- #1
hello478
- 165
- 14
- Homework Statement
- pic below
- Relevant Equations
- newtons 3rd law...
my answer was B
because wont it B be as its the normal reaction force? due to the newtons 3rd law??
correct answer is A...
w= is by box on earthOrodruin said:Two forces being a third law pair means that they are forces between two objects - one the force from the first object on the second and the other the force from the second on the first. W is the force of gravity on the box exerted by the Earth. B is the contact force from the Earth's surface exerted on the box.
Because it does not exert a force on a spring scale that is not at rest under it.PeroK said:So, if the box is in freefall, why is it called weightless?
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:B = the force exerted by the surface of the Earth on the box.
W = the force exerted by the entire Earth on the box.
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:B = the force exerted by the surface of the Earth on the box.
W = the force exerted by the entire Earth on the box.
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:w= is by box on earth
b= by earth on box
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:my answer was B
because wont it B be as its the normal reaction force? due to the newtons 3rd law??
correct answer is A...
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:My usual example is to imagine pushing with your hand on top of the box. The weight and the normal force from the Earth on the box are no longer of the same magnitude so they cannot possibly be a 3rd law pair.
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: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.
No. W and B act on the same object so they can't be a third-law pair.hello478 said:my answer was B
because wont it B be as its the normal reaction force? due to the newtons 3rd law??