PHYSIC QUESTION!plz help!

• 0lala

  10 Sep 06, 03:35

  theres 320g of radioactive substance...with a half life of 4yrs....how much is left after 12 yrs?
  
  plz i need help
• tt3

  10 Sep 06, 03:41

  i dunch noe...
• kenshin_himura

  10 Sep 06, 03:45

  Originally posted by 0lala:

  theres 320g of radioactive substance...with a half life of 4yrs....how much is left after 12 yrs?
  
  plz i need help

  320g convert to no of molecules..... use N= No*e^-lamda*t
  then u can find how much is left after 12 yrs
  
• a-Lost-9uY

  10 Sep 06, 03:47

  I dont know too,
  Phyics 2 me is History.
  Mayb u can try asking ur frens?
  Or u try pressing the 'window key' on ur keyboard cum 'up arrow' key
  n press 2times 'enter'.
  A clearer pic will appear.
• ipjh79

  10 Sep 06, 04:09

  40g...i think so....the last time i studied this was 1 year ago...
• ^tamago^

  10 Sep 06, 04:21

  Originally posted by 0lala:

  theres 320g of radioactive substance...with a half life of 4yrs....how much is left after 12 yrs?
  
  plz i need help

  320g × (1/2)^(12/4) = 40g.
• the.owl

  10 Sep 06, 05:51

  Originally posted by ^tamago^:

  320g × (1/2)^(12/4) = 40g.

  correct.
• ^tamago^

  10 Sep 06, 05:53

  Originally posted by the.owl:

  correct.

  thank u~
• missqi

  10 Sep 06, 06:23

  walao.
  rudimentary u also dont know.
  man oh man.
  good luck.
  u really need it.
• Classyfied

  10 Sep 06, 12:05

  take 120 / 40 = 3
  
  then (1/2) ^ 3 * 320 = 40
  
  
• popmynutsy

  10 Sep 06, 13:01

  lemme try.
  320 1st yr.
  1 half life = 4 yrs.
  so = 160 at the 4th yr.
  =80 at the 8th yr
  and 40 at the 12th yr. i think is like dat.
• SixSentinels

  10 Sep 06, 14:15

  320, half life = 160 units after 4 years and the equation is (new)=(original)e^k(time) so it becomes 160=320e^k(4), then 1/2=e^k4, and to find the value after 12 years u use this: y = 320e^k(4)3 [since 4x3=12], then since we already know 1/2=e^k(4), then we substitute into the general equation and it becomes Y = [320(1/2) ^ 3] and use the calculator to find the answer. solved =)
• TwinTurbo_Supra

  10 Sep 06, 20:07

  Originally posted by Classyfied:

  take 120 / 40 = 3
  
  then (1/2) ^ 3 * 320 = 40
  
  

  yup this is right.
• SBS9828X

  22 Sep 06, 20:04

  
• nightzip

  22 Sep 06, 20:09

  Originally posted by a-Lost-9uY:

  I dont know too,
  Phyics 2 me is History.
  Mayb u can try asking ur frens?
  Or u try pressing the 'window key' on ur keyboard cum 'up arrow' key
  n press 2times 'enter'.
  A clearer pic will appear.

  what's this?
• ^tamago^

  22 Sep 06, 20:09

  
• OO_OO_OO

  22 Sep 06, 20:10

  N/No= (1/2)^(t/t1/2)
• JennTS

  22 Sep 06, 22:41

  Isn't it just...divide the 320g by two 3 times, since it's halflife is every 4 years? (12/4=3)
  
  
• ^tamago^

  22 Sep 06, 22:46

  i wonder how he's going to pass 'O' levels...
  
  all of us could write it down off-hand...
  
  
• bowler200482

  22 Sep 06, 23:02

  Originally posted by 0lala:

  theres 320g of radioactive substance...with a half life of 4yrs....how much is left after 12 yrs?
  
  plz i need help

  320 x 1/2 x 1/2 x 1/2 (3 half-lifes) = 320 x 1/8
  = 40g
• alexkusu

  22 Sep 06, 23:02

  as above
• Darkness_hacker99

  22 Sep 06, 23:06

  Originally posted by 0lala:

  theres 320g of radioactive substance...with a half life of 4yrs....how much is left after 12 yrs?
  
  plz i need help

  Lolx. so easy
  
  Year 1 = 320g
  year 2 = 160g
  year 3 = 80g
  year 4 = 40g
• freedom4ever

  22 Sep 06, 23:28

  Originally posted by Darkness_hacker99:

  Lolx. so easy
  
  Year 1 = 320g
  year 2 = 160g
  year 3 = 80g
  year 4 = 40g

  lol. you teaching the wrong thing.
  half life of 4 years not 1 year.
• de_middle

  22 Sep 06, 23:35

  Originally posted by ^tamago^:

  i wonder how he's going to pass 'O' levels...
  
  all of us could write it down off-hand...
  
  

  he's bringing his laptop to the exams & when he has a question, just ask here & VIOLA......the answers will appear
• peeehead

  23 Sep 06, 00:19

  Originally posted by 0lala:

  theres 320g of radioactive substance...with a half life of 4yrs....how much is left after 12 yrs?
  
  plz i need help

  12 years divide by 4 years to get 3 half-lives.
  
  320 > 160 > 80 > 40
  
  *each > represents 1 half life.
  
  so there's 40g left.