Correct Answer to Question 10

A scale measures the force which pushes up, in other words, the normal force. Thus, in the free-body diagram on the right, if we sum the forces and set them equal to m a, from Newton's law, we can solve for Fn:

Fn - W = m a

Using a = 0.81 m/s^2, and the weight of the briefcase W, we can solve for Fn (note, we can find the mass of the briefcase m, from the weight 25 N since we know that is the weight when g = 9.8 m/s^2).

This gives the value 27 N for Fn, which is what the scale reads. It is important to get the sign of the acceleration of the elevator consistent with the direction of the forces.

If not, you will end up subtracting instead of adding.

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