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

    16
    ReputationRep:
    PS Reviewer
    a book slides down a rough desk, incline = 30, and falls to the ground which is 0.8 below the edge of the lid. The coefficient of friction is 1/4. Given thatthe books starts from rest 0.4m from the bottom of the lid which is hinged at the edge of the desk, find the speed at it which it hits the floor.


    So, did:

    gh - 1/4 R * 0.4 = KE
    9.8 * 0.4sin30 - 1/4 R * 0.4


    However, how can I work out R if there is no mention of the mass of the book?
    Offline

    17
    ReputationRep:
      R  = mgcos30
    • PS Reviewer
    • Thread Starter
    Offline

    16
    ReputationRep:
    PS Reviewer
    But I don't know what 'm' is and hence cannot calculate R
    Offline

    14
    ReputationRep:
    (Original post by SilverstarDJ)
    But I don't know what 'm' is and hence cannot calculate R
    It "m" term will cancel.
    • PS Reviewer
    • Thread Starter
    Offline

    16
    ReputationRep:
    PS Reviewer
    (Original post by electriic_ink)
    It "m" term will cancel.
    I can't see where it cancels out --- could you elaborate on that please?
    • PS Reviewer
    • Thread Starter
    Offline

    16
    ReputationRep:
    PS Reviewer
    ahhh yes, I see where they cancel out. Thanks.

    However, I'm not sure how to calculate the speed.

    I tried mgh - 1/4 mgcos30 * 0.4 = 1/2 m v * v

    And the the value for v, I put into the suvat equation, but the I get an incorrect answer. Any advice on what I have done wrong?
    Offline

    17
    ReputationRep:
    Show me your suvat equation
    • PS Reviewer
    • Thread Starter
    Offline

    16
    ReputationRep:
    PS Reviewer
    v * v = u * u + 2as
    Offline

    17
    ReputationRep:
    (Original post by SilverstarDJ)
    v * v = u * u + 2as
    >__> I meant your working with it
    • PS Reviewer
    • Thread Starter
    Offline

    16
    ReputationRep:
    PS Reviewer
    (Original post by 2710)
    >__> I meant your working with it
    0.5mv*v = mgh - friction * distance
    0.5 v*v = 9.8sin30 - 0.25 * 0.4* 9.8 cos30
    v * v = 4.051....

    Into suvat:

    v * v = 4.051... + 2x 9.8 x 0.8 = 19.73
    v= 4.41

    (4.23 is the answer on the answer sheet)
    Offline

    17
    ReputationRep:
    (Original post by SilverstarDJ)
    0.5mv*v = mgh - friction * distance
    0.5 v*v = 9.8sin30 - 0.25 * 0.4* 9.8 cos30
    v * v = 4.051....

    Into suvat:

    v * v = 4.051... + 2x 9.8 x 0.8 = 19.73
    v= 4.41

    (4.23 is the answer on the answer sheet)
    The bolded parts are wrong. Firstly, you are missing the actually height of 0.4. It's meant to be 0.4x9.8sin30. It will then give you the right answer
    • PS Reviewer
    • Thread Starter
    Offline

    16
    ReputationRep:
    PS Reviewer
    Thank you very much for your help. It is much appreciated.
 
 
 
  • 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
    Would you rather give up salt or pepper?
    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

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