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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: kenhung123 on June 05, 2010, 08:15:56 pm

Title: Ambiguity
Post by: kenhung123 on June 05, 2010, 08:15:56 pm: When we use a formula like Pi=Pf
mv=mv

where the 2 m's are not equal and 2 v's are not equal, does it matter in physics or its best to write like m1v1=m2v2?
Title: Re: Ambiguity
Post by: cameron_15 on June 05, 2010, 08:43:34 pm: Write it how you've done it the second time with the subscripts. Otherwise it likely to be assumed that the masses are equal, which may not be the case...
Title: Re: Ambiguity
Post by: olly_s15 on June 05, 2010, 09:31:16 pm: do them separately and equate

you are farrrr less likely to screw it up this way
Title: Re: Ambiguity
Post by: ks04 on June 05, 2010, 09:40:37 pm: I write them like m₁v₁+m₂v₂=m₃v₃
Makes it clear otherwise it implies they are equal.
Title: Re: Ambiguity
Post by: kenhung123 on June 05, 2010, 09:48:28 pm: Yea I do them seperately normally...does that mean its ok?
Title: Re: Ambiguity
Post by: ryan on June 05, 2010, 11:29:34 pm: I'm also interested...

I usually write $\Delta Pi = \Delta Pf$

Still not sure if it's correct; can anyone tell me?
Title: Re: Ambiguity
Post by: ghadz7 on June 05, 2010, 11:30:27 pm: Do you do UMEP ryan?
Title: Re: Ambiguity
Post by: Cthulhu on June 05, 2010, 11:43:13 pm: Quote from: kenhung123 on June 05, 2010, 08:15:56 pm
When we use a formula like Pi=Pf
mv=mv

where the 2 m's are not equal and 2 v's are not equal, does it matter in physics or its best to write like m1v1=m2v2?
Yes. You need to define what these masses are. You cannot say "mv = mv" if the two m and v values are different.
Title: Re: Ambiguity
Post by: Cthulhu on June 06, 2010, 12:26:40 am: Quote from: ryan on June 05, 2010, 11:29:34 pm
I'm also interested...

I usually write $\Delta Pi = \Delta Pf$

Still not sure if it's correct; can anyone tell me?
This is correct as long as when you write them in terms of mv you define what m and v are for each case.
I.e
$<br />P_i = P_f<br />$
$m_i v_i = m_f v_f$
Where $m_f = m_i + m_x$
$m_x$ is just some random mass that gets added/taken away from the system

Edit: Sorry. I think you mean $P_i = P_f$ in which case $\Delta P = 0$
Title: Re: Ambiguity
Post by: ryan on June 06, 2010, 09:49:34 pm: Quote from: Cthulhu on June 06, 2010, 12:26:40 am
Quote from: ryan on June 05, 2010, 11:29:34 pm
I'm also interested...

I usually write $\Delta Pi = \Delta Pf$

Still not sure if it's correct; can anyone tell me?
This is correct as long as when you write them in terms of mv you define what m and v are for each case.
I.e
$<br />P_i = P_f<br />$
$m_i v_i = m_f v_f$
Where $m_f = m_i + m_x$
$m_x$ is just some random mass that gets added/taken away from the system

Edit: Sorry. I think you mean $P_i = P_f$ in which case $\Delta P = 0$

Oh yeah haha dunno why I put delta t___t
Title: Re: Ambiguity
Post by: Chavi on June 06, 2010, 09:55:43 pm: Placing a delta in front of a variable just means "change in"

e.g.
$\Delta Pi = \Delta Pf$ just means change in initial momentum = change in final momentum
Title: Re: Ambiguity
Post by: ryan on June 06, 2010, 10:37:06 pm: Quote from: ghadz7 on June 05, 2010, 11:30:27 pm
Do you do UMEP ryan?

Nope don't do uni maths
Title: Re: Ambiguity
Post by: /0 on June 06, 2010, 10:47:27 pm: Quote from: Chavi on June 06, 2010, 09:55:43 pm
Placing a delta in front of a variable just means "change in"

e.g.
$\Delta Pi = \Delta Pf$ just means change in initial momentum = change in final momentum

When you have a 'change' in a quantity it means that, along a certain trajectory, that quantity can occupy different values.

But $P_i$ are $P_f$ can only have unique values along a single trajectory in a defined interval of time, so it doesn't make sense to have a 'change' in each of them. They are not 'variables' in the sense that $P$ is.