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Old May 27th 2013, 09:43 AM   #1
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Newtons 3rd law

Hi I just have a general question aout the 3rd law that I hope someone could shed some light on.

If I push an object (in space) with my hand, my hand exerts a force on that object, and that object exerts an equal force on my hand. The object will accelerate for as long as I keep pushing it.
But how can there be acceleration if there is no net force?

More generally my question is that if every force is opposed by an equal and opposite force, why don't they all just cancel out?
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Old May 27th 2013, 11:16 AM   #2
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No doubt.

Forces act on different system.e.g. in your example you forced the bag so it accelerate,the repulsive force acts on you,not on the bag.This repulsive force tries to accelerate you.But its not enough to do so.
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Old May 27th 2013, 02:32 PM   #3
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Originally Posted by asifikbalsahin View Post
Forces act on different system.e.g. in your example you forced the bag so it accelerate,the repulsive force acts on you,not on the bag.This repulsive force tries to accelerate you.But its not enough to do so.
Ok but in order for there to be a force F on the object, doesn't my hand need to have a force F acting on it in the same direction (to cause the force on the object) which would be cancelled out by the reaction force on my hand in the opposite direction?
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Old May 27th 2013, 04:57 PM   #4
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Originally Posted by kinhew93 View Post
Ok but in order for there to be a force F on the object, doesn't my hand need to have a force F acting on it in the same direction (to cause the force on the object) which would be cancelled out by the reaction force on my hand in the opposite direction?
As asifikbalsahin noted the 3rd Law force pairs act on different objects. To put it Mathematically say we have that F(ab) is the force on object a by object b. Then Newton's 3rd says that F(ba) = - F(ab). So talking about each individual object, there is a force acting separately on object b and object a, then there is a force equal in magnitude to F(ab).

If we look at the system composed of both objects together then the 3rd Law pairs cancel out.

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Old May 27th 2013, 06:01 PM   #5
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What about this example:

If an object rests on a flat table, the forces on the object will be it's weight and the normal reaction force of the table, making the resultant force on the object 0. But if the resultant force on the object is 0, how can it exert a force on the table?
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Old May 27th 2013, 08:23 PM   #6
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Originally Posted by kinhew93 View Post
What about this example:

If an object rests on a flat table, the forces on the object will be it's weight and the normal reaction force of the table, making the resultant force on the object 0. But if the resultant force on the object is 0, how can it exert a force on the table?
By Newton 3, since the table exerts a normal force N up on the object, the object exerts a normal force N down on the table. The weight of the object does NOT act on the table.
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Old Aug 17th 2013, 04:49 AM   #7
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Hello! An object can exert a force on another object even if the net force that acts on it is zero.

For example:
Friend A and Friend B are pulling on a string in a tug-of-war using the same amount of Force, F.

The forces acting on the string are:
F(as) and F(bs)

According to Newton's Third Law, the string also pulls on friend A the same time so F(as) = -F(sa).

Here, we see that the string exerts a force on friend A but the string does not move because the force that A exerts on the string, F(as) is cancelled out by the force that b exerts on the string F(bs)

Summation of Forces on the three object for reference:

Friend A:
F(sa) + friction = 0

string:
F(as) + F(bs) = 0

Friend B:
F(sb) + friction = 0
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Old Aug 17th 2013, 10:01 AM   #8
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please tell me what will be the tension in a string(tug of war) if force is same on both of its side. i.e 10 N
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Old Aug 17th 2013, 11:59 PM   #9
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Originally Posted by jamshaidalamkhan View Post
please tell me what will be the tension in a string(tug of war) if force is same on both of its side. i.e 10 N
F(as) = 10N, and F(bs) = 10N

but the act towards opposite directions, one is towards the negative x direction and one is towards the positive x direction. So, all the forces that act on the string will cancel out.

The string also pulls on friend A at the same time with a magnitude of 10N but on opposite direction. Since friend A is stationary, the sum of forces acting on him/her must be equal to zero. Thus, another force cancels the string's pull. This opposing force must be of the same magnitude but opposite direction. The normal force and gravitational pull on friend A acts along the y-axis so the opposing forces must be the friction of let's say friend A's shoes against the ground.

Same thing goes for friend B.
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Old Aug 18th 2013, 07:55 AM   #10
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Originally Posted by jamshaidalamkhan View Post
please tell me what will be the tension in a string(tug of war) if force is same on both of its side. i.e 10 N
String tension will be 10 N.
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