Hey there! Sign in to join this conversationNew here? Join for free

Calculating tension in a wire/string - AS Physics (Mechanics) Watch

Announcements
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    How do you do tension calculations in AS physics? Seriously stuck on exam questions to do with it, particularly this one right now but also in general:

    Name:  16409247_1024284441010697_2024837574_o.jpg
Views: 77
Size:  72.7 KB

    Thanks in advance
    Offline

    3
    ReputationRep:
    Okay, first just think about the mass in the left hand side.

    We know it is in equilibrium, therefore all forces have to be balanced.

    Now let's think what those forces are. Obviously the mass experiences the downwards pull of it's weight (W = mg), and it will also experience and upwards pull from the wire.

    If we define up to be positive, and remember that all forces are balanced, we get the equation:

    T - W = 0
    T = W
    T = mg
    m = 2.0, g = 9.81
    Therefore T = 2.0 * 9.81
    Offline

    3
    ReputationRep:
    If you wouldn't mind replying to this thread and quoting me so that I get a notification, I'll write a short paragraph on how to tackle the questions generally when I get back to my room later
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    (Original post by Darth_Narwhale)
    Okay, first just think about the mass in the left hand side.

    We know it is in equilibrium, therefore all forces have to be balanced.

    Now let's think what those forces are. Obviously the mass experiences the downwards pull of it's weight (W = mg), and it will also experience and upwards pull from the wire.

    If we define up to be positive, and remember that all forces are balanced, we get the equation:

    T - W = 0
    T = W
    T = mg
    m = 2.0, g = 9.81
    Therefore T = 2.0 * 9.81
    Ah okay yeah that was easy, should have got that on my own tbh. Guess I was just confused by the 1.6m and 30 degree angle?

    Next part of the question asks: "Show that the mass of the bar is approximately 3.5kg." How would I go about answering this one?

    Thanks for your help by the way
    Offline

    3
    ReputationRep:
    Have you done anything on taking moments before? If so, take the moment around B and that'll give you an equation you can use to find out mass
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    (Original post by Darth_Narwhale)
    Have you done anything on taking moments before? If so, take the moment around B and that'll give you an equation you can use to find out mass
    We have but I didn't really understand them. Would this be right?:

    1.6 x 9.81 x m = 19.62 x cos(30)

    m = (19.62 cos(30)) / (1.6 x 9.81)
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    Never mind it's definitely not
    Offline

    3
    ReputationRep:
    (Original post by NiamhM1801)
    Never mind it's definitely not
    Haha, remember moment = force * perpendicular distance from force to pivot
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    (Original post by Darth_Narwhale)
    Haha, remember moment = force * perpendicular distance from force to pivot
    19.62 x cos(30) x 1.6 = m x 9.81 x 1.6 ?
    Offline

    3
    ReputationRep:
    (Original post by NiamhM1801)
    19.62 x cos(30) x 1.6 = m x 9.81 x 1.6 ?
    Almost. Weight acts through the centre of mass of an object, which here is gonna be... (I'm letting you figure thiss out for yourself obvs)
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    (Original post by Darth_Narwhale)
    Almost. Weight acts through the centre of mass of an object, which here is gonna be... (I'm letting you figure thiss out for yourself obvs)
    0.8 :facepalm:

    so it'll be:

    19.62 x cos(30) x 1.6 = m x 9.81 x 0.8

    m = (19.62 x cos(30) x 1.6) / (9.81 x 0.8)

    which gives me 3.46
    Offline

    3
    ReputationRep:
    (Original post by NiamhM1801)
    0.8 :facepalm:

    so it'll be:

    19.62 x cos(30) x 1.6 = m x 9.81 x 0.8

    m = (19.62 x cos(30) x 1.6) / (9.81 x 0.8)

    which gives me 3.46
    exactly
    • TSR Support Team
    • Clearing and Applications Advisor
    • Thread Starter
    Offline

    21
    ReputationRep:
    (Original post by Darth_Narwhale)
    exactly
    Thanks for the help
 
 
 
  • 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.

  • Will you be richer or poorer than your parents?
  • 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.