# Newton's Third Law -- Normal vs. Tensile as Opposite of mg?

• gravityripple
In summary, the normal force is not the third law force pair with the gravitational force. Force pairs always act on different bodies and the third law partner of the gravitational force on one of the masses is the force with which that mass is acting on the Earth.
gravityripple
Hello, I am returning to college after a ten year hiatus and am taking an online course on edx to try and refresh my knowledge a bit before the fall. I read a few other posts on Newton's Third law, but it seems I am falling short on this one concept.

In the case of an object, m_1, which is at rest on some frictionless table. There is a rope connecting this object to another object, m_2, which is hanging over the edge. To be clear, the rope connects m_1 to m_2, m_1 is on the table and m_2 hangs in the air off the edge of the table. There could be a pulley if you like on the edge of the table assisting with the direction change of the rope.

Assume I am drawing the free body diagrams for m_1 and m_2 separately, *not* as a single system. Also for simplifying explanation, assume a cartesian coordinate system with +y in the vertical at 90 and -y at 270, and +x at 0 and -x at 180.

I place a downward arrow on each diagram for the F_mg. In the case of m_1, I have a opposite Normal force with an upward arrow and an arrow in the +x direction for the tension force on m_1 by the rope. It makes sense to me that the Normal force in this case is opposing the gravitational force.

Why is it correct that m_2 only has the downward F_mg force (as mentioned) and an upward T force on m_2 from the rope? Why is the normal force not included here? As it stands, I am just "playing along" that the force just needs something to oppose it, so we have the Normal force and with m_2, the Tension force is sufficient, but I don't really understand.

Is the Normal force only present in cases where there needs to be a "mathematical correction" and could any other provide the opposing force? What if there was a person pushing down on a box? There would be the arrow downward for F_mg, and would the applied contact force be sufficient as an opposing force where as there would be no normal force?

The normal force is not the third law force pair with the gravitational force. Force pairs always act on different bodies and the third law partner of the gravitational force on one of the masses is the force with which that mass is acting on the Earth.

Normal forces only appear when they are needed to satisfy physical constraints. For example, a force is necessary to stop m1 from falling through the table. The third law partner of this force is the force with which the mass m1 pushes the table.

Bottom line is that you will never draw any third law pairs on the same free body diagram. If you draw two free body diagrams for two objects which affect each other with a force, the third law partner of the force on object 1 from object 2 will be an equal an opposite force on object 2 from object 1.

Orodruin said:
Normal forces only appear when they are needed to satisfy physical constraints. For example, a force is necessary to stop m1 from falling through the table. The third law partner of this force is the force with which the mass m1 pushes the table.

This is exactly what I was confused about. Thank you very much!

It sounds like I should read up a bit more on Third Pairs. Any other suggestions?

## 1. What is Newton's Third Law?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object will exert an equal force in the opposite direction.

## 2. How does Newton's Third Law apply to normal and tensile forces?

In the context of "normal vs. tensile as opposite of mg," normal force refers to the force exerted by a surface on an object in contact with it, while tensile force refers to the force exerted by a string or cable on an object attached to it. According to Newton's Third Law, the normal force and the tensile force are equal and opposite, as they are both reactions to the weight of the object (mg) pulling down.

## 3. What is the difference between normal and tensile forces?

The main difference between normal and tensile forces is their direction. Normal forces act perpendicular to the surface of contact, while tensile forces act in the direction of the string or cable. Additionally, normal forces are caused by objects in contact, while tensile forces are caused by objects being pulled or stretched.

## 4. How do normal and tensile forces affect an object's motion?

Normal and tensile forces do not affect an object's motion directly, as they are both reaction forces. However, they do affect the net force on an object, which can change its motion according to Newton's Second Law (F=ma). If the normal and tensile forces are balanced, the object will remain at rest or continue moving at a constant velocity. If the forces are unbalanced, the object will accelerate in the direction of the net force.

## 5. Can normal and tensile forces cancel each other out?

Yes, normal and tensile forces can cancel each other out if they are equal and opposite. This means that the object will experience a net force of zero, and will either remain at rest or continue moving at a constant velocity.

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