You are Here: Home >< Maths

# Hard pulley question m1 watch

1. in part d of this question, I am stuck. The answer for this is two times bigger than mine. Attachment 512945512947 why is the answer 2tcos30 instead of just tcos30? I remember there are questions like this but normally it just involves one tension. So why is it different in this one? Cheers.
Attached Images

2. (Original post by coconut64)
Cheers.
Watching this may prove helpful. He does a simpler example first and then moves on to your situation at around 3:15 - but I would advise watching the entire thing!
3. (Original post by coconut64)
in part d of this question, I am stuck. The answer for this is two times bigger than mine. Attachment 512945512947 why is the answer 2tcos30 instead of just tcos30? I remember there are questions like this but normally it just involves one tension. So why is it different in this one? Cheers.
If you consider the portion of the string in contact with the pulley at any instant, it feels a tension T pulling in the direction of the "top" portion of the string, and a tension T pulling it in the direction of the "side" portion of the string. In the absence of any other force, it would accelerate in the direction of the resultant R of those forces, by Newton II.

However, it does not accelerate in that direction, hence the pulley must exert a nett reaction force of magnitude R on that portion in the opposite direction to the resultant. By Newton III, that portion of the string exerts on the pulley a force of equal magnitude and opposite direction to the pulley's reaction i.e. i.e. it exerts on the pulley a force equal to the resultant R.
4. (Original post by Zacken)
Watching thismay prove helpful. He does a simpler example first and then moves on to your situation at around 3:15 - but I would advise watching the entire thing!
Thank you! This does explain the question however I normally use pythagorus to solve it and it works. Now I am really muddled. for example on this question I used pythagorus to solve it and this gives me the same answer as the other method .Attachment 512973512975 1 is the method in the video but I used method 2. What is the difference cuz both give me the same answer. Thanks.
Attached Images

5. (Original post by coconut64)
Thank you! This does explain the question however I normally use pythagorus to solve it and it works. Now I am really muddled. for example on this question I used pythagorus to solve it and this gives me the same answer as the other method .Attachment 512973512975 1 is the method in the video but I used method 2. What is the difference cuz both give me the same answer. Thanks.
Pythagoras will work if the two parts of the string are at right angles to each other, but not otherwise.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 15, 2016
Today on TSR

### A-Level OCR Biology Unofficial Markscheme

Find out how you've done here

### 2,785

students online now

Exam discussions

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams