# Homework Help: General motion of a particle

1. Apr 8, 2012

### jiboom

a particle moves in the xy plane such that at a time t it is at a point (x,y) where

x=2cos(at)
y=2sin(at)

prove particle moves in a circular path with constant angular velocity. prove that the acceleration of the particle at time t is in the direction of the radius from the particle to the centre of its path.

so i have

R=[2cos(at),2sin(at)]

V=[-2asin(at),2acos(at)]

A=[-2a^2cos(at),-2a^2sin(at)]

not sure how to show angular is constant.

i can take |V| and that s constant but im not using angular velocity.

for the second part:

if if i take direction of A= (d2y/dt2)/(d2x/dt2)=tan(at) where d2y/dt2 is second derivative of y wrt t

does this answer the question as motion is circle centre (o,0) so any radius will have tan (at) as its gradient?

2. Apr 8, 2012

### tiny-tim

hi jiboom!

(try using the X2 button just above the Reply box )
that's fine … now use v = rω (and r is constant)
erm

isn't it obvious that A is parallel to (x,y)?

3. Apr 8, 2012

### jiboom

now you point it out, yes it is:)

i was going to use v = rω but as i am taking |v| am i not just getting angular speed not velocity?

4. Apr 8, 2012

### jiboom

think im being thick.

is it that as angular velocity only depends on the angle we have |ω|=ω

just as v=|v| for 1-d motion

5. Apr 8, 2012

### jiboom

sorry but im now confused again.

the question actually says
x=2cosωt
y=2sin ωt

so

V=V=[-2ωsin(ωt),2ωcos(ωt)]

so |V|=2ω

so using v=rω only gives me r=2????

6. Apr 8, 2012

### HallsofIvy

No, you knew from the original position vector that the mass was moving in a circle with radius 2: if $x=2 cos(\omega t)$and $y= 2 sin(\omega t)$, then $x^2+ y^2= 4 cos^2(\omega t)+ 4 sin^2(\omega t)= 4$.

The calculation you have done is the derivation of $v= r\omega$!

7. Apr 8, 2012

### jiboom

so what do i need to say to justify angualr velocity is constant?

8. Apr 8, 2012

### tiny-tim

hi jiboom!

(been out for the evening )

yup, x=2cosωt y=2sin ωt means r = 2 and angular speed = ω !
again, it's obvious!

go to sleep on it :zzz:, and it'll all be obvious in the morning!

9. Apr 8, 2012

### jiboom

lol. wont be sleeping til i get this.

i dont have much here to play with.

v=rω

now is ω angular velocity or angular speed?

the v i have is not angular velocity and not constant

r is 2

now in your first post you said to use v=rω but then i just get r=2 so that does not show
angular velocity = constant

think its going to be long night unless i get a big nudge here,its not obvious to me due to a lack of understanding about velocity v speed |v| angular velocity ω and angular speed | ω|

10. Apr 8, 2012

### tiny-tim

what's the difference??

we're in 2D, so they're the same
but you know that v is constant (from the formula), so it does

i think you're over-thinking this

v is velocity, v is speed, v = |v|

ω is angular velocity, ω is angular speed, ω = |ω|

v = rω

v = ω x r

Last edited: Apr 9, 2012
11. Apr 8, 2012

### jiboom

i see what you are saying but im thinking along these lines:

ω

then how can i then say |v| is constant? |v| is 2ω so is only constant if ω is but that is what im trying to show

and i dont understand

v = ω x r

lhs is a vector,but rhs is vector product of 2 scalars ?

12. Apr 8, 2012

### jiboom

tiny-tim has gone off-line. can you help explain what they were getting at?

13. Apr 9, 2012

### tiny-tim

hi jiboom!

(just got up :zzz:)
v = |v| is constant because the coordinate formula for v shows it is
no, ω and r are both vectors, the rhs is a vector product of two vectors

(technically, ω is a pseudovector )

v = ω x r is the definition of the angular velocity vector for a rigid body

Last edited: Apr 9, 2012
14. Apr 9, 2012

### jiboom

im afraid im totally lost here.

v = |v| is equal to 2ω so how are you saying this is constant?

im trying to show ω is constant which you seem to be saying,forgive me if im misunderstanding, is equal to ω

so i dont see how the constancy of v follows without assuming what im trying to show, ie ω is constant.

v = ω x r: still dont see what the vector is for ω. i thought the r was radius,but i now think its the postion vector of the particle.

15. Apr 9, 2012

### tiny-tim

hi jiboom!
it follows from the following coordinate equations …
… what is |v| ?
ω is the vector of magnitude ω pointing along the axis of rotation

and yes, r is the position vector of the particle

important

v = ω x r is an equation you need to be completely familiar and happy with

you need to study it in your books or wikipedia or other internet site (google "angular velocity" and "rigid body") until you are

16. Apr 9, 2012

### jiboom

as i say |v|=2ω, but if ω is the angular velocity then i cant say this is constant without assuming what i want to prove.

i should say,i have changed that a in my op to ω as that is the letter the question uses.

17. Apr 10, 2012

### jiboom

anyone ?

18. Apr 12, 2012

### jiboom

hi. dont give up on me just yet :)

i do appreciate your help and just need one last shove to get there.

all i need convincing off now is that the omega=angular speed =|v| is constant.

does this follow from the question? if this is so,then for sure i am happy |v|=constant => angular velocity is.

19. Apr 12, 2012

### tiny-tim

in your original post, you wrote …
so |V| = … ?

20. Apr 12, 2012

### jiboom

hi,thanks for coming back.

forget the a,the letter used in the question was omega.

i know what |V| is,it is just 2omega

but....

if this omega (a in my original post) is the angular speed i cant say |v| is constant UNLESS i know omega in the question is constant?

maybe i should be clearer, when in question i have

R=(2cos wt,2sinwt)

as this w is not said to be constant in the question i could have w=w(t) and should really have

V=(-2sin(wt)(d(wt)/dt),2cos(wt)(d(wt)/dt)

21. Apr 12, 2012

### jiboom

maybe an easier question would be to clear up what the w is in the question

so from

R=(2coswt,2sinwt)

is the w the same as |w| the angular speed?

if so,is it just to be assumed its a number and not a function of some variable such as time as it's not writtem w(t)?

22. Apr 12, 2012

### tiny-tim

oh i see!!! :tongue2:

no, i think it's safe to assume that the ω in the question is constant (in exactly the same way as you would if it was a instead of ω ) …

it never occurred to me to think otherwise!​

stop imagining that the questions are more difficult than they really are!
yes!!

23. Apr 12, 2012

### jiboom

great,thanks for your time with this.