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

Someone answer this kinematics question for me? Watch

    • Thread Starter
    Offline

    8
    ReputationRep:
    I have found answers myself but I don't want to post them in case they're embarrassingly wrong lmao.

    The question:

    A stone is thrown vertically upwards with speed 16 ms^-1 from a point h metres above the ground. The stone hits the ground 4s later. Find:

    a) the value of h
    b) the speed of the stone as it hits the ground
    Offline

    18
    ReputationRep:
    (Original post by emilyrose2001)
    I have found answers myself but I don't want to post them in case they're embarrassingly wrong lmao.

    The question:

    A stone is thrown vertically upwards with speed 16 ms^-1 from a point h metres above the ground. The stone hits the ground 4s later. Find:

    a) the value of h
    b) the speed of the stone as it hits the ground
    You will learn more by posting embarassingly wrong answers so that other people can see what you don't understand.

    You could split the problem into two phases of motion, upward travel where g slows the stone to 0 m/s and then free falling under the acceleration of g BUT this is very fiddly. You should remember that s is displacement so much more simply the displacement of the stone from it's start point is h in a downward direction.
    Choosing the downward direction to be positive and s=ut+0.5at^2 you can easily determine h (note that since the initial velocity is upwards u=-16)

    For b) what kinematics equation connects v, u, a and s?
    • Thread Starter
    Offline

    8
    ReputationRep:
    So I've ended up with :
    a)
    Spoiler:
    Show

    Displacement downwards s = 0.5(v+u)t
    so s= 0.5(23.25)2.37
    Displacement downwards = 27.55
    27.55-13.04 = 14.51
    So h = 14.51m

    b)
    Spoiler:
    Show


    t= (v-u)/a, so t= -16/-9.81 = 1.63s travelling upwards
    Which means for the stone travelling upwards, s= 1/2 * 16 * 1.63 = 13.04m
    T for the stone falling back down is 4-1.63 = 2.37
    so v= 0+(9.81*2.37) = 23.25ms^-1


    And I've kind of done the question backwards and am now confused
    Offline

    18
    ReputationRep:
    (Original post by emilyrose2001)
    So I've ended up with :
    a)
    Spoiler:
    Show


    Displacement downwards s = 0.5(v+u)t
    so s= 0.5(23.25)2.37
    Displacement downwards = 27.55
    27.55-13.04 = 14.51
    So h = 14.51m


    b)
    Spoiler:
    Show



    t= (v-u)/a, so t= -16/-9.81 = 1.63s travelling upwards
    Which means for the stone travelling upwards, s= 1/2 * 16 * 1.63 = 13.04m
    T for the stone falling back down is 4-1.63 = 2.37
    so v= 0+(9.81*2.37) = 23.25ms^-1



    And I've kind of done the question backwards and am now confused
    Other than rounding issues introduced when you rounded t early in your working that method is fine. I would use the exact result for t for the upward period of motion. My suggestion regarding displacement is less fiddly.
    • Thread Starter
    Offline

    8
    ReputationRep:
    (Original post by gdunne42)
    Other than rounding issues introduced when you rounded t early in your working that method is fine. I would use the exact result for t for the upward period of motion. My suggestion regarding displacement is less fiddly.
    Thank you
 
 
 
  • 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?
    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.