Why don’t action and reaction forces on a person swimming in water cancel each other out?
A. Because they act on the same object.
B. Because they act on different objects.
C. They do if the amounts of force are different.
D. Cannot be determined without the specific amount of force.

Question

Why don’t action and reaction forces on a person swimming in water cancel each other out?
  A. Because they act on the same object. 	
  B. Because they act on different objects. 	
  C. They do if the amounts of force are different. 	
  D. Cannot be determined without the specific amount of force.

theeric · Answer

This is a tricky question, honestly. It's just going for Newton's third law understanding, but swimming is a complicated process! I can rule out one, sort of rule out another, I'm for one but it seems to contradict what I typically think of when considering "action and reaction" force pairs, and another one seems like a catch-all to the situation in which the question is asking what I typically think contradicts the force pair idea.

Do you have any thoughts?

anonymous · Answer

I think its B?

theeric · Answer

That's what I was thinking too... In this situation your hands and legs push the water behind you, which pushes you forward by reaction.
Then, since you move forward, you will push the water forward with your body, which will in turn push you back.
You can move forward because you push yourself forward more than you are pushed back.

My issue is that action-reaction force pairs are paired. So, you push the water with your hands and are pushed by it. That's a pair. The other pair is the water on your body and your body on the water. So, these pairs are always on the same spot.... But B seems alright.

D is also true, but it seems like a catch-all and doesn't answer the question (unless it's a trick question.

C is contradictory, it must be false. Unequal forces will not cancel out.

A seems correct about action-reaction pairs, but doesn't answer the question really.

So, I favor B most of all, but am not 100%.

anonymous · Answer

I did a little more research and came upon A, but I do understand where you are coming from with B. Thank you, I will just see my outcome.

theeric · Answer

Okay! Good luck! I wish you the best!

A is not informative, but I agree that action-reaction pairs will be on the same spot, when pushing.

theeric · Answer

A doesn't capture why movement occurs, because action-reaction pairs are equal and opposite and so they "cancel" at the site.

Yet, B doesn't really capture what an action-reaction pair is!

Sorry you got a weird question!

anonymous · Answer

In case of fluids, it is not force which matters, it is shearing force. But here the safe answer is B

anonymous · Answer

It is B.