TITLE: Is there an action reaction pair of forces when an object accelerates due to a pull/push?
QUESTION [0 upvotes]: I see that Newton's Third Law of Motion explains that forces always come in action-reaction pairs. The Third Law states that for every action force, there is an equal and opposite reaction force.
I understand that there is an action-reaction pair of forces when an object is sitting stationary. For example if I am to pull on a crate and it doesn't move then I know that there is an action-reaction pair of forces where the pull from me is resisted by friction. The force applied by me is equal to but opposite to the frictional force from the crate.
If I am to now pull on a cart and the cart accelerates is there still an action-reaction pair of forces between myself and the cart? Since the crate accelerates that means the action-reaction pair is no longer equal right?

REPLY [0 votes]: In Newtonian physics Newton’s third law always holds for every interaction at every moment regardless of the motion of the objects. 

if I am to pull on a crate and it doesn't move then I know that there is an action reaction pair of forces where the pull from me is resisted by friction

This is not an example of an action reaction pair. Third law pairs always act on different objects. So in this case the third law pair of the pull from you is they equal and opposite force that the crate is exerting on you pulling you back. Similarly, the third law pair for the friction force is the equal and opposite friction force acting in the ground from the crate. 
The friction force and the pulling force are not a third law pair because they act on the same object. Each third law pair represents a single interaction between two objects.
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