Deriving Projectile Motion Equations (1 Viewer)

[kpq_sniper017]

suppose a question in an exam only involves a basic situation involving projectile motion i.e. starts from O, projected at an angle @ etc.
is it still necessary to derive all the equations....path, time of flight etc. or can u just quote them?
or is derivation only required when u have a variation, such as horizontal projection, point of projection not O etc.?

 

[Xayma]

You only have to derive the equations that you will be using in the exam (ie dont derive the x displacement if it asks purely for max height)

 

[mojako]

Agree.
You need to derive the things you're using... with the "integration" and "+ C" spelled out too according to my teacher, who might've said that just because he wants us to do it.
By "integration" I mean you need to say something like
a (acceleration) = 9.8
v = integral of a . dt
...
x = integral of v . dt
And I think you can write integral of v . dt instead of integral of 9.8t . dt, which is longer.

 

[gman03]

Originally posted by Xayma
You only have to derive the equations that you will be using in the exam (ie dont derive the x displacement if it asks purely for max height)

Originally posted by mojako
Agree.
You need to derive the things you're using... with the "integration" and "+ C" spelled out too ...

Originally posted by pcx_demolition017
can u just quote them?

In conclusion: don't quote them, derive them.

 

[:: ck ::]

onli derive the part of the q that it is asking

dont derive the part which leads to the part you are finding.... if that makes sense.. hahaha

 

[CrashOveride]

I dont think it does ryan = p

 

[BFEPNS]

wah??????

 

[mojako]

onli derive the part of the q that it is asking

dont derive the part which leads to the part you are finding.... if that makes sense.. hahaha

It does to me, after thinking for 5 seconds...

So, for example, a question has 2 parts.
Part(a):
Show that the equation for the horizontal displacement is this: blablabla..
Then you need to derive it properly.

Now,
Part(b):
Find the maximum height of the projectile (i.e. when the velocity is perfectly horizontal).
You don't need to fully derive the equation of the vertical velocity.

 

[CrashOveride]

I dont see how that follows from what he said ? anywayz

IF you dont do that, so do you just assume the s = ut + 1/2at² equation ?
From what i recall, i dont think your allowed to assume anything.

 

[CM_Tutor]

Don't assume Physics formulae like s = ut + (1 / 2)at² in projectile motion in Maths. You are supposed to be able to derive and solve the problems mathematically - the Physics formulae are not appropriate.

 

[mojako]

I dont see how that follows from what he said ? anywayz

IF you dont do that, so do you just assume the s = ut + 1/2at² equation ?
From what i recall, i dont think your allowed to assume anything.

I was giving an example...
In part (a) of the hypothetical question that I made,
we are asked to derive the equation for the horizontal component of the displacement.
In part (b), we are asked to find the maximum height. To find this maximum height we don't actually need to use an expression for horizontal displacement. [In real exams part (a) usually relates to part (b) though] One way of doing part (b) is to use the expression for the vertical component of the velocity and an expresion for vertical component of displacement. Find when the vertical velocity equals zero and that's when it reaches maximum height. Then put t = {that time} into the expression for vertical displacement.

Not the physics formula. What ryan was saying (from what I understand anyway) is that you can use/quote the general formulae, vertical-velocity = -g t + V sin@ and y = -0.5 g t^2 + V t sin@.
But I I were you I would just derive the formulae... I'm not sure if what ryan is saying is correct.

EDIT: on a second look those 2 formulae look very much like the physics formulae anyway... u = V sin@ or V cos@ depending on whether it's the vertical or horizontal part. If my interpretation on what ryan said is true, you still need to write the V and @ part.

 

[kpq_sniper017]

onli derive the part of the q that it is asking

dont derive the part which leads to the part you are finding.... if that makes sense.. hahaha

so if, for example, u were asked to find the range of the particle, could u just quote the path equation and go from there?
so it isn't necessary to derive the horizontal and vertical components first and find the path equation before proceeding to find the range?

 

[mojako]

so if, for example, u were asked to find the range of the particle, could u just quote the path equation and go from there?
so it isn't necessary to derive the horizontal and vertical components first and find the path equation before proceeding to find the range?

Well since none of us here is a teacher, I think you shouldn't rely too much on this..
and remember that teachers may have different ways of marking so it's best to avoid the risk.
But, as a guide, the number of marks allocated should give you a hint.
If it's 4 marks then you obviously NEED to derive the formula.
If it's 1 mark then you can quote it.

 

[kpq_sniper017]

Well since none of us here is a teacher, I think you shouldn't rely too much on this..
and remember that teachers may have different ways of marking so it's best to avoid the risk.
But, as a guide, the number of marks allocated should give you a hint.
If it's 4 marks then you obviously NEED to derive the formula.
If it's 1 mark then you can quote it.

true....
but i'd just like to know for sure.....just for future questions.

btw. in ur signature....don't the two Is and the + make I+I? that's not really an H is it??

 

[:: ck ::]

so if, for example, u were asked to find the range of the particle, could u just quote the path equation and go from there?
so it isn't necessary to derive the horizontal and vertical components first and find the path equation before proceeding to find the range?

if it were me

derive the time of flight equation (psht takes like 3 lines)

sub it into x velocity = vtcos@

go from there ...

 

[acmilan]

Sometimes they let you assume formulae. If they ask something like write down the formula for... and only give one mark for it (like in one of the recent CSSA trials) then you can just write it down without deriving. You have to look at the question to see what they want you to do.

 

[mojako]

true....
but i'd just like to know for sure.....just for future questions.

btw. in ur signature....don't the two Is and the + make I+I? that's not really an H is it??

hhmmmm...
originally I want to write "1+1=H", with 1 being written as "l" like in most ppl's handwriting. But the computer one doesn't look realy nice so I wrote "I" (and this new icon looks awful unfortunately....)

 

[kpq_sniper017]

hhmmmm...
originally I want to write "1+1=H", with 1 being written as "l" like in most ppl's handwriting. But the computer one doesn't look realy nice so I wrote "I" (and this new icon looks awful unfortunately....)

|+| ???
no bars at the top.

 

[kpq_sniper017]

Sometimes they let you assume formulae. If they ask something like write down the formula for... and only give one mark for it (like in one of the recent CSSA trials) then you can just write it down without deriving. You have to look at the question to see what they want you to do.

yea.
coz some guys in my class went through the whole "integrating from first principles" process when they were asked to prove that dN/dt was a solution of N in a "Newton's law of cooling" question....waste of time (and they didn't get the answer anyway)
so i guess the best way is just to look at the mark value...i'll look through some past papers and see what they ask for....might give me a better indication of what to do.

 

[mojako]

|+|???
no bars at the top.

Sooo trueee...
how come I didn't realise that? even when I was writing l + l in my previous post... [I guess this is what meant by daydreaming in my post titled "interpretation"]
BTW I used the letter l (el) in my previous post and you used the vertical bar thing.. el doesn't look like a straight line in Times New Roman. I (i) looks like straight line in Arial and I think that's why I used it.

Also from you:
>> coz some guys in my class went through the whole "integrating from first principles" process when they were asked to prove that dN/dt was a solution of N in a "Newton's law of cooling" question....waste of time (and they didn't get the answer anyway) <<
didn't you mean, N was a solution to dN/dt?