Lucia20613978
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Heya!
I’m quite stuck on this mechanics question and was wondering if anyone could help me out. I have annotated the diagram too with info but can’t work it out :/ I would really appreciate some help.Name:  B0F4D200-58FC-4A3D-833E-B103F9AE2EC0.jpeg
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davros
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(Original post by Lucia20613978)
Heya!
I’m quite stuck on this mechanics question and was wondering if anyone could help me out. I have annotated the diagram too with info but can’t work it out :/ I would really appreciate some help.Name:  B0F4D200-58FC-4A3D-833E-B103F9AE2EC0.jpeg
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What are your thoughts so far?

Can you set up any equations relating the forces involved?
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Lucia20613978
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(Original post by davros)
What are your thoughts so far?

Can you set up any equations relating the forces involved?
So, I was thinking that using Force= moment/distance might be something used here where 0.9(5g)+ 1.6(2g)=1.8TP
And then, 0.9(5g)+0.2(2g)=1.8TQ
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davros
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(Original post by Lucia20613978)
So, I was thinking that using Force= moment/distance might be something used here where 0.9(5g)+ 1.6(2g)=1.8TP
And then, 0.9(5g)+0.2(2g)=1.8TQ
So you've basically got 2 unknowns here TP and TQ, so you are going to need to set up 2 equations to find them.

You will find it helps to organize your thoughts (and people marking your work) if you write down the principles you're using when you set up the equations.

So in one case you might say "the vertical forces balance each other, so resolving forces vertically gives..."

And for another case you know there is no net turning effect on the rod, so taking moments about a point will give you another equation relating TP and TQ.

It looks like you're trying to use 2 moment equations, which is fine also.

Can you say about which point you're taking moments in each case, and check your distances carefully?
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Lucia20613978
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I am not fully sure about the distances.
I said that the 5Kg was at the Centre of mass so thats how I got the 0.9m and then from that for both sides I did 0.9(5g). Is this the case or would you ignore the 5Kg because it is equal on both sides?
0.2m is the distance from B to C so that was the 0.2(2g)
If the vertical forces balance each other, the resolving forces vertically would give zero as they are equal.
If there’s no net turning effect the moments will be equal for P and Q?
I’m really sorry that I have made this confusing, I just started the topic today and in my notes I wasn’t given anything on Tensions.
Usually, when doing tensions, I would use F=ma where F is equal to the downwards mass- the tension in the string.
However, the topic was on moments so I assumed using moments equations was the way to go :/
Thanks,
Lucia
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(Original post by Lucia20613978)
I am not fully sure about the distances.
I said that the 5Kg was at the Centre of mass so thats how I got the 0.9m and then from that for both sides I did 0.9(5g). Is this the case or would you ignore the 5Kg because it is equal on both sides?
0.2m is the distance from B to C so that was the 0.2(2g)
If the vertical forces balance each other, the resolving forces vertically would give zero as they are equal.
If there’s no net turning effect the moments will be equal for P and Q?
I’m really sorry that I have made this confusing, I just started the topic today and in my notes I wasn’t given anything on Tensions.
Usually, when doing tensions, I would use F=ma where F is equal to the downwards mass- the tension in the string.
However, the topic was on moments so I assumed using moments equations was the way to go :/
Thanks,
Lucia
I thought post 3 was basically right, but your P and Q tensions should be swapped?
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Lucia20613978
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(Original post by mqb2766)
I thought post 3 was basically right, but your P and Q tensions should be swapped?
Really? I’m not too sure. This question is really a pain in the butt for me lol.
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(Original post by Lucia20613978)
Really? I’m not too sure. This question is really a pain in the butt for me lol.
Moment about A has TQ on the right (of the equation). And moment about B has TP on the right. Just work out their values. You simply mislabelled the tensions in the two moment equations. Other than that they're fine.
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Lucia20613978
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(Original post by mqb2766)
Moment about A has TQ on the right. And moment about B has TP on the right. Just work out their values.
So, does that mean that they’re right, I just got them mixed up?
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(Original post by Lucia20613978)
So, does that mean that they’re right, I just got them mixed up?
Yes, you simply misspelled them. You could have called then TA and TB, so the moment about A would involve TB and vice versa.
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davros
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(Original post by Lucia20613978)
I am not fully sure about the distances.
I said that the 5Kg was at the Centre of mass so thats how I got the 0.9m and then from that for both sides I did 0.9(5g). Is this the case or would you ignore the 5Kg because it is equal on both sides?
0.2m is the distance from B to C so that was the 0.2(2g)
If the vertical forces balance each other, the resolving forces vertically would give zero as they are equal.
If there’s no net turning effect the moments will be equal for P and Q?
I’m really sorry that I have made this confusing, I just started the topic today and in my notes I wasn’t given anything on Tensions.
Usually, when doing tensions, I would use F=ma where F is equal to the downwards mass- the tension in the string.
However, the topic was on moments so I assumed using moments equations was the way to go :/
Thanks,
Lucia
(Original post by Lucia20613978)
Really? I’m not too sure. This question is really a pain in the butt for me lol.
Yes I think you have the 2 mixed up - I only had chance to have a quick look earlier but it looked like your distance was wrong in each case with the notation you're using.

Let's take an example if you're working with moments. Take moments about the point A. Since TP acts through A its moment is 0.

In an anticlockwise sense you have the tension TQ pulling the rod upwards acting at a perpendicular distance of 1.8m from A, so the anticlockwise moment about A is 1.8TQ.

In a clockwise sense you have 2 weights: 5g acting at 0.9m from A and 2g acting at 1.6m from A (because the particle is 0.2m from the other end), so the clockwise moment is (5g x 0.9) + (2g x 1.6).

So our moments equation taking moments about A becomes:

1.8TQ = (5g x 0.9) + (2g x 1.6)

You can do the same for TP with the appropriate distances.

As a check, there is no net vertical force, so when you have worked out TP and TQ you should find that TP + TQ = 5g + 2g because the tensions acting vertically upwards are balanced out by the weights acting vertically downwards.

Does this help to clarify things?
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Lucia20613978
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(Original post by davros)
Yes I think you have the 2 mixed up - I only had chance to have a quick look earlier but it looked like your distance was wrong in each case with the notation you're using.

Let's take an example if you're working with moments. Take moments about the point A. Since TP acts through A its moment is 0.

In an anticlockwise sense you have the tension TQ pulling the rod upwards acting at a perpendicular distance of 1.8m from A, so the anticlockwise moment about A is 1.8TQ.

In a clockwise sense you have 2 weights: 5g acting at 0.9m from A and 2g acting at 1.6m from A (because the particle is 0.2m from the other end), so the clockwise moment is (5g x 0.9) + (2g x 1.6).

So our moments equation taking moments about A becomes:

1.8TQ = (5g x 0.9) + (2g x 1.6)

You can do the same for TP with the appropriate distances.

As a check, there is no net vertical force, so when you have worked out TP and TQ you should find that TP + TQ = 5g + 2g because the tensions acting vertically upwards are balanced out by the weights acting vertically downwards.

Does this help to clarify things?
Ahhh yes this helps tons! Thank you so much. I’m really really sorry I made this such trouble hahah. I really appreciate the help so much
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Lucia20613978
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(Original post by mqb2766)
Yes, you simply misspelled them. You could have called then TA and TB, so the moment about A would involve TB and vice versa.
Ohhh thank you thats great hahah I appreciate you taking the time to look at it!
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davros
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(Original post by Lucia20613978)
Ahhh yes this helps tons! Thank you so much. I’m really really sorry I made this such trouble hahah. I really appreciate the help so much
That's OK

Are you reading ahead of this in preparation for starting mechanics in the autumn?
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mathstutor24
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When deciding at which point to take moments about, consider what you are trying to find and what info you have.

In this example, if you want to find TQ, then you would need to take moments about point A, since that will eliminate TP.
Similarly, if you want to find TP, then take moments about point B to eliminate TQ.
If you took moments about C, for example, you will have 2 unknowns: TP and TQ.

If it is in a state equilibrium, the upward forces are balanced by the downward forces (equal to each other). So you could take moments about either A or B to find TQ or TP, then calculate the total upward and downward forces and work out the remaining tension from that.

As mqb2766 said, when writing your moments calculations, you swapped the tension labels.
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Lucia20613978
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(Original post by davros)
That's OK

Are you reading ahead of this in preparation for starting mechanics in the autumn?
I am starting A2 in September but am starting it now to get a bit ahead to keep on top of things .
I know some of the stuff about tensions was in AS but It was one of the last things we did and I missed the last week before schools closed due to my mum being at such high risk. Hopefully, with more practise I will be able to get used to these ones haha.
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Lucia20613978
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(Original post by mathstutor24)
When deciding at which point to take moments about, consider what you are trying to find and what info you have.

In this example, if you want to find TQ, then you would need to take moments about point A, since that will eliminate TP.
Similarly, if you want to find TP, then take moments about point B to eliminate TQ.
If you took moments about C, for example, you will have 2 unknowns: TP and TQ.

If it is in a state equilibrium, the upward forces are balanced by the downward forces (equal to each other). So you could take moments about either A or B to find TQ or TP, then calculate the total upward and downward forces and work out the remaining tension from that.

As mqb2766 said, when writing your moments calculations, you swapped the tension labels.
Thank you for this, you’ve really helped me to understand this I appreciate the help so much.
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mathstutor24
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Lucia20613978 - you're very welcome! There are lots of us here who are more than happy to help.

Out of interest, what books/resources are you using to prep for A2? Which exam board and course are you studying?
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Lucia20613978
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(Original post by mathstutor24)
Lucia20613978 - you're very welcome! There are lots of us here who are more than happy to help.

Out of interest, what books/resources are you using to prep for A2? Which exam board and course are you studying?
Heya,
I’m glad to hear that
I am studying maths A2 for WJEC and I have been using resources our teacher gave us with the WJEC mathematics A2 revision book by Sophie Goldie & Rose Jewell. I have the WJEC maths book for Pure maths for that section and some resources on Statistics too which are also in the book I mentioned . Would you happen to know any good resources for this?
Lucia
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(Original post by Lucia20613978)
Heya,
I’m glad to hear that
I am studying maths A2 for WJEC and I have been using resources our teacher gave us with the WJEC mathematics A2 revision book by Sophie Goldie & Rose Jewell. I have the WJEC maths book for Pure maths for that section and some resources on Statistics too which are also in the book I mentioned . Would you happen to know any good resources for this?
Lucia
I'm not familiar with the WJEC spec, but after a quick look at the applied unit (A2 Unit 4), it's very similar to Edexcel. The reason why I'm mentioning Edexcel is that I really like the CGP student workbooks for A level maths self-study. I dislike the order in which the official Edexcel textbooks teach mechanics, and past students have really struggled to get to grips with certain concepts without certain prior knowledge. These are the books I recommend when self-studying / getting ahead before class: CGP student books

I don't know how comfortable you are with the AS applied unit, or how WJEC teach the first year of mechanics. If you wanted to use the CGP books, it may be that you can go straight to the Y2 Applied book.

As I said, always feel free to ask anything here, or to private message for help. Personally, I believe it is really important for students to understand everything correctly, so if you don'tunderstand something, or want to check your understanding - ask away!
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