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Force Questions Help

  1. Mar 6, 2016 #1
    1. The problem statement, all variables and given/known data
    Brian whose mass is approximately 70 kg, is in an elevator accelerating upwards at 2m/2^2. Determine the normal force acting on his body. What weight would the scale register when Brian is going up?

    Melanie is flying east in her new private jet at a constant altitude. The lift force on the plane is 100,000 N. The inital velocity of the plane is 100m/s[E], when it begins to accelerate at a constant rate. If the air resistance on the plane is 40,000 N and the thrust is 52,000 N, find the time it takes her plane to reach a velocity of 115m/2[E[?

    2. Relevant equations

    3. The attempt at a solution
    For the first questions I simply calculated the normal force, which was 750 N. Then I calculate the force of gravity, for which I got 686. Then I subtracted the force of gravity from the normal force (750 N -686 N= 64N, which is 64kg?) Am I going in the right direction?

    I do not get the second question at all
  2. jcsd
  3. Mar 7, 2016 #2


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    I am not sure if you need to subtract gravity for the normal force. However, for what the scale would read, it should be more than he would weigh if he weren't moving.
  4. Mar 7, 2016 #3
    your conclusion is that man will be lighter but the usual experience in a lift going up is opposite- so think over it.
  5. Mar 7, 2016 #4

    Simon Bridge

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    You have not shown how you calculated the normal force, which is the first question.
    Weight is 70kg x 9.8N/kg = 686N well done
    N>mg OK
    64N < 686N ... so this result says the net force is 10% of his weight pointing up.
    64N is not 64kg ...

    You should draw a free-body diagram for Brian, then apply Newton's laws.
    To see how these normal forces and weights turn into the reading on a scale, have a look at the forces on the scale. How do scales tell you your weight?
    Last edited: Mar 7, 2016
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