Newton's third law and weight

[hello478]

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...

 

[Orodruin]

No, W and B are not a 3rd law pair. 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.

 

[hello478]

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.

w= is by box on earth
b= by earth on box
so...?

 

[kuruman]

B = the force exerted by the surface of the Earth on the box.
W = the force exerted by the entire Earth on the box.

If there were a hole at B, i.e. no surface, the box will fall in and get closer to the center of the Earth because W is still acting on the box.

This explains, among other things, which coins dropped down a wishing well end up at the bottom of the well where there is a firm surface.

 

[PeroK]

So, if the box is in freefall, why is it called weightless?

 

[kuruman]

So, if the box is in freefall, why is it called weightless?

Because it does not exert a force on a spring scale that is not at rest under it.

 

[PeroK]

Because it does not exert a force on a spring scale that is not at rest under it.

He just grinned and shook my hand. "No", was all he said!

 

[Orodruin]

B = the force exerted by the surface of the Earth on the box.
W = the force exerted by the entire Earth on the box.

This.

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.

 

[jbriggs444]

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 $F=ma$.

 

[Orodruin]

w= is by box on earth
b= by earth on 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.

 

[haruspex]

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...

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.

 

[kuruman]

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.

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.

 

[jbriggs444]

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.

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.

 

[Herman Trivilino]

my answer was B
because wont it B be as its the normal reaction force? due to the newtons 3rd law??

No. W and B act on the same object so they can't be a third-law pair.

W is the gravitational force exerted on the box by Earth.

A is the gravitational force exerted on Earth by the box.