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Welcome to our Physics lesson on What is the Condition that Two Forces Obey the Action-Reaction Principle?, this is the third lesson of our suite of physics lessons covering the topic of Newton's Third Law of Motion, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Not all forces that are equal in magnitude and opposite in direction can qualify to be part of action-reaction principle. The condition is that they must be of the same nature (type). For example, despite the moving force of a car may be equal in magnitude and opposite in direction to the frictional force exerted between the car's tires and the road, these forces do not represent the action-reaction principle because the moving force is caused by the push of the engine to the tire's axis, while frictional force has an intermolecular nature. It depends on the properties of the surfaces in contact and the car's weight.
Another example in this regard is the gravitational force and the normal force acting on an object that rests on a horizontal table. The gravitational force caused by the Earth on the object is a field force as explained in the tutorial "What causes Motion? The Meaning of Force" while Normal Force is a contact force caused by the resistance provided by the ground against deformations when an object pushes it.
But when two people are pulling each other by hands, the forces are of the same nature. Therefore, one of these forces is taken as active (action force) and the other as reactive (reaction force).
Only half of all forces involved in a system is shown in a force diagram. This is because we are interested only in the active forces as they cause motion. Indeed, even if we include the reactive forces in the diagram and the calculations, we would obtain the same result, only that the solution would be longer.
You have reached the end of Physics lesson 4.7.3 What is the Condition that Two Forces Obey the Action-Reaction Principle?. There are 4 lessons in this physics tutorial covering Newton's Third Law of Motion, you can access all the lessons from this tutorial below.
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