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

    15
    ReputationRep:
    STRUGGLING WITH MOMENTS IN PHYSICS

    A uniform metre rule of mass 80g is balanced on 2 knife edges place at the 10cm and 65cm marks
    A block of metal of mass 100g is placed on the metre rule

    Calculate upward force on the rule by each knife edge when the block is placed at the left hand end of the rule
    (ii) at the 50cm mark
    Offline

    8
    ReputationRep:
    Let's call the point at 10cm A and 65cm B.

    The centre of mass of the ruler is at 50cm, and the mass of the block is at i) the left and ii) the middle.

    i) taking moment around point A, we have 100x0.1 to the left and 80x0.4 to the right, for a total of (32-10) to the right (=22)

    65-10 = 55, 22/0.55 = 40 so B is supporting 40g. 180-40 = 140 so A is supporting 140g

    ii) again taking moment around point A, we have 180x0.4 = 72 to the right.

    72/0.55 = 131 (roughly) so B is supporting 131g and A is supporting 49g

    If I've skipped over any steps you need to know feel free to ask.
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by DaftVader)
    Let's call the point at 10cm A and 65cm B.

    The centre of mass of the ruler is at 50cm, and the mass of the block is at i) the left and ii) the middle.

    i) taking moment around point A, we have 100x0.1 to the left and 80x0.4 to the right, for a total of (32-10) to the right (=22)

    65-10 = 55, 22/0.55 = 40 so B is supporting 40g. 180-40 = 140 so A is supporting 140g

    ii) again taking moment around point A, we have 180x0.4 = 72 to the right.

    72/0.55 = 131 (roughly) so B is supporting 131g and A is supporting 49g

    If I've skipped over any steps you need to know feel free to ask.
    I got something similar to you but the answers say FA = 1.68N AND FB = 0.09N

    ii. FA = 0.59N FB = 1.18N
    Offline

    8
    ReputationRep:
    Oh, my answers are in grams. If the solutions are in newtons then divide by 1000 and multiply by 9.8.
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by DaftVader)
    Oh, my answers are in grams. If the solutions are in newtons then divide by 1000 and multiply by 9.8.
    ohhh thats what i did as well
    i got so confused lol
    do you know good physics a level resources online?
    Offline

    8
    ReputationRep:
    (Original post by AsianG99)
    ohhh thats what i did as well
    i got so confused lol
    do you know good physics a level resources online?
    sorry, no but there are some threads on this that can probably help you out.
 
 
 
  • 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
    Did TEF Bronze Award affect your UCAS choices?
  • 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.