Moving a box, how is there an equal and opposite reaction?

I push a box horizontally. My hands push against the box with force F. The box accelerates forward.

How is it that the box is pushing back on my hand with force F?

Yet for there to be acceleration, or change in velocity, there is an unbalanced force.

With every action there is an equal and opposite reaction. Where is the reaction force? I've read its me pushing backward on the earth and friction of the box? I still don't find this intuitive :( Help :(


And with a sledgehammer I break a block of concrete. How is it that the block of concrete I have broken, overcome, destroyed, annihilated acts back on the sledgehammer with the same force during impact? In my mind, it must be less thus is why it is broken. :(

Update 2:

OOOH :D I see! I think I just realized, when considering the net force on a specific object consider the forces ON that specific object!!! Only the arrows pointing to it!!! The forces/arrows pointing from the object outwards, fridge toward floor and fride back toward pusher is not considered! Thankyou NCS!

2 Answers

  • NCS
    Lv 7
    3 years ago
    Favorite Answer

    Many students don't find Newton's Third intuitive. The problem is that they (and apparently you) fail to grasp that the two opposing forces occur on two (opposing) objects.

    Let's say you push a refrigerator with 200 N of force, and that there is a friction force on the refrigerator of 100 N.

    Consider these pairs:

    you/refrigerator: You push with 200 N; the refrigerator pushes back with 200 N.

    floor/refrigerator: The floor exerts a horizontal force of 100 N against the direction of motion; the refrigerator pushes back with 100 N IN the direction of motion.

    you/floor: You push backwards on the floor with a force of 200 N (assuming you are not accelerating); the floor pushes forwards on you with that same force.

    To find the NET HORIZONTAL FORCE on the refrigerator, consider the horizontal forces on it --

    200N - 100N = 100 N

    But note that the two forces on the LHS are parts of two different action/reaction pairs.

    As for the hammer:

    Yup, it seems like the hammer exerts much more force on the concrete than the other way around. BUT IT DOESN'T! You have to use other methods to explain why the concrete basically explodes and the hammer doesn't (strength of materials, impulse, etc.).

    Source(s): Please revisit this old nugget:
  • Luca
    Lv 6
    3 years ago

    The net force isn't zero because the forces are acting on different bodies. You push the box with force F, the box pushes you back. Yet, the box is on a (almost) frictionless floor, while you're standing still on the ground due to high friction and much more weight of course, so with even force the box moves, while you don't.

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