Projectile Motion (1 Viewer)

[danz90]

I'm a little confused about when it is needed to use the formulas:
v = u + at
s = ut + 1/2.at^2
v^2 = u^2 + 2as

Are they only for motion under constant acceleration?

So when I'm doing a projectile motion question.. i should use:

x''= 0
x' = v.cos(t)
x = vt.sin(t)

y'' = -g
y' = -gt + v.sin(t)
y = (-gt^2 / 2) - vt.cos(t)

....right?
I would really appreciate some explanation to simplify it... can get confusing.

 

[AkaiHanabi]

The first three formulas are only used in Physics. If you use them in maths, I think you can only use them if they are given to you, ie the question says you may assume. Otherwise, when they test you in Projectile motions, derive from x" = 0 and y" = -g

 

[steer-rider]

When doing horizontal and vertical displacment in projectile motion (that is, 'x' and 'y') you need to remember that there is a contant present, however it is quite often equal to zero. It should be given to you, either as a pronumeral (often a capital letter) or a number, during the question and is required to be used. That is why, when you are integrating from x' or x'' (and similarly y' and y'') that the answer has a constant, and requires a substitution of the given values (these are normally initial conditions). Be careful for tricks, such as x=-4 at t=0, or a skydiver is oushed out of a plane by a deranged instructor at 1.5 km high (which is y=1500 when t=0). In the failure of them to give you any conditions, assume that at t=0, x=0 and y=0.

It is a similar story for other motions, always REMEMBER THE CONSTANT!!

 

[steer-rider]

Also, don't assume that g=10, because it is not nessecarily a question with terrestial applications. Definitely don't do it in extension two, unless you want a pointless argument with your teacher over whether there are tables on Mars or not (and why on Mars they are there...)

 

[wendus]

i don't think you can use physics equations in maths at all. you need to derive them like all the above have said.

 

[bachem]

when i finish the physics hsc exam, im going to burn the physics textbook plus my folders

I HATE PHYSICS WITH A PASSION!!!!

 

[wendus]

me too. so glad i didn't choose it. i didn't even understand basic year 10 physics... lol

 

[danz90]

those equations are wrong, you need to use a different pronumeral for the angle. also your y and x is wrong. they should be:

x''= 0
x' = v.cos@
x = vt.cos@

y'' = -g
y' = -gt + v.sin@
y = (-gt^2 / 2) + vt.sin@

that might save some confusion

ahhh yes thanks heaps... we look at vcos@ and vsin@ as constants.. so when integrating w.r.t t it becomes vtcos@ and vtsin@ respectively..
stupid me lol

arghh i hate projectile motionssss

 

[vds700]

when i finish the physics hsc exam, im going to burn the physics textbook plus my folders

I HATE PHYSICS WITH A PASSION!!!!

me too, except replace "physics" with "english" Haha.

BTW if the questionn says "state" the equations for horizontal and vertical displacement, you dont need to derive them. So its still a good idea to memorise them, although in most cases, they will ask u to derive them.

 

[Aerath]

when i finish the physics hsc exam, im going to burn the physics textbook plus my folders

When people say that, do they actually literally mean burning it?

 

[omniscience]

When people say that, do they actually literally mean burning it?

Why not? I have seen few videos showing people burning their own HSC tasks.

But in my case, I would donate all my books to library.