A Ball Is Thrown Vertically Upwards, Find The Ratio of PE to KE

  • #26
look at the SUVAT equation

(1) v = u + at

at the top of the balls flight, v = 0

so

t = u/a = 1.63 s

half this time is 0.815 s

now plug that back in to the equation (1)

a*t = 8 which is half the intial speed
Ohhh! Thank you! The graph and the calculation made it clearer!
 
  • #27
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deleted...accidental reptition
 
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  • #28
haruspex
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v^2-u^2=2as
v^2=2 x -9.81 x 9.789
What happened to u2?

This problem illustrates why it is much better to keep everything symbolic as long as possible. If you ignore the given numbers (except the "/2") and just use symbols for the variables you can get to the answer purely in algebra, barely any arithmetic required. There are many other benefits too.
 
  • #29
haruspex
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you can do it the long way:
William, I know you are keen to be very helpful to the posters, and no doubt they are grateful, but the way the homework forums work is that we do not post complete solutions (unless the poster has already found one). We ask questions, provide hints, point out mistakes. In the long term, we believe this is of greater benefit.
 
  • #30
What happened to u2?

This problem illustrates why it is much better to keep everything symbolic as long as possible. If you ignore the given numbers (except the "/2") and just use symbols for the variables you can get to the answer purely in algebra, barely any arithmetic required. There are many other benefits too.
I had assumed it was 0. But I get it now, u=16m/s.
Thanks!
 
  • #31
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William, I know you are keen to be very helpful to the posters, and no doubt they are grateful, but the way the homework forums work is that we do not post complete solutions (unless the poster has already found one). We ask questions, provide hints, point out mistakes. In the long term, we believe this is of greater benefit.
yes, I totally understand, but OP was lost after a week of help; and the help was getting more an more abstract.

One piece of helpful advice was "Note that the answer is independent of g, m and u and, in fact, there was no need for any numerical calculations"

Which, is totally useless for somebody that is lost. Why is the answer independent? Why is there no need for calcs? There was no explanation. That was just left there, hanging. Fine if you are au fait with projectile motion - utterly bewildering if you are not. If the OP was already lost, this "smart" help help just made them even more lost!

Surely there is a point where somebody needs to be shown how to do something so they can move forwards? I know that is certainly true for me - especially after a week of frustration trying to figure something out! Isn't that what teachers do?
 
  • #32
PeroK
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yes, I totally understand, but OP was lost after a week of help; and the help was getting more an more abstract.

One piece of helpful advice was "Note that the answer is independent of g, m and u and, in fact, there was no need for any numerical calculations"

Which, is totally useless for somebody that is lost. Why is the answer independent? Why is there no need for calcs? There was no explanation. That was just left there, hanging. Fine if you are au fait with projectile motion - utterly bewildering if you are not. If the OP was already lost, this "smart" help help just made them even more lost!

Surely there is a point where somebody needs to be shown how to do something so they can move forwards? I know that is certainly true for me - especially after a week of frustration trying to figure something out! Isn't that what teachers do?

@William White I can't say I'm happy with this unsolicited abuse. If you're unhappy with what someone else has said to you, then leave me it of, mate!
 

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