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Help with Simple Force Problems

  1. Feb 10, 2010 #1
    1. 1. A hockey player hits a puck so it starts to slide across the ice. Assume the ice is perfectly smooth so that there is no friction, and that the normal force from the ice cancels the force of gravity. Thus, after the puck leaves the stick, the resultant force acting on the puck is zero. What happens to the puck after it leaves the stick?

    2. Estimate (to the nearest order of magnitude) the amount of force you need to pick up a 1.5 kg mass and the amount of force you need to exert to pick up a 1,054 kg mass.

    3. On Earth, Javier has a mass m and a weight w. Which of the following would be true if Javier went to the Moon?

    He would have a mass of exactly m and weight of exactly w.
    He would have a mass of exactly m and weight less than w.
    He would have a mass less than m and weight of exactly w.
    He would have a mass of less than m and a weight of less than w.

    4. Explain your answer to the multiple choice question above.



    2. Relevant equations w=mg, F(x)=ma



    3. The attempt at a solution1. It continues to move at a constant velocity in a straight line.

    2. For the 1.5 Kg mass, you would need any force larger than 14.7 N, and for the 1,054 Kg mass, you would need any force larger than 10329.2 N.

    3. He would have a mass of exactly m and weight less than w.

    4. Your mass is constant no matter where you are, but your weight depends on the gravity of where you are. The gravity on the moon is less than the force of gravity on Earth, and according to the equation of weight (mass * gravity), Javier's weight should be less on the Moon.
     
  2. jcsd
  3. Feb 10, 2010 #2
    :) you actually dont need any help , you did everything right so well done ..
     
  4. Feb 10, 2010 #3
    Really? Thanks, anyone else?
     
  5. Feb 11, 2010 #4
    no.1 is correct because according to newtons first law, the object remains at rest (or moving with constant speed) if there is no force acting upon(net force equals zero)..

    And no.2 is also correct because if you want to pick up an object of mass m you need to exert a force atleast equal to the weight of that mass(mg) ..

    And no.3 you gave a perfect answer ..

    So no need to wait for another member to confirm that your answers are correct .. :) as i told you , you dont need any help ..
     
  6. Feb 11, 2010 #5

    Redbelly98

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    While you did the weight calculations correctly, note the question asks for an estimate to the nearest order of magnitude. You could simply use g=10 N/kg to do this in your head. And then round your answer off to the nearest power of 10, i.e. 1N, 10N, 100N, etc.
     
  7. Feb 11, 2010 #6
    So, should I say the answers are "any force on the order of magnitude of 10 N" and "any force on the order of magnitude of 10000 N"?
     
  8. Feb 11, 2010 #7

    Redbelly98

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    Correct, though it's even simpler than that. You could just say "10 N" and "10,000 N". (And show your work, of course.)
     
  9. Feb 11, 2010 #8
    Thanks!
     
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