OCR Physics A G484 Jan 2012 - The Newtonian World

Original post by shorty.loves.angels

Why did I not know that?? Grr, not gona forget it now!

Argh you lot are making me panic about this exam.

I'm doing a past paper tonight :colonhash:

Haha, to be fair, that particular question is unlikely to pop up :tongue:

But you know now :biggrin:

Reply 101

Ralphus J

Original post by wibletg

Haha, to be fair, that particular question is unlikely to pop up :tongue:

But you know now :biggrin:

Its come up before "describe the difference between frequency and angular frequency...... that was the question. :rolleyes:

Reply 102

shorty.loves.angels

16

Original post by Ralphus J

Its come up before "describe the difference between frequency and angular frequency...... that was the question. :rolleyes:

I hate wordy questions. I'm not good at wording things... well I am but not on the spot. I could describe them using absolutely no key words, but then I'd get absolutely no marks.

Reply 103

shorty.loves.angels

16

In momentum questions, I never know whether I'm supposed to be adding or subtracting. What's the deal? :sad:

If I'm calculating total momentum before and after, then I do m1v1 - m2v2 right? Remembering that if object '2' is travelling in the opposite direction then its velocity should be a minus... and hopefully by doing so I'll know what direction the resultant momentum is in.

Or according to the mark scheme you don't make object 2's velocity minus. Ugh, don't know what's what here at all.

(edited 12 years ago)

Reply 104

Ralphus J

Original post by shorty.loves.angels

In momentum questions, I never know whether I'm supposed to be adding or subtracting. What's the deal? :sad:

If I'm calculating total momentum before and after, then I do m1v1 - m2v2 right? Remembering that if object '2' is travelling in the opposite direction then its velocity should be a minus... and hopefully by doing so I'll know what direction the resultant momentum is in.

Right?

Dont you need to know if the two objects stick together or not?

Reply 105

Ralphus J

Is this guy wrong, i swear it should be T^2 / r^3 = k ?? not what hes put?

http://www.youtube.com/watch?v=DJfu9R-4new

Reply 106

wibletg

3

Original post by shorty.loves.angels

In momentum questions, I never know whether I'm supposed to be adding or subtracting. What's the deal? :sad:

If I'm calculating total momentum before and after, then I do m1v1 - m2v2 right? Remembering that if object '2' is travelling in the opposite direction then its velocity should be a minus... and hopefully by doing so I'll know what direction the resultant momentum is in.

Or according to the mark scheme you don't make object 2's velocity minus. Ugh, don't know what's what here at all.

m1v1 + m2v2 is the original form of the equation.

however if m2v2 is travelling in the opposite direction - your equation would look like: m1v1 - m2v2.

Reply 107

shorty.loves.angels

16

Original post by Ralphus J

Dont you need to know if the two objects stick together or not?

That's usually in the question or diagram.

Original post by wibletg

m1v1 + m2v2 is the original form of the equation.

however if m2v2 is travelling in the opposite direction - your equation would look like: m1v1 - m2v2.

Ah I get it. I swear they just change the signs in the equation in the books without telling you why for the fun of it.

Cheers :yy:

Reply 108

Jukeboxing

8

Original post by shorty.loves.angels

In momentum questions, I never know whether I'm supposed to be adding or subtracting. What's the deal? :sad:

If I'm calculating total momentum before and after, then I do m1v1 - m2v2 right? Remembering that if object '2' is travelling in the opposite direction then its velocity should be a minus... and hopefully by doing so I'll know what direction the resultant momentum is in.

Or according to the mark scheme you don't make object 2's velocity minus. Ugh, don't know what's what here at all.

I was getting confused with this too, then my teacher said before starting, give directions a +/- ve sign, like let the direction to the right be positive.

Then start your calculations, give the values there +ve or -ve sign. And another thing is, sometimes, write your equations for before and after seperately, then make them equal if necessary, remembering that +ve and -ve signs of the values remain.

Reply 109

Ralphus J

Original post by Ralphus J

Is this guy wrong, i swear it should be T^2 / r^3 = k ?? not what hes put?

http://www.youtube.com/watch?v=DJfu9R-4new

bump please?

Reply 110

wibletg

3

Original post by Ralphus J

bump please?

I think he's correct :s-smilie:

Reply 111

Ralphus J

Original post by wibletg

I think he's correct :s-smilie:

but keplers 3rd law is T ^2 is directly proportional to r ^3
so by re arranging T^2 / r^3 = k surely? not r^3 / T^2 = k

Reply 112

XiaoXiao1

Original post by wibletg

Angular frequency is 2pi times the frequency :tongue:

daftest definition ever.

and amplitude does only decrease with damping in SHM.

Damping only ever affects the frequency during resonance (the resonant frequency shifts to the left very slightly) :smile:

Correct me if wrong, but I think damping (at least the light damping type which leads to an exponential decay in amplitude), changes the natural frequency of oscillation, which causes the frequency at which resonance occurs to decrease.

Reply 113

wibletg

3

Original post by Ralphus J

but keplers 3rd law is T ^2 is directly proportional to r ^3
so by re arranging T^2 / r^3 = k surely? not r^3 / T^2 = k

Whoops, just did that there using gravitational force = centripetal force and you're right :tongue:

Original post by XiaoXiao1

Correct me if wrong, but I think damping (at least the light damping type which leads to an exponential decay in amplitude), changes the natural frequency of oscillation, which causes the frequency at which resonance occurs to decrease.

Are you on about damping during SHM here? :smile:

@ wibletg: yes.

Original post by XiaoXiao1

@ wibletg: yes.

The way I understand it -

Damping during forced oscillations leads to decrease in amplitude due to small decrease in natural frequency of system.

Damping during free oscillations leads to decrease in amplitude due to decrease in energy in the system.

Feel free to correct me :tongue:

Reply 116

XiaoXiao1

So if you have an oscillating system (e.g. a mass on a spring), it will oscillate at a natural angular frequency w0 in the absence of friction, but if damping is present then the frequency decreases, as well the amplitude, I think the effect is the same is the oscillations were forced.

Which means that for resonance to occur in a damped system, the driving force needs to have a lower frequency than if the damping wasn't there, which means the graph of amplitude against frequency becomes skewed.

Sorry if that wasn't very clear... :tongue:

Reply 117

XiaoXiao1

*By decreases I meant relative to the natural undamped frequency, not with time.