Newton's Third Law of Motion

Identifying action and reaction
Now a stick of dynamite can produce a great deal of force when it explodes. But does a stick of dynamite contain force? No, unlike energy, force is not something an object has or can store. Force is an interaction between two objects. So when trying to identify the action and reaction forces it easier to identify the two interacting objects.

For example, lets look at a boulder falling towards the earth.
Step 1 Identify the two objects that are interacting. In this case it is the boulder and the earth.
Step 2 The action and reaction forces can be stated in this way.

Action is the force exerted by the boulder on earth
Reaction is the force exerted by the earth on the boulder

It does not matter which force you call the action or reaction.

Lets look at the earth's gravitational pull on the moon. Consider the question below.
The earth attracts the moon by its gravitational force. Does the moon also pull on the earth and if so does this form an action-reaction pair of forces?

Which force is stronger?

Step 1 Identify the two objects that are interacting. In this case it is the moon and the earth.
Step 2 The action and reaction forces can be stated in this way.

Action is the force exerted by the moon on earth
Reaction is the force exerted by the earth on the moon

Since the action and reaction forces are of equal magnitude but in opposite directions, both the force exerted by the moon on the earth and the force exerted by the earth on the moon are equal in strength.
Consider a car traveling along the road. Identify the action and reaction forces.

Step 1 Identify the two objects that are interacting. In this case it is the road and the tyres of the vehicle.
Step 2 The action and reaction forces can be stated in this way.

Action is the force exerted by the tyre on the road.
Reaction is the force exerted by the road on the tyre.
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