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October 22, 2020, 06:07:03 pm

### AuthorTopic: VCE Physics Question Thread!  (Read 322505 times) Tweet Share

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#### rozmaaate

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##### Re: VCE Physics Question Thread!
« Reply #2370 on: September 06, 2020, 07:45:46 pm »
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Hey guys I’m super confused with this question as I thought in order to calculate change in velocity in 2 dimensions you are suppose to add the 2 vectors like this, v+(-u), However this isn’t shown in the solution for this questions, as the drawing shown should be drawn head to tail , so 16m/s south west then -u which is 20m/s north west.

#### Owlbird83

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##### Re: VCE Physics Question Thread!
« Reply #2371 on: September 06, 2020, 08:30:27 pm »
+3
Hey guys I’m super confused with this question as I thought in order to calculate change in velocity in 2 dimensions you are suppose to add the 2 vectors like this, v+(-u), However this isn’t shown in the solution for this questions, as the drawing shown should be drawn head to tail , so 16m/s south west then -u which is 20m/s north west.

If the ball moved in a straight line forwards or back you could do do the calculations like you said, but because it's going in the south west then north west directions you have to draw out the vectors in a triangle, and calculate the resultant vector (the one connecting from the start of the initial to end of the final) using pythagoras to find the change in velocity. And because velocity is a vector you need the direction too, so to find the direction of the resultant vector/change in velocity, you must find the angle in the triangle in the picture in the answers, then add 45deg, because you know the initial velocity is 45deg from south (south-west).
I hope that answers your question? Let me know if you are still confused.
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#### rozmaaate

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##### Re: VCE Physics Question Thread!
« Reply #2372 on: September 06, 2020, 09:08:31 pm »
+1
If the ball moved in a straight line forwards or back you could do do the calculations like you said, but because it's going in the south west then north west directions you have to draw out the vectors in a triangle, and calculate the resultant vector (the one connecting from the start of the initial to end of the final) using pythagoras to find the change in velocity. And because velocity is a vector you need the direction too, so to find the direction of the resultant vector/change in velocity, you must find the angle in the triangle in the picture in the answers, then add 45deg, because you know the initial velocity is 45deg from south (south-west).
I hope that answers your question? Let me know if you are still confused.

Thanks for your time , unfortunately I’m still confused here’s what I have so far

#### Owlbird83

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##### Re: VCE Physics Question Thread!
« Reply #2373 on: September 06, 2020, 09:38:38 pm »
+2
Thanks for your time , unfortunately I’m still confused here’s what I have so far

All good!
Sorry, when I referred to the angle in the triangle in my last post, I muddled it up a little.
Although the answers suggest calculating the angle on the right side of the triangle, I reckon it's easier to do it like this:
(also I hope this isn't confusing, I'm calculating a different theta than they are in the answers, cos it seems more simple to me).
my drawing
I hope my drawing makes it a little easier to understand.
You want to find the direction of the final velocity, and you can find the angle that I found in my picture, and add this (38.7) to 45 which equals S83.7W.

Spoiler
Alternatively, you have found the angle on the right side correctly, so since you know there's 180deg in a triangle you can do 180-90-51.3=38.7 which will be the angle on the left side
I can get a little confused with direction stuff too, and I found it useful to draw the little axis to orient the diagram which helped!

Let me know if you want more clarification!

Edit: oh no I did this the wrong way around, ahhh I'm sorry, I hope I didn't make you more confused?
I feel bad, ignore what I said cos I did it all wrong and am also confused
« Last Edit: September 07, 2020, 11:27:32 am by Owlbird83 »
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#### ImamB1

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##### Re: VCE Physics Question Thread!
« Reply #2374 on: September 07, 2020, 07:02:13 am »
0
Hi,

Does anyone have a list of the questions removed from the past VCAA Physics examinations. I’ve only found a list from 2017 onwards but I’m looking to go much prior to that.

#### Newton is Nice

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##### Re: VCE Physics Question Thread!
« Reply #2375 on: September 24, 2020, 09:09:16 pm »
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Hello, I was just curious to know how an AC generator reverses the current of the coil even though it only has slip rings? I though only a split ring commutator can reverse the current of the coil.
Anyone's help would be greatly appreciated.

#### Bri MT

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##### Re: VCE Physics Question Thread!
« Reply #2376 on: September 24, 2020, 10:21:28 pm »
+1
Hello, I was just curious to know how an AC generator reverses the current of the coil even though it only has slip rings? I though only a split ring commutator can reverse the current of the coil.
Anyone's help would be greatly appreciated.

Hey!

In an AC generator the reason you have current going in the opposite direction sometimes is because the change in flux is in the opposite direction & therefore the induced current is in the other direction so that the change is still opposed; the change in direction is definitely not because of slip rings "swapping it" or anything. I would recommend not saying "an AC generator reverses the current of the coil" because then the person marking you might think you're getting confused with how split ring commutators work.

I recommend looking at diagrams of flux with the coil diagrams and trying to figure out what the direction of current should be at different points to consolidate this info.

I hope this helps, please feel free to reply with any further questions

#### Newton is Nice

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##### Re: VCE Physics Question Thread!
« Reply #2377 on: September 25, 2020, 11:13:25 am »
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Hey!

In an AC generator the reason you have current going in the opposite direction sometimes is because the change in flux is in the opposite direction & therefore the induced current is in the other direction so that the change is still opposed; the change in direction is definitely not because of slip rings "swapping it" or anything. I would recommend not saying "an AC generator reverses the current of the coil" because then the person marking you might think you're getting confused with how split ring commutators work.

I recommend looking at diagrams of flux with the coil diagrams and trying to figure out what the direction of current should be at different points to consolidate this info.

I hope this helps, please feel free to reply with any further questions
Thanks for replying. So essentially, it s the magnets that are reversing, right?

#### Newton is Nice

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##### Re: VCE Physics Question Thread!
« Reply #2378 on: October 01, 2020, 03:26:37 pm »
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- Why is the frictional force equal to accelerative force?

#### Bri MT

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##### Re: VCE Physics Question Thread!
« Reply #2379 on: October 01, 2020, 03:29:21 pm »
+1
Thanks for replying. So essentially, it s the magnets that are reversing, right?

No worries. The relative position of the magnets and coil are cycling yeah.

- Why is the frictional force equal to accelerative force?

More context is needed for this question

#### Newton is Nice

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##### Re: VCE Physics Question Thread!
« Reply #2380 on: October 02, 2020, 09:22:05 pm »
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No worries. The relative position of the magnets and coil are cycling yeah.

Hi thank you once again for replying. So, if commutators reverse the current in the coil, but AC generators do not use commutators, so they have the reversion for current due to the change of direction of magnets (relative) right? If so, why do DC generators require split ring commutator for reversion of current and AC generators do not?

#### Newton is Nice

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##### Re: VCE Physics Question Thread!
« Reply #2381 on: October 09, 2020, 09:40:01 pm »
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Does anyone understand Einstein's first postulate and can explain it?

#### S200

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##### Re: VCE Physics Question Thread!
« Reply #2382 on: October 09, 2020, 11:16:20 pm »
+3
So, if commutators reverse the current in the coil, but AC generators do not use commutators, so they have the reversion for current due to the change of direction of magnets (relative) right?
An AC motor really only works because the magnetic field changes in sync with the phase of the AC current.
Because the magnetic field flips 180 degrees you dont have the deceleration issue that affects DC motors.

Challenge
Once you have that locked away, try to work out how to do an AC motor with permenant magnets.

If so, why do DC generators require split ring commutator for reversion of current and AC generators do not?
DC Motors require split ring commutators due to the constant magnetic field given by the permenant magnets.
Under Lenz's law, the rotor would end up completely perpendicular to the magnetic field if you didn't have a split-ring commutator.
« Last Edit: October 21, 2020, 06:18:15 pm by S200 »
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