

$x = \frac{-b \pm \sqrt {b^2-4ac}}{2a}$

Unknown008 · Sep 13, 2008, 01:51 AM

But, but, but Capuchin, I said that I was not sure! This was only my opinion, but now that you're here, you corrected me and led me as well! Anyway, at my level, i don't think that i have ever done such a question. All those which i have done include an equation already given, either equation for velocity, distance or acceleration for a certain time $t$ .

Capuchin · Sep 13, 2008, 02:14 AM

Originally Posted by Unknown008

But, but, but Capuchin, I said that I was not sure! This was only my opinion, but now that you're here, you corrected me and led me as well! Anyways, at my level, i don't think that i have ever done such a question. All those which i have done include an equation already given, either equation for velocity, distance or acceleration for a certain time $t$ .

Sorry! Here you're either right or wrong! :) You did some weird thing where you split time up into seconds - you should look at zeno's paradoxes to see some of the problems with doing that.

Unknown008 · Sep 13, 2008, 02:21 AM

You're in UK right? So you'll perhaps understand that I'll be taking my SC exams this year, and I know nothing about zeno's paradoxes. The only subject that I do which has this kind of complicated problems is Add Maths, and I don't think that I will have such questions. My level is perhaps too low... :(

Capuchin · Sep 13, 2008, 02:31 AM

Originally Posted by Unknown008

You're in UK right? So you'll perhaps understand that i'll be taking my SC exams this year, and i know nothing about zeno's paradoxes. The only subject that i do which has this kind of complicated problems is Add Maths, and i don't think that i will have such questions. My level is perhaps too low...:(

I'm not sure what SC exams are, but I wish you good luck anyway! Please don't take offense to the reddie, in the science forums we use them just to make sure that the OP realises a certain answer is incorrect (it's not like they matter any more as the site doesn't keep a tally). I for one appreciate you having a go, as I can help you and clear up any misunderstandings.

Zeno was a greek philosopher. His paradoxes and their solutions are very useful to think about when learning about the laws of motion:
Zeno's paradoxes - Wikipedia, the free encyclopedia

Unknown008 · Sep 13, 2008, 02:41 AM

Ok, SC stands for School Certificate, given by Cambridge when you pass its exams. And thanks for the help!:)

Capuchin · Sep 13, 2008, 02:45 AM

Originally Posted by Unknown008

Ok, SC stands for School Certificate, given by Cambridge when you pass its exams. And thanks for the help!:)

What level is this at? GCSE?

Unknown008 · Sep 13, 2008, 03:10 AM

Some sort. It's, I think, another syllabus, but I think that it's the same level as GCSE.

Capuchin · Sep 13, 2008, 03:16 AM

Originally Posted by Unknown008

Some sort. It's, i think, another syllabus, but i think that it's the same level as GCSE.

Sounds about right from where you are in your learning, OK well if you ever have any problems then feel free to ask! Stuff at that level is very logical and once you get the method and understanding down it's something you never forget.

Unknown008 · Sep 13, 2008, 03:21 AM

Yeah, I'm fine for now... just making the last adjustments and 'polishing' if you know what I mean. See you later, I need to go now.

serenitymantra · Sep 16, 2008, 04:28 PM

Originally Posted by Capuchin

For $ax^2+bx+c=0$ :

$x = \frac{-b \pm \sqrt {b^2-4ac}}{2a}$

after scratching my head for a while longer, I think I was able to get it. Since the gravitational pull is 9.8m/s2 and the velocity was (-) & (+) 20m/s. I managed to calculate it as 4.08 s, well 4.1s.

The explanations helped to get my wheels turning.. lots of dust & cobwebs there!

Thanks!

Capuchin · Sep 16, 2008, 04:40 PM

Originally Posted by serenitymantra

after scratching my head for a while longer, I think I was able to get it. Since the gravitational pull is 9.8m/s2 and the velocity was (-) & (+) 20m/s. I managed to calculate it as 4.08 s, well 4.1s.

The explanations helped to get my wheels turning.. lots of dust & cobwebs there!

Thanks!

I just worked it out quickly and got the same answer as you.

It's always nice to hear someone who appreciates us working through the method with them, rather than giving them the answer, so thanks for that!