The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

the following calculations all involve physical quantities

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

Scroll to see replies

Reply 1
Avatar for Zacken
Zacken
22
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.
Reply 2
Avatar for p29
p29
OP
12
image.jpg
Original post by Zacken
Post a picture of the question.
Reply 3
Avatar for p29
p29
OP
12
any ideas?
Reply 4
Avatar for Zacken
Zacken
22
Original post by p29
image.jpg


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?
Reply 5
Avatar for p29
p29
OP
12
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:smile:
what if there was negative time, would that be invalid?
Reply 6
Avatar for Zacken
Zacken
22
Original post by p29
ohhh, thanks:smile:
what if there was negative time, would that be invalid?


Yep! Precisely, it makes no sense (in most scenarios) to have a negative time.
Reply 7
Avatar for Gman786
Gman786
3
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.
Reply 8
Avatar for p29
p29
OP
12
Original post by Zacken
Yep! Precisely, it makes no sense (in most scenarios) to have a negative time.


once again, thank you:smile:
Original post by p29
once again, thank you:smile:


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.
Reply 10
Avatar for p29
p29
OP
12
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
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.
Reply 12
Avatar for Zacken
Zacken
22
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. :tongue:

But, are you sure you are subtracting times? And not s−1s^{-1}? Because that's what rate usually means (from what I know).
Original post by Zacken
...I'm glad I didn't do chemistry. :tongue:

But, are you sure you are subtracting times? And not s−1s^{-1}? Because that's what rate usually means.


you can talk about half lives with s^1
Reply 14
Avatar for p29
p29
OP
12
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
Reply 15
Avatar for Zacken
Zacken
22
Original post by ValerieKR
you can talk about half lives with s^1


But does a negative half life make any sense?
Reply 16
Avatar for Zacken
Zacken
22
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.
Original post by Zacken
But does a negative half life make any sense?


You could find the difference between two half lives
Reply 18
Avatar for Gman786
Gman786
3
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"
Reply 19
Avatar for Zacken
Zacken
22
Original post by ValerieKR
You could find the difference between two half lives


Fair enough. :tongue:

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you