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Q & A: Newton's third law

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Most recent answer: 07/25/2015
Q:
What is the concept of action and reaction forces?
- Amy
Conotton Valley, OH
A:
Say that one object (A) exerts a force on another object (B). Then ALWAYS B will exert a force of the same strength on A, but pointing the opposite way. If you want to call these forces action and reaction, it doesn’t matter at all which one you call action and which one you call reaction.

So if A starts to accelerate East, then B will accelerate West, assuming nothing else is exerting forces on them. When the two objects have similar masses, the accelerations are comparable and easily noticeable. If one object is small (say a tennis ball) and the other big (say the Earth), then the force will make the small object accelerate lots more than the big object. When that happens, it looks like only one object is feeling the force, but if you could measure carefully you’d see that actually the force still goes both ways.

Sometimes people get confused about all this when one of the objects is a person. Then it’s tempting to think of the person as somehow different from the other object- say a basketball. They are different, of course, but this law (called Newton’s third law of motion) applies equally to both of them.

Mike W.

(published on 10/22/2007)

Follow-Up #1: equal and opposite reaction

Q:
If every force has an equal and opposite reaction, how can anything move? Wouldn't the reaction force cancel out the applied force since they are equal in magnitude and opposite in sign? To answer this question, I'm guessing that the reaction force comes AFTER the applied force; hence, an object stops accelerating after the applied force has been removed. Am I correct to think of it like this, or is there another explanation to my question?
- Jonathan (age 23)
Tavares, Florida
A:
Similar questions have come in before, so I've marked this as a follow-up. The issue is so important and so often misunderstood that it's worth revisiting.

Those two forces are NOT both acting on the same object. One force acts on one object. The equal and opposite force acts on the other object.

Once you get used to this universal fact, you actual may stop thinking of them as two forces but rather start to think that a force between two objects simply means that they're trading momentum. Newton is just saying that the total momentum doesn't change. Whatever one object loses, the other gains.

What if there were only one object? Then your picture would follow. The object can't exert a net force on itself, so it won't start accelerating.  that's sometimes called the principle of inertia (Newton's First Law), and you can see how it's really just a special case of conservation of momentum (Newton's Third Law).

Mike W.

(published on 06/25/2011)

Follow-Up #2: 3rd law for a bullet hitting a target?

Q:
According to the 3rd law a bullet when hitting a human should bounce back due to opposite reaction of action, but it doesn't. Why?
- krish (age 16)
varanasi, up , india
A:

The 3rd law holds in your case as both the bullet and the target body will experience the same forces. However, the law only states that the reaction will be equal in magnitude, but it does not say anything about what that equal magnitude should be. The trajectory of the bullet will depend on the amount of force applied to it, which is in this case mostly friction and some from elastic compression, ensuring that the total linear momentum of the universe is conserved. As the bullet passes through the body, it will decelerate due to this force and lose energy. If force is small there will be an exit wound. Since the friction force is bigger than the spring force from compression, a larger force will often make the bullet stop somewhere in the body.

Now suppose you are firing the gun against a shield, or an armadillo. The compressive force  can be much higher. Instead of entering the body and getting slowed by friction, the bullet and the shield both deform, like springs.  Therefore very little energy would be dissipated by friction but rather stored as elastic potential energy that now can be harvested to accelerate the bullet back.

Tunc


(published on 07/25/2015)

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