The discussion clarifies the distinction between weight (W) and the normal force (B) in the context of Newton's Third Law. W represents the gravitational force exerted on a box by the Earth, while B is the contact force from the Earth's surface acting on the box. These forces cannot be considered a third-law pair because they act on the same object. The correct understanding is that W is the force of gravity, and B is the normal force, which varies depending on the situation, such as freefall or contact with a surface.
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w= is by box on earthOrodruin said:
Because it does not exert a force on a spring scale that is not at rest under it.PeroK said:
He just grinned and shook my hand. "No", was all he said!kuruman said:Because it does not exert a force on a spring scale that is not at rest under it.
This.kuruman said:
Two equal and opposite forces on the same object that result in a stationary object remaining stationary are what I like to call a "second law pair". The net force is zero, so the acceleration is zero by ##F=ma##.kuruman said:
Also, this is simply wrong. Both W and B are forces from the Earth on the box (with the further subdivision into Earth surface/entire Earth given above). W is the gravitational force of the Earth on the box, not the gravitational force of the box on the Earth.hello478 said:
Others have explained why W and B are not a second law pair. There is also clue in the question: "forces of the same type". The type distinction being made here is gravitational forces versus contact forces.hello478 said:
That's another good way to put it. In everyday life, a bathroom scale "measures your weight". In the physics classroom it measures the normal force exerted on its top surface when the scale is at rest with respect to the center of the Earth.Orodruin said:
Or, slightly more precisely, the normal force exerted on its top surface when its base remains at rest with respect to the rotating frame in which the Earth is stationary.kuruman said:
No. W and B act on the same object so they can't be a third-law pair.hello478 said: