1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding Newton's Third Law

  1. Sep 27, 2007 #1

    I'm in year 12 at the moment doing my AS level. I'm having some troube understanding Newton's Third Law.

    Would anyone mind fully explaining it to me please?

    Thanks :)
  2. jcsd
  3. Sep 27, 2007 #2
    is there anything specific that's worrying u
  4. Sep 27, 2007 #3
    All of it :s
  5. Sep 27, 2007 #4

    Doc Al

    User Avatar

    Staff: Mentor

  6. Sep 27, 2007 #5


    Staff: Mentor

    Or punch a wall. The dent in the wall and the bruises on your knuckles are due to an action-reaction pair.

    But Doc Al's suggestion is probably less painful :-)
  7. Sep 27, 2007 #6
    well one thing u need to jnow is it is a law and it talka about the nature of somwthing called "Force " whose definition(some would say Pseudo) was given in the 2nd law.
    well there are two forms of newton's third law the wak form and the strong form.The former claims the existence of action and reaction pairs and remeber they act on different bodies.
    The latter form says the force between any two bodies acts along the line joinig those.
    well remeber as i said it is just a law it is more of an experimental act than a logical necessity.undertsand it intutively but don't take that to be obvious.
  8. Sep 28, 2007 #7
    Take this example: If you walk off of a small boat onto a pier, the boat will move back some. This is because the force that it takes to move your body onto the pier is then equal to the force put on the boat that pushes it back.
  9. Sep 28, 2007 #8
    Yes its a necessity, if you take as fundamental, that the future must be predicted from the present.
    The other option is that things happen at random.
  10. Sep 28, 2007 #9
    well if u think that's a logical necessity then u should be ready for some surprises it doesn't hold 'directly' in electromagnetism.u need to introduce something called field momentum .Newton's third law doesn't hold there directcly in the form it is
  11. Sep 28, 2007 #10

    Doc Al

    User Avatar

    Staff: Mentor

    Newton's 3rd law is related to the more general law of conservation of momentum, which is a consequence of translational (spatial) symmetry.
  12. Sep 28, 2007 #11
    Last edited by a moderator: May 3, 2017
  13. Sep 28, 2007 #12

    Doc Al

    User Avatar

    Staff: Mentor

  14. Sep 28, 2007 #13
    Can you tell me the difference between:

  15. Oct 3, 2007 #14
    Okay... I understand basic third law situations when objects are not accelerating (ie normal force vs gravity), but what about when objects are accelerating? Can someoene help me understand the following question?

    "A small compact car is giving a push to a large van that has broken down, and they travel along the road with equal velocities and accelerations. While the car is speeding up, is the force it exerts on the van larger than, smaller than, or the same magnitude as the force the van exerts on it? Which object, the car or the van, has the largest net force on it, or are the net forces the same? Explain."

    I know that the van must have the largest net force because it has the largest mass. If both objects are to have the same acceleration, the one with the larger mass has to have the greater net force. The thing I don't understand is where this extra force comes from in the free body diagram. If it all comes from the car, then how does the car still accelerate? Wouldn't the van push back on the car with the same force the car pushes on the van with (Newton's third law)?
    Last edited: Oct 3, 2007
  16. Oct 4, 2007 #15

    Doc Al

    User Avatar

    Staff: Mentor


    From the road pushing on the tires of the car--that's how any car accelerates.
    Is the force that the car exerts on the van equal and opposite to the force that the van exerts on the car? Yes! Is that the only force on the car? No!

    Forces on car:
    (1) The ground pushes the car forward (as a result of the engine turning the wheels)
    (2) The van pushes the car back.

    Forces on van:
    (3) The car pushes the van forward.

    (Let's ignore friction and any other forces acting on the van.)

    Forces (2) and (3) are equal and opposite. As you realize, force (1) had better be larger than force (2), otherwise the car won't accelerate.

    To put some totally made up numbers to it, let's say that the forces are (1) = 100 units, (2) = 80 units, and (3) = 80 units. So the net force on the car would be 100-80 = 20 units; the net force on the van, 80 units.
  17. Oct 7, 2007 #16
    Thanks! I just made a FBD using that info and I understand now :).
  18. Oct 14, 2007 #17
    Newton's Third Law, Conceptual Post

    When an object collides with another object, the second object exerts and equal and opposite force upon the first.

    When objects interact, they only do it at the same moment. One object cannot hit another without the other one not hitting it. There is no such thing as action, then reaction. (At least, in the simultaneous sense.)

    One object cannot touch another without being touched. When two objects touch, one cannot touch the other even just a little bit more than the other.

    One object cannot push on another object more than the other pushes back. The amount of force exerted by one, is the amount of force exerted by the other. If this were not so, then one would have to touch the other longer without be touched, so that extra force could be applied. This never happens, because force cannot occur between objects, unless they touch. Try pushing on space, neglecting air resistance and gravity. Feel any force?

    If you tried to push on an object but your force was less than the force that the object pushed on you, then part of the object you push on would not move. It would not move because the force necessary to move it would not be there. It would have to do something like magically pass through your hand, which never happens.

    Hope that helps.
    Last edited: Oct 14, 2007
  19. Oct 15, 2007 #18
    key point is to take each object individually. think: you pushing a key on a keyboard. individually, let's look at you. you push the key with a force. now look at the key, the key pushes your finger with equal force in opposite direction. both forces are similar because both are forces that push the atoms together - interaction forces.

    another example. moon and earth. earth pulls the moon. the moon pulls on the earth. both are gravitational forces. equal in size (magnitude) and opposite in direction.

    the common problem is the quesion, 'if the forces are equal and opposite, why do things still move?' well imagine an ice skater pushes on a wall and moves back. forces between the ice-skater and wall are equal and opposite interactive forces, but the wall pushing on the skater is in this case larger than the friction between skates and ice which is pushing in opposite direction. if the skate and ice are replaced with a shoe and wooden floor, then the frictional force that causes shoe to slide on wood is much greater than the skaters push on the wall (and the push of the wall on the skater), and the feet therefore don't slip.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook