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

    2
    ReputationRep:
    A bottle on a table is stationary
    ---------------------------------------------------------------------------------------
    A student wrote this incorrect explanation

    The force of gravity pulls the bottle down.
    The bottle pushes down on the table, so by Newton’s first law,
    the table pushes up with an equal and opposite force.
    According to Newton’s third law, if the forces are balanced,
    nothing can move

    why this is incorrect? everything he said makes a perfect sense
    Offline

    10
    (Original post by reb0xx)
    A bottle on a table is stationary
    ---------------------------------------------------------------------------------------
    A student wrote this incorrect explanation

    The force of gravity pulls the bottle down.
    correct

    The bottle pushes down on the table, so by Newton’s first law 3rd Law,
    the table pushes up with an equal and opposite force. On the bottle.
    According to Newton’s third law, if the forces are balanced,
    nothing can move
    Newton's 3rd Law doesn't say that.

    why this is incorrect? everything he said makes a perfect sense
    Newton's 3rd law states that the action and reaction are on different objects.
    The earth pulls down on the bottle and the bottle pulls up on the earth. These are the action and reaction pair for gravity.

    The bottle pushes down on the table and the table pushes up on the bottle.
    These are the action/reaction pair for the bottle and table.

    How many forces act on the bottle? If the bottle doesn't move, then the forces on it are balanced. There is gravity pulling it downwards and the table pushing upwards. These are not action and reaction in the Newton 3 sense.
    The student is getting confused with "normal reaction".
    Offline

    15
    ReputationRep:
    (Original post by reb0xx)
    A bottle on a table is stationary
    ---------------------------------------------------------------------------------------
    A student wrote this incorrect explanation

    The force of gravity pulls the bottle down.
    The bottle pushes down on the table, so by Newton’s first law,
    the table pushes up with an equal and opposite force.
    According to Newton’s third law, if the forces are balanced,
    nothing can move

    why this is incorrect? everything he said makes a perfect sense
    I would correct the "force of gravity" by saying it's the "force of weight"
    Offline

    0
    ReputationRep:
    (Original post by gozatron)
    I would correct the "force of gravity" by saying it's the "force of weight"
    Well, actually weight has the dimension of \mathrm{kg\, m\, s^{-2}}, and is a force. So there's no need to say something like "force of a force".
    Offline

    15
    ReputationRep:
    (Original post by jaroc)
    Well, actually weight has the dimension of \mathrm{kg\, m\, s^{-2}}, and is a force. So there's no need to say something like "force of a force".
    Yes saying "force of weight" is just defining which force your talking about, surely?
    Offline

    0
    ReputationRep:
    (Original post by gozatron)
    Yes saying "force of weight" is just defining which force your talking about, surely?
    Well, I'm certainly not an expert on English language, but I suspect that as you would say 'thrust', not 'force of thrust', or 'drag', not 'force of drag', you would say 'weight', not 'force of weight' alike.
 
 
 
  • 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
    Brexit voters: Do you stand by your vote?
  • 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.