Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    16
    ReputationRep:
    Name:  1457987185939-1542586955.jpg
Views: 127
Size:  277.9 KB 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
     
    Offline

    22
    ReputationRep:
    (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!
    Offline

    11
    ReputationRep:
    (Original post by coconut64)
    Name:  1457987185939-1542586955.jpg
Views: 127
Size:  277.9 KB 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.
    • Thread Starter
    Offline

    16
    ReputationRep:
    (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. Name:  1457990529268-1542586955.jpg
Views: 84
Size:  310.2 KB 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
     
    Offline

    14
    ReputationRep:
    (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. Name:  1457990529268-1542586955.jpg
Views: 84
Size:  310.2 KB 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.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    What's your favourite Christmas sweets?
    Useful resources

    Make your revision easier

    Maths

    Maths Forum posting guidelines

    Not sure where to post? Read the updated guidelines here

    Equations

    How to use LaTex

    Writing equations the easy way

    Student revising

    Study habits of A* students

    Top tips from students who have already aced their exams

    Study Planner

    Create your own Study Planner

    Never miss a deadline again

    Polling station sign

    Thinking about a maths degree?

    Chat with other maths applicants

    Can you help? Study help unanswered threads

    Groups associated with this forum:

    View associated groups
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.