Ambiguity

[kenhung123]

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?

[cameron_15]

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...

[olly_s15]

do them separately and equate

you are farrrr less likely to screw it up this way

[ks04]

I write them like m₁v₁+m₂v₂=m₃v₃
Makes it clear otherwise it implies they are equal.

[kenhung123]

Yea I do them seperately normally...does that mean its ok?

[ryan]

I'm also interested...

I usually write

Still not sure if it's correct; can anyone tell me?

[ghadz7]

Do you do UMEP ryan?

[Cthulhu]

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.

[Cthulhu]

I'm also interested...

I usually write

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


Where
is just some random mass that gets added/taken away from the system

Edit: Sorry. I think you mean in which case

[ryan]

I'm also interested...

I usually write

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


Where
is just some random mass that gets added/taken away from the system

Edit: Sorry. I think you mean in which case

Oh yeah haha dunno why I put delta  t___t

[Chavi]

Placing a delta in front of a variable just means "change in"

e.g.
just means change in initial momentum = change in final momentum

[ryan]

Do you do UMEP ryan?

Nope don't do uni maths

[/0]

Placing a delta in front of a variable just means "change in"

e.g.
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 are 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 is.