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SI Units equation help!

Hi everyone

I've got a question that I've answered, but I have a feeling my answer is completely wrong. Can anybody help point out where I've gone wrong, if I have?

The Question:
Work out the SI base units of y.

y = ax^2 / b

a = a constant with the units N (Newtons)
b = a constant with the units m (metres)
x = the units s (seconds)

My working out:
base units of N = kg m s^-2

y = (N) x (s^2) / m
y = (kg m s^-2) x (s^2) / m
= (s^-2) x (s^2) = s
= (kg m) / m = kg
= kg + s = kg s

units of y = kg s

Thats what I've got. Any help would be appreciated!
Reply 1
Avatar for B_9710
B_9710
18
Looks right.
Original post by thegallowsgirl

y = (N) x (s^2) / m
y = (kg m s^-2) x (s^2) / m
= (s^-2) x (s^2) = s

Both of the equalities on the last line are incorrect. For the first you've forgotten the kgmm\frac{\mathrm{kgm}}{\mathrm{m}} and for the second you haven't cancelled properly.

= (kg m) / m = kg

I agree with this although I don't know how you got it from the previous line.

= kg + s = kg s
This line doesn't make any sense. How did you get kg = kg + s? You can't add quantities with different dimensions so kg + s doesn't make sense. kg = kgs doesn't make sense either(not mentioning kg + s = kgs for the aforementioned reason). How did you get it?
(edited 8 years ago)
I didn't think it made sense either! But it's what I was left with at the end, thats why I thought I was doing something wrong.
I know the kg / m part is right, but I don't know about the (s^-2) x (s^2) - am I doing something wrong there when I get the answer (s) for that? Is there any way you anybody could walk me through it?
Reply 4
Avatar for Zacken
Zacken
22
Original post by thegallowsgirl
I didn't think it made sense either! But it's what I was left with at the end, thats why I thought I was doing something wrong.
I know the kg / m part is right, but I don't know about the (s^-2) x (s^2) - am I doing something wrong there when I get the answer (s) for that? Is there any way you anybody could walk me through it?


s^(-2) x s^2 = no units = 1.
So if I'm doing it correctly, then cancelling that out and doing the rest, the final answer would be something like kg (1), instead of kg s? Or am I getting confused!
Reply 6
Avatar for Zacken
Zacken
22
Original post by thegallowsgirl
So if I'm doing it correctly, then cancelling that out and doing the rest, the final answer would be something like kg (1), instead of kg s? Or am I getting confused!


Yep, kg(1) - otherwise known as just kg.

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