Mechanics 2 question?!!

Could someone help me with this please? I though it was Tcos60-5cos30=0.3g but the mark scheme says 5cos30=0.3g+Tcos60. Why is this?
http://www.thestudentroom.co.uk/attachment.php?attachmentid=519343&d=1459807059

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Reply 1

Vanilla Poison

If you resolve vertically there is only the component of tension of AP acting upwards. The forces acting downwards are the weight and the vertical component of the tension in the string PB. We assume P is in equilibrium, so the upwards forces must equal the downwards forces.

Reply 2

swagmister

Original post by Vanilla Poison

For similar questions I used the equation I thought and I got the answers write, e.g.

Attachment not found

is this there something different about the question?

image.jpg302.8KB

Reply 3

Vanilla Poison

I don't know if you've got it right, but your first equation by resolving doesn't make any sense. It looks as though you're taking the forces acting down as positive (weight), but then the component of T2 is acting upwards. You'd realise this mistake if you brought all the forces to one side.

Reply 4

swagmister

Original post by Vanilla Poison

It was a past paper question and the mark scheme said it was right and my teacher showed me the first equation, I don't know why it doesn't work for this question

Bump

Original post by swagmister

It was a past paper question and the mark scheme said it was right and my teacher showed me the first equation, I don't know why it doesn't work for this question

Can you clarify the issue? Perhaps post a link to the markscheme?

Reply 7

Duke Glacia

Original post by swagmister

Can u explain why u think ur ans is right ?
try resolving upwards, and remember gravity acts downwards

(edited 8 years ago)

Reply 8

swagmister

Original post by Zacken

Can you clarify the issue? Perhaps post a link to the markscheme?

http://www.ocr.org.uk/Images/62961-mark-scheme-june.pdf
Mechanics 2 question 6i for the second question I posted, I got question right, this is how I thought you work out the tension however for the first question I posted the mark scheme shows a different way which meant I got this question wrong.
Qusetion 6i http://www.ocr.org.uk/Images/62955-mark-scheme-june.pdf
I don't understand why they worked out the tension differently for the first question.

Reply 9

swagmister

Original post by Duke Glacia

Can u explain why u think ur ans is right ?
try resolving upwards, and remember gravity acts downwards

I thought the vertical components of the tensions should be equal to the weight

(edited 8 years ago)

Reply 10

Zacken

Original post by swagmister

You seem to be saying that Tcos 60 acts in the opposite direction to weight? Why do you think this?

Reply 11

swagmister

Original post by Zacken

You seem to be saying that Tcos 60 acts in the opposite direction to weight? Why do you think this?

Yep because its vertical component is up and I've treated up as negative and down as positive

(edited 8 years ago)

Reply 12

swagmister

Original post by Zacken

You seem to be saying that Tcos 60 acts in the opposite direction to weight? Why do you think this?

I just realised this is for the first question I posted, for that one I thought 5cos30 was in the opposite direction to the weight and Tcos60 was in the same direction as the weight

Reply 13

Zacken

Original post by swagmister

Yep because its vertical component is up and I've treated up and negative and down as positive

The (vertical component of) tension in the string BP acts upwards and the (vertical component of) tension in the string AP acts upwards. So why do they have opposite signs in your equation?

Reply 14

swagmister

Original post by Zacken

The (vertical component of) tension in the string BP acts upwards and the (vertical component of) tension in the string AP acts upwards. So why do they have opposite signs in your equation?

I thought Tcos60 (BP) was downwards and 5cos30 (AP) was upwards?

Reply 15

Zacken

Original post by swagmister

I thought Tcos60 (BP) was downwards and 5cos30 (AP) was upwards?

Urgh, sorry. Craning my neck isn't helping. Apologies. :lol:

Reply 16

Zacken

Original post by swagmister

I thought Tcos60 (BP) was downwards and 5cos30 (AP) was upwards?

5 cos 30 acts upwards. (AP)

T cos 60 acts downards. (BP)

Weight acts downards.

Upwards = downwards.

5 cos 30 = T cos 60 + weight

Your equation should be -5 cos 30 + T cos 60 + 3g = 0