# Thread: Solving For Half Life Question

1. Activity: 240Bq
After 1.5 hours it is 30 Bq
What is the half life?

2.

3. Originally Posted by postiveion
Activity: 240Bq
After 1.5 hours it is 30 Bq
What is the half life?
This is homework, no?

Unfortunately we have a policy against doing other people's homework for them. What can be done, is you show us what you have done so far or tell us what specifically you are having difficulty in understanding and then we can nudge you in the right direction. Homework assignments are there for your own good. You'll develop a much better functional understanding of what you are learning if you mostly figure it out by yourself.

4. Okay .

I had attempted at doing it but it really confused me and I am not sure if I did it right so I wanted to check.

My attempt at working it out.

240 - 120 - 60 - 30
1 2 3 4
X + 2x +3x + 4X = 10

240/10 = 24 min

5. No, that's wrong. By looking at the first line of what you wrote, you can see that the amount of radioactive material has gone through 3 half-lives to go from 240 to 30. If 3 half lives is 90 minutes, what's one half-life? This one is easy to do by inspection but in the general case you would use the formulas given here:
http://en.wikipedia.org/wiki/Half-li...onential_decay
You will end up with an equation with exponentials and you will have to solve it using logarithms.

6. Originally Posted by postiveion
Activity: 240Bq
After 1.5 hours it is 30 Bq
What is the half life?
240Bq 2^(-1.5hr/h) = 30Bq

When you solve this for the half life h you get a nice number. In retrospect the ratio between 240Bq and 30Bq shows me why this is the case.

7. Try halving 240 several times and see what you get.

Bear in mind the term halflife.

8. I just have a question about radiometric dating. It assumes that whatever material you are using to determine how much decay has occured, uranium, strontium, or whatever, you have to ensure that it has only started to decay at the time it entered whatever material you are measuring. That's one thing I have never understood, because why would it only start to decay once it had entered a bowl or a body or a rock, or whatever you are measuring? Could someone please answer this questino for me? Thanks, GH

9. Originally Posted by Gary Hart

10. Thank you Harold. I went there and some people were asking about could Uranium be leached out of rock, and would that change the dates, etc., and it seems like they were having a hard time just getting a unanimous, straight answer on THAT question, but I guess I'll forge on ahead in my lonely search for the truth....is radiometric dating a science, or the biggest scam ever devised by man? Tune in next week....

11. Originally Posted by Gary Hart
Thank you Harold. I went there and some people were asking about could Uranium be leached out of rock, and would that change the dates, etc., and it seems like they were having a hard time just getting a unanimous, straight answer on THAT question, but I guess I'll forge on ahead in my lonely search for the truth....is radiometric dating a science, or the biggest scam ever devised by man? Tune in next week....
Have you checked the obvious?

Scientists really want to find out how old stuff is and lying about it serves no purpose. They don't generally really care what creationists think, because most of them are untrained and use unscientific methods to arrive at their conclusions. The situation is often portrayed as an even battle between the two groups, while the "battle" part mostly comes from creationists. Scientists don't do science for the sole purpose of debunking religion!

12. That's good to know. I'm not sure many people are aware of it, but both Isaac Newton and Albert Einstein were (maybe I should say are) Christians. I don't think anyone would accuse them of being stupid. Also I have read some books by Henry Morris, you may have heard of him? GH :wink:

13. I'm not sure many people are aware of it, but both Isaac Newton and Albert Einstein were (maybe I should say are) Christians
Albert Einstein was a Jew, my dear friend. That was why he was forced to flee from Germany in the Second World War.

Also, Isaac was what we may call a heretic. He was troubled by the Bible, and did not accept it as the truth, although no one may deny that he was fervently devoted to God.

Both scientists believed in God; Einstein in Spinoza's version of a God who does not interfere in human activities, and Newton, who believed that it was necessary for God to keep the universe in check. He had to: his theory of gravity predicted that the universe was not stable, but instead expanded or contracted. We now know this is correct; however, this was such an outlandish prospect that until Hubble nobody seriously accepted it.

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