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# the following calculations all involve physical quantities

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1. identify any calculations that are meaningless and evaluate those that are valid?
63s+52s-34s-41s
i dont know what i am supposed to do with that, it doesen't even equal anything, i dont understand the question either. please help
2. (Original post by p29)
identify any calculations that are meaningless and evaluate those that are valid?
63s+52s-34s-41s
i dont know what i am supposed to do with that, it doesen't even equal anything, i dont understand the question either. please help
Post a picture of the question.
3. (Original post by Zacken)
Post a picture of the question.
4. any ideas?
5. (Original post by p29)
Alright, so the first one is valid because you are subtracting and adding units of time to get an overall positive time. Nothing fishy with that.

However, some of them are completely invalid, how can you add amperes to volts like in 3??

You get the gist?
6. (Original post by Zacken)
Alright, so the first one is valid because you are subtracting and adding units of time to get an overall positive time. Nothing fishy with that.

However, some of them are completely invalid, how can you add amperes to volts like in 3??

You get the gist?
ohhh, thanks
what if there was negative time, would that be invalid?
7. (Original post by p29)
ohhh, thanks
what if there was negative time, would that be invalid?
Yep! Precisely, it makes no sense (in most scenarios) to have a negative time.
8. Lol.. Think of units as different fruits. You cant add a banana to an orange and vice versa. You cant add a mV to a mA as volts and amperes measure two different things. A mJ and J can be added because a milli joule is 1/1000ths of a joule. The units needs to be homogeneous to be added/ minused.
9. (Original post by Zacken)
Yep! Precisely, it makes no sense (in most scenarios) to have a negative time.
once again, thank you
10. (Original post by p29)
once again, thank you
I would say it doesn't invalidate it because if you're referring to relationships between times in a rates of reaction experiment, say, you may want to subtract times.
11. (Original post by ValerieKR)
I would say it doesn't invalidate it because if you're referring to relationships between times in a rates of reaction experiment, say, you may want to subtract times.
oh god
12. (Original post by p29)
oh god
The only other questionable one (than those mentioned) is Newtons being a vector rather than scalar, but again it does not invalidate it because there are situations where it makes sense.
13. (Original post by ValerieKR)
I would say it doesn't invalidate it because if you're referring to relationships between times in a rates of reaction experiment, say, you may want to subtract times.
...I'm glad I didn't do chemistry.

But, are you sure you are subtracting times? And not ? Because that's what rate usually means (from what I know).
14. (Original post by Zacken)
...I'm glad I didn't do chemistry.

But, are you sure you are subtracting times? And not ? Because that's what rate usually means.
you can talk about half lives with s^1
15. (Original post by Gman786)
Lol.. Think of units as different fruits. You cant add a banana to an orange and vice versa. You cant add a mV to a mA as volts and amperes measure two different things. A mJ and J can be added because a milli joule is 1/1000ths of a joule. The units needs to be homogeneous to be added/ minused.
homogenous? plz explain
16. (Original post by ValerieKR)
you can talk about half lives with s^1
But does a negative half life make any sense?
17. (Original post by p29)
homogenous? plz explain
The same up to a constant.

So you can add mJ and J because they're essentially joules, just that mJ has a constant factor attached to the Joules.
18. (Original post by Zacken)
But does a negative half life make any sense?
You could find the difference between two half lives
19. (Original post by Zacken)
The same up to a constant.

So you can add mJ and J because they're essentially joules, just that mJ has a constant factor attached to the Joules.
Yh exactly what zacken said. Homogeneous means "same"
20. (Original post by ValerieKR)
You could find the difference between two half lives
Fair enough.

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