AAMC #5 Half life

[Godric]

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I thought since half life is the time it takes for the conc to decrease by half that I could go in reverse and knowing that 18,000 years ago it was created and that the half life of C14 is 6000 years, I could do this

18,000/6,000= 3 half lifes
so

1000----->1500----->2250----->3375

I did 1000/2=500+1000=1500/2=750+1500=2250/2=1125+2250=3375

The answers use 2^#of half lives, Ive never seen this before, does this work always ? and when can I use it.

 

[Sandstorm]

I thought since half life is the time it takes for the conc to decrease by half that I could go in reverse and knowing that 18,000 years ago it was created and that the half life of C14 is 6000 years, I could do this

18,000/6,000= 3 half lifes
so

1000----->1500----->2250----->3375

I did 1000/2=500+1000=1500/2=750+1500=2250/2=1125+2250=3375

The answers use 2^#of half lives, Ive never seen this before, does this work always ? and when can I use it.

No, you double not halve and add
It. It should be 1000 at the end, go back 6000 years you would have 2000, go back another 6000 years (12000 years) you would have 4000, go back another 6000 years (18000 years) you would have 8000

It would be 1000--->2000---->4000---->8000


Easiest way to solve these is just use your scratch paper to map it out and don't worry about tricks or equations

 

[LoLCareerGoals]

Your definition is more like a 2/3ds - life. Half means half is gone, not 1/3 is gone.

 

[shaboobly]

you know you have 1000 and that there were originally x amount 18000 years ago. 6000 half life was prob given in the passage so you can do 3 half lives in 18000 years. this means the measured amount will double, 2000, then double again, 4000, then double a 3rd time for 8000. that arithmetic you were doing.. idk.. seems like you had no idea about half life. hope this makes it easier though.

oh i see what you did, you took half of the original amount of the previous number and added it.. yea.. i dont think this is used for anything.

 

[sciencebooks]

I thought since half life is the time it takes for the conc to decrease by half that I could go in reverse and knowing that 18,000 years ago it was created and that the half life of C14 is 6000 years, I could do this

18,000/6,000= 3 half lifes
so

1000----->1500----->2250----->3375

I did 1000/2=500+1000=1500/2=750+1500=2250/2=1125+2250=3375

The answers use 2^#of half lives, Ive never seen this before, does this work always ? and when can I use it.

Remaining amount after 3 half lifes = 1,000. So original amount = 1,000 x 2^3 = 8,000.

 

[Godric]

No, you double not halve and add
It. It should be 1000 at the end, go back 6000 years you would have 2000, go back another 6000 years (12000 years) you would have 4000, go back another 6000 years (18000 years) you would have 8000

It would be 1000--->2000---->4000---->8000


Easiest way to solve these is just use your scratch paper to map it out and don't worry about tricks or equations

WTF why did I do what I did. Last time I am taking a FL at 3am.
Thanks everyone! I knew I needed to do the opposite, and I knew that half life is the time it takes to reduce to half the conc, but my for some reason I didnt do the opposite (double instead of halving)