Join TSR now and get all your revision questions answeredSign up now
    • Thread Starter
    Offline

    0
    ReputationRep:
    Jack and Jill are playing on a see-saw made from a uniform plank AB of length 5 m pivoted at M, the mid-point of AB. Jack has mass 35 kg and Jill has mass 28 kg. Jill sits at A. Where must Jack sit for the plank to be in equilibrium when horizontal?

    Let d = distance from M to Jack
    R1 = Reaction force for Jill
    R2 = Reaction force for Jack

    What I done:

    0 = (28g x 2.5) + (R2 x d) - (35g x d) - (R1 x 2.5)

    28g + 35g = R1 + R2

    I dont how to find out d!
    Help :confused:
    Offline

    16
    ReputationRep:
    (Original post by Seatbelt)


    0 = (28g x 2.5) + (R2 x d) - (35g x d) - (R1 x 2.5)
    What was the basis for this equation
    • Thread Starter
    Offline

    0
    ReputationRep:
    Sum of clockwise = Sum of anti-clockwise, I think
    Offline

    16
    ReputationRep:
    (Original post by Seatbelt)
    Sum of clockwise = Sum of anti-clockwise, I think
    If you are taking moments you need to do so about a point

    what point did you use
    • Thread Starter
    Offline

    0
    ReputationRep:
    I used M, thats the only point which you can take moments in right??
    Offline

    16
    ReputationRep:
    (Original post by Seatbelt)
    I used M, thats the only point which you can take moments in right??
    Sorry misread
    • Study Helper
    Offline

    13
    I think the important thing is that you don't need to be considering the reaction forces R1, R2. These are not forces acting on the plank.

    The plank is presumably stationary when horizontal and in equilibrium.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by TenOfThem)
    Why would that be the only point?

    I would have taken moments about A
    why?? I thought moments only act on pivots?
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    I think the important thing is that you don't need to be considering the reaction forces R1, R2. These are not forces acting on the plank.

    The plank is presumably stationary when horizontal and in equilibrium.
    If I exclude R1 and R2 then I've no idea how to work it out, my methods so far is based on what my teacher had done in lesson :confused:
    • Study Helper
    Offline

    13
    (Original post by Seatbelt)
    If I exclude R1 and R2 then I've no idea how to work it out, my methods so far is based on what my teacher had done in lesson :confused:
    If you leave out R1 and R2 in your first equation, then you only have one unknown "d".

    0 = (28g x 2.5) - (35g x d)
    Offline

    0
    ReputationRep:
    Just like ghostwalker said; the reaction forces R1 and R2 are unnecessary to consider in this question.
    You know how far Jill sits from the pivot and the force she applies at that point. You know the force Jack would provide as you know his mass, you just don't know the distance.
    It's then a case of anticlockwise moments = clockwise moments with only 1 unknown
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by ghostwalker)
    If you leave out R1 and R2 in your first equation, then you only have one unknown "d".

    0 = (28g x 2.5) - (35g x d)
    I dont understand why we can exclude the reaction forces, according to my teacher there's always a reaction force acting on an object if it is touching another object?
    • Study Helper
    Offline

    13
    (Original post by Seatbelt)
    I dont understand why we can exclude the reaction forces, according to my teacher there's always a reaction force acting on an object if it is touching another object?
    The reaction forces is acting on Jack and Jill (and stopping them from falling to earth with a bump); they're not acting on the plank.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Name:  ImageUploadedByStudent Room1363870967.618731.jpg
Views: 47
Size:  42.8 KB what I done


    Posted from TSR Mobile
    Offline

    16
    ReputationRep:
    (Original post by Seatbelt)
    Name:  ImageUploadedByStudent Room1363870967.618731.jpg
Views: 47
Size:  42.8 KB what I done


    Posted from TSR Mobile
    As has been said those R values do not act on the plank so they are not included when you take moments
    • Thread Starter
    Offline

    0
    ReputationRep:
    The answer in the book is 0.5
    0 = (28g x 2.5) - (35g x d)
    From this equation I get d=2??
    • Study Helper
    Offline

    13
    (Original post by Seatbelt)
    The answer in the book is 0.5
    0 = (28g x 2.5) - (35g x d)
    From this equation I get d=2??
    Depends how the question/answer is phrased.

    2m from M is 0.5m from B.
 
 
 
Poll
If you won £30,000, which of these would you spend it on?
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

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.