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Explaining Newton's First Law

  1. Oct 12, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi everyone. I am a grade 11 physics teacher. This week, I will be teaching about Newton's first law in class. I'm having trouble explaining one thing.

    Students generally understand that a hockey puck moving on ice will continue to move with constant velocity. In this case, there is a complete absence of force.

    However, when there are balanced forces, students don't get it. For example, in an elevator moving with constant speed, the force due to gravity and the tension on the elevator are balanced.

    2. Relevant equations

    3. The attempt at a solution
    Typically, I tell students that if the gravity force were bigger than the tension force, the elevator would accelerate down. And if the tension force were bigger, the elevator would accelerate up.

    However, this doesn't seem to rectify the misunderstanding. Does anyone have a better way of explaining this?

    Thanks in advance!
  2. jcsd
  3. Oct 12, 2015 #2


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    Get them to think about the beginning and end of the ride, when you feel heavier (lighter) and lighter (heavier) respectively, if travelling up (down). Those are the only times when the forces are unbalanced, as a result of which the lift accelerates up or down, and they can actually feel Newton's Law in action.

    They could contrast those interesting phases with the middle phase of the ride when their weight is normal, because the forces are balanced and there is no acceleration.

    I like to jump up and down in elevators during those beginning and end phases, to savour the sensation of reduced and increased weight.
  4. Oct 12, 2015 #3


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    Perhaps the example of pushing a heavy box on a rough surface. Once you get it moving you can keep it moving at a constant speed because you are balancing the friction force but if you slack off, it will slow down and if you push harder it will speed up.
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