# Are Newton's first and third laws contradictory?

Newton's first law states that given an ideal situation, any force applied on a body would change its state of motion and the object would continue to pursue that state, given no other force acts on the body. However, Newton's third law says that for every action force, there is an equal and opposite reaction force, so the assumption of the ideal and frictionless scenario is wrong, since we apply an action force, so according to Newton's third law there must inherently be friction present. Doesn't this nullify the presumption of the first law?

(Sorry if my thoughts do not appear clear, I tried my best to express them in words, and I would appreciate any feedback on my line of thought. Also, I've viewed similar questions but none seem to clarify this doubt of mine)

• Aug 8, 2020 at 5:56

## 3 Answers

They are not contradictory. Contradictory would mean that they both apply in the same situation and have effects that cannot both be true together.

Instead, they apply for different situations. The first law applies when there is no force and the third law applies when there is a force. In other words, the first law describes the motion of non-interacting objects, and the third describes the dynamics of interacting pairs of objects.

Notice however, that a more modern understanding of Newton’s laws may be helpful here. Newton’s 1st law can be understood as defining inertial frames. Then Newton’s 3rd law characterized interactions between bodies in that inertial frame. This is an alternative view where again they are not contradictory but additionally they are now both active at the same time.

• Ahh, this makes more sense, thanks for the clarification, I didn't realize that they are applicable for different situations, I just assumed all three were applicable to all scenarios at all times Aug 8, 2020 at 4:34
• @Dale Can you explain further? I don't see how they are not contradictory. N1 says an object at rest will remain at rest unless another force acts on it. That obviously means if there is a force, it will be compelled to move. Which is contradictory to N3, which says for every action there is an equal and opposite reaction (and since it is equal and opposite, nothing should happen, no net movement should occur? Like if you push a person and they push back exactly just as hard, then neither of you would move. But N1 says you will move. Feb 18 at 13:18
• @Ihavemanyquestions it seems like what you want is a discussion forum, not a Q&A forum. You should ask this question at physicsforums.com where I am also active. There they encourage back and forth discussion. What you ask is already answered above, so you will need a discussion to see the answer, but those are discouraged here
– Dale
Feb 18 at 13:23

Newton's first law states that an object with no forces acting on it does not change it's velocity. Newton's third law states that if you apply a force to an object, then that object applies a force of equal magnitude and opposite direction to you. Since Newton's first law applies in the absence of force, and Newton's third law applies when a force is applied, they are not in contradiction.

N3 applies to two different bodies interaction, i e the action is on body B and reaction on body A, so what is the problem ??