A Common Misconception about Newton’s Third Law Force Pairs (or Action-Reaction Pairs)
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A Common Misconception about Newton’s Third Law Force Pairs (or Action-Reaction Pairs)


mr.p: I would like to address
a misconception about Newton’s Third Law Force Pairs. Bo: Hey guys. Billy: Hey Bo.
Bobby: Hi Bo. ♫ flipping physics ♫ mr.p: Because this concept
is an extension of my video introducing Newton’s Third
Law and Third Law Force Pairs, I suggest you watch that video first by clicking on the link
in the description, or the link right here that
is embedded in this video. Now, I need a free-body
diagram of all the forces acting on the book. Bo? Bo: The force normal is
perpendicular to the table and a push, so it is up. And the force of gravity,
as always, is down. mr.p: Correct. And when we sum
the forces in the y-direction on the book, which we’ve done many times we can show that the force
normal and the force of gravity are equal in magnitude, and
from our free-body diagram, you can see that the force
normal and the force of gravity are opposite in direction. Therefore, when we include
vectors in our equation the force normal is equal in magnitude and opposite in direction
to the force of gravity. However, the force normal
and the force of gravity are not a Newton’s Third Law Force Pair. I need to know why not. Bo: Because the two forces act
on the same object: the book. Newton’s Third Law Force Pairs
act on different objects. mr.p: Correct. The force normal
and the force of gravity do not form a Newton’s
Third Law Force Pair because they act on the same object. There is a downward normal
force, caused by the book on the table, which is the
Newton’s Third Law Force Pair to the upward normal force, which is caused by the table on the book. There is an upward force of gravity, caused by the book on the Earth, which forms the Newton’s
Third Law Force Pair with the downward force of gravity, caused by the Earth on the book. Every Newton’s Third Law
Force Pair is forces on two different objects:
object one and object two. Because, for every force caused
by object one on object two, there exists an equal but opposite force, caused by object two on object one, where both forces are vectors. Thank you very much for
learning with me today. I enjoyed learning with you.

22 Comments

  • Michael Smitten

    Absolutely right and I frequently challenge students, when I drop a pen, as to whether the earth moves towards the pen. Of course mostly they will say, "No", but it does. However the relative masses of the pen and the earth are so vast as to make no difference in engineering. But this point still needs to be made, as a fact of nature.

  • Jackalakalaka

    Also, to ascertain that it's a 3rd law force pair, just check if the subject and direct object of the statement "the book pulls up on the earth" are switched. Just another way to think about it

  • ATA 2012

    can you say that a force pair is the gravity from the table acting on the book and the gravity of the book acting on the table?

  • Quantum Zain

    I spent one year living with this misconception of the Beautiful 3rd law and due to that I grew many many confusions and even paradoxes… And now finally these are cleared because of what I thought that I understood completely

    I thought I got Newton's laws but turned out I didn't! I never the idea of a book at rest on a table is a horrible example of the 3rd law

    Finally it makes sense!!

    Thank you so much, I really appreciate it… There are tears in my eyes… Man Physics is cool and even more when proven that you are wrong about something you thought is undoubtedly true …

  • Ashim Rai

    Hell of a video! I've had the misconception recently…and I had a really hard time looking it up on google..but couldn't get a proper explanation…Then I found your video..Now I think my misconception is erased…thanks to your informational video…keep it up

  • FZ500

    I think the Third Law misconception is related to how free-body diagrams are generally drawn in physics textbooks. All supports (=adjacent bodies) should be removed before the forces are drawn, otherwise it is not unambiguous which body the forces are exerted on.

    You make this "mistake" in the video. You should consider drawing separate free-body diagrams for the book and the table.

  • Parth Prajapati

    Sir,Q.1if the force of gravity and and the normal force are a pair as you said in last then why they donot cancel out each other ?And why the body is not flying ?As in space l.
    Q.2 Firstly you said that the force of gravity and normal force is not the pair but in the case of earth you …Why?Can you pls ans …

  • Hugo Lachs

    The normal force has nothing to do with gravity. It's only between objects contacting each other. How is the normal force the same as gravity?

  • Kevin Sun

    I'm still a little confused. Isn't the book exerting a force onto the table which causes the table to exert a force back onto the book.

  • Dat DAO Tien

    I would like to ask you a question, if newton's third law is true, which hopefully it is, how do we move around, because if every force has an equal opposite reaction, shouldn't the air resistance, or water resistance, or really any force you're facing, be enough to stop you from moving, since the forces should be strong enough to push the force you're exerting back, and therefore stopping you from moving completely.
    Again, obviously, I know this isn't true but, I can't come up with a way to explain it

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