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F=ma Qtn

  1. Feb 22, 2014 #1
    If a car is travelling towards a bridge at a constant speed of 20ms-1. The bridge takes 30s to cross entirely. The car creates a force of 3000N. Calculate the mass of the car when it is on the bridge.


    a=(v2-v1)/t
    F=ma

    I figured that a = 0 due to the fact that the car is travelling at constant speed.
    If a = 0 then isnt it impossible for the car to create a force of 3000N? I am very confused?
     
  2. jcsd
  3. Feb 22, 2014 #2

    adjacent

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    Gold Member

    First,the speed is expressed as m/s not ms-1.
    When does the car create a force of 3000N?On the start of bridge?

    Edit:Oh I forgot.Welcome to PF :smile:
     
    Last edited: Feb 22, 2014
  4. Feb 22, 2014 #3

    NascentOxygen

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    Hi Lee80! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    I'm a little confused by the question, too. Perhaps the bridge is curved, maybe semicircular? It is not clear whether this 3000N is a vector sum of multiple forces, including also that force known as the vehicle's weight.

    Are you sure you have quoted the question details in full?
     
    Last edited by a moderator: May 6, 2017
  5. Feb 22, 2014 #4
    Thanks for the welcome! :)
    I have written the question exactly the way it is on the page including the ms-1, which i know is supposed to be superscript but isn't it interchangable with m/s? Anyway the only extra info given was a second part to the question which asks "The bridge can support a mass of 5 tonne or less safely. Is it safe for the car to travel across the bridge?"
    I don't think the question is correct? I think it needs more info. The question is supposed to be aimed at year 11 physics.
     
  6. Feb 22, 2014 #5

    NascentOxygen

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    Pity, because it sounded like the basis of a good question: "If the bridge is of level height but curves around the coastline in a semicircular arc, determine that radius of curvature, and the length of the bridge ..." :smile:

    Sure is.

    There is either too much information provided, or else not enough. :wink: But something's amiss.
     
  7. Feb 22, 2014 #6

    SammyS

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    Yes, if the -1 is a superscript then they're interchangeable.

    Use the X2 icon above the message box for superscripts.


    m/s is ms-1 .
     
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