When a certain calibrated laboratory mass is placed a the pan balance as seen in the photograph at the left below, we see that its weight is approximately 10 newtons, as read on the scale. If the same weight is suspended from an identical spring scale, we see in the picture in the center below that it requires about 10 newtons of force for the upper spring scale to suspend the mass.
Now suppose that while the lab mass is resting on the lower scale we start to lift it with the upper scale.
When the upper scale reads 6 newtons, as seen in the picture at the right, the lower scale will read:
- (a) 10 newtons.
- (b) 8 newtons.
- (c) 6 newtons.
- (d) 4 newtons.
- (e) 2 newtons.
- (f) 0 newtons
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The answer is (d) 4 newtons, as can be seen in the photograph below, where the mask has been removed from the spring scale face.
This is an example of equilibrium of forces; the total force upward on the lab mass, equal to its weight of 10 newtons, can be applied by either the lower or the upper scale, or a combination thereof.