August 14, 2020, 05:30:10 pm

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#### Bri MT

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##### Re: QCE Specialist Maths Questions Thread
« Reply #15 on: March 22, 2020, 10:02:54 am »
+3
Sorry, I have another question!
I'm not quite sure how to do conditional probability for exponential probability distributions and I'm stuck on part c) of attached question.
Hopefully, that's my last question for now.
Much appreciated!

Hey,

Recall that:
$p_{x|y} = \frac{p_{X,Y} (x,y)}{p_X (x)}$

Let's think about what this means. We know that something being greater than 5 AND greater than 10, really just means it's greater than 10. So this allows us to find $p_{X,Y} (x,y)$  (use your result from step a)

Let me know if you're still confused and I'm happy to step you through it but it would be great to see your working/thoughts

#### A.Rose

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##### Re: QCE Specialist Maths Questions Thread
« Reply #16 on: June 01, 2020, 08:38:14 pm »
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Hi!
I have a question from Unit 4 Topic 2 and its about forces. This chapter brings in a lot of vectors and I'm quite rusty on Vectors from Unit 3 last year so I need some help with this question.
I was trying to do vector projection for this question but I was having trouble. I would greatly appreciate the help!
The question I am stuck on is part b.
Thank you!!
« Last Edit: June 02, 2020, 07:44:26 am by A.Rose »

#### Bri MT

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##### Re: QCE Specialist Maths Questions Thread
« Reply #17 on: June 02, 2020, 08:54:26 am »
+3
Hi!
I have a question from Unit 4 Topic 2 and its about forces. This chapter brings in a lot of vectors and I'm quite rusty on Vectors from Unit 3 last year so I need some help with this question.
I was trying to do vector projection for this question but I was having trouble. I would greatly appreciate the help!
The question I am stuck on is part b.
Thank you!!

Hey!

You know how for part a you can make triangles from each of the vectors to EF by adding in a vertical line, then applying the trig formulas?
For part b they've done the same thing except the triangle is red line + blue line + line perpendicular to the blue line. The angle between red line and blue line is 40 + 15.

Let me know if this clarifies things or if you still have questions

#### A.Rose

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##### Re: QCE Specialist Maths Questions Thread
« Reply #18 on: June 02, 2020, 09:46:48 am »
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Oh ok, Thank you.

That took me a little while to get my head around. For some reason, I had it in my head to do vector projection as the textbook example switched between vector projection and trig methods throughout.
So for these questions, you have to almost treat the direction vector as if it were horizontal and resolve the other vectors in terms of the direction vector? Sort of? Looking at it that way made sense for 11b but is there a more accurate interpretation?
Thanks again

#### Bri MT

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##### Re: QCE Specialist Maths Questions Thread
« Reply #19 on: June 02, 2020, 10:19:51 am »
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Oh ok, Thank you.

That took me a little while to get my head around. For some reason, I had it in my head to do vector projection as the textbook example switched between vector projection and trig methods throughout.
So for these questions, you have to almost treat the direction vector as if it were horizontal and resolve the other vectors in terms of the direction vector? Sort of? Looking at it that way made sense for 11b but is there a more accurate interpretation?
Thanks again

No worries!

You could also view the same maths I described as the dot product of the red line with a unit vector in the direction of the blue line. Both interpretations are numerically equivalent.

I think the trick here for you might be recognising that we don't want the magnitude of the red line influencing our answer, we just want to be using the direction. Therefore, if we use the dot product we use the red line divided by its magnitude, or we can use the trig method instead.

Hope this helps

Edit:

with the horizontal stuff, there's nothing in the question or any reference frame in the diagram to indicate that horizontal is meaningful or special - we could make anything "horizontal" if we wanted
« Last Edit: June 02, 2020, 10:23:55 am by Bri MT »

#### orla007

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##### Re: QCE Specialist Maths Questions Thread
« Reply #20 on: July 14, 2020, 06:16:23 pm »
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Hi! I just need some help with this question. Thanks in advance

#### 1729

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##### Re: QCE Specialist Maths Questions Thread
« Reply #21 on: July 14, 2020, 07:36:41 pm »
+5
Hi! I just need some help with this question. Thanks in advance
Hey orla007!
You want to find the horizontal and vertical components of the force such that the net horizontal accel is 0.25g draw an FBD first and foremost, aka as in how do you draw it or what are the forces.
VCE
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#### Bri MT

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##### Re: QCE Specialist Maths Questions Thread
« Reply #22 on: July 15, 2020, 12:10:59 pm »
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Hey!

To build on the guidance 1729 has given, there are some key things to remember:

- the normal reaction force acts perpendicular to the surface the block is on (the table is horizontal so this force is up), and the block is initially at rest because this force is cancelling out the weight force.
- You can split up the 45 degree angle into horizontal and vertical components using trigonometry (in this case pythag works but often you'll want to use SOH CAH TOA)
- net force = mass * acceleration
- for net force you add forces in the same direction together and subtract forces in the opposite direction (i.e. frictional force is subtracted)

Have you drawn a force body diagram before?