# ForceWatch

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Thread starter 7 years ago
#1
Why is normal reaction force not equal to weight? I thought the reaction force and weight are action reaction pairs

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0
7 years ago
#2
Are you talking about this in relation to circular motion?
0
7 years ago
#3
Weight acts on an object with mass. It is the gravitational force of the earth on the object. Its Newton third law pair is thus the gravitational force of the object on the earth, not the reaction force.

The pair to the reaction force of the object on the earth is the reaction force of the earth on the object. Draw a force diagram and consider these various forces carefully - it's well worth it as it is amazing how much confusion this causes people.
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7 years ago
#4
(Original post by 3nTr0pY)
Weight acts on an object with mass. It is the gravitational force of the earth on the object. Its Newton third law pair is thus the gravitational force of the object on the earth, not the reaction force.

The pair to the reaction force of the object on the earth is the reaction force of the earth on the object. Draw a force diagram and consider these carefully.
Yeah thats true actually, they cant be a third law pair because they are of a different type
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7 years ago
#5
(Original post by Cephalus)
Yeah thats true actually, they cant be a third law pair because they are of a different type
What are you on about?

Are we talking about a mass and Earth and the forces between them?
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7 years ago
#6
(Original post by TheGrinningSkull)
What are you on about?

Are we talking about a mass and Earth and the forces between them?
Well if there is circular motion, there must be an inwards centripetal force, so there must be a resultant force downwards thus the weight is slightly more than the reaction force (say if you were standing on scales)

It depends on the context really, but if you were considering a person standing on earth then this would apply. But the OP has not provided any context
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7 years ago
#7
(Original post by Cephalus)
Well if there is circular motion, there must be an inwards centripetal force, so there must be a resultant force downwards thus the weight is slightly more than the reaction force (say if you were standing on scales)

It depends on the context really, but if you were considering a person standing on earth then this would apply. But the OP has not provided any context
I understand what you mean now.

Even then, the force is gonna be mv/r

m say is 100 kg, v I've calculated to be 231 m/s (2x107 / 86400)
r is 6.4x106

Giving F to be around 0.004N

So negligible for human weight

Although with larger masses such as rockets, I see why there is concern
0
7 years ago
#8
(Original post by TheGrinningSkull)
I understand what you mean now.

Even then, the force is gonna be mv/r

m say is 100 kg, v I've calculated to be 231 m/s (2x107 / 86400)
r is 6.4x106

Giving F to be around 0.004N

So negligible for human weight

Although with larger masses such as rockets, I see why there is concern
Yeah it is negligible. Physicists do have a tendency to round things at the end (which is a great shame). This problem is annoying me now.
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Thread starter 7 years ago
#9
Wait i am just talking about dynamics,newton's third law of motion. simple in general,"the normal force on an object always equals the weight of an object". I chose true but it is actually false.the answer says both are not action rxn pairs but i dont get WHy they arent

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7 years ago
#10
(Original post by Vadevalor)
Wait i am just talking about dynamics,newton's third law of motion. simple in general,"the normal force on an object always equals the weight of an object". I chose true but it is actually false.the answer says both are not action rxn pairs but i dont get WHy they arent

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Newton's Third Law pairs are always of the same type. But the reaction force is an electrostatic force, and the weight is a gravitational force hence cannot be a third law pair. Say you had an object which was on the earth, you have two third law pairs of forces on the object

Pair 1: An electrostatic force (reaction force) due to the earth on the object AND its pair force, the electrostatic force on the object due to the earth

Pair 2: A gravitation force from the earth onto the object AND its pair force, the gravitational force from the object onto the earth.
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Thread starter 7 years ago
#11
(Original post by Cephalus)
Newton's Third Law pairs are always of the same type. But the reaction force is an electrostatic force, and the weight is a gravitational force hence cannot be a third law pair. Say you had an object which was on the earth, you have two third law pairs of forces on the object

Pair 1: An electrostatic force (reaction force) due to the earth on the object AND its pair force, the electrostatic force on the object due to the earth

Pair 2: A gravitation force from the earth onto the object AND its pair force, the gravitational force from the object onto the earth.
Concise ;straight to the point! Understood immediately thanks!

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