Suppose that I were to jump on the surface of the Earth. The moment I hit the surface, shouldn't the force Earth is applying on me be greater than the force I am applying to it so that I stop completely? If the forces were equal, wouldn't I still be moving?
Asked
Active
Viewed 93 times
0
-
Note that you can apply the exact same argument in the opposite direction - the moment the earth's surface hits you, shouldn't the force you're applying to the earth be greater than the force it's applying to you so that it stops completely? Both you and the earth come to a stop, and we clearly can't argue that both forces are greater than the other depending on our choice of reference frame. – Nuclear Hoagie Jul 06 '23 at 19:46
-
"shouldn't the force Earth is applying on me be greater than the force I am applying to it [...] If the forces were equal, wouldn't I still be moving?" - the two forces in the N3L action-reaction pair aren't both acting on you. The force that is exerted on the Earth by you does not affect how you move. As for the forces that are simultaneously acting on you, N3L doesn't say anything about them. – Filip Milovanović Jul 06 '23 at 19:58
-
Consider this scenario: there's an equal an opposite reaction force while you're in freefall. – Filip Milovanović Jul 06 '23 at 20:01
-
This question does indeed remind one of "Given Newton's third law, why are things capable of moving?" but it isn't really a duplicate. – Philip Wood Jul 07 '23 at 15:14
-
@NuclearHoagie You are looking at it classically. Quantum Physically you have to be observer-dependent (frame-of-reference dependent). From that thinking, two forces can be greater than each other at the same time in different frames-of-reference. You see, distance from here to alpha centauri can be anything, from zero to whatever, depending who is observing, as per that line of thinking. If distance can vary forces can vary too, it takes zero force to never travel / cross a zero distance. – Atif Jul 07 '23 at 20:18
-
@Atif The frames I'm describing are inertial frames, which are by definition indistinguishable. Third law forces pairs are third law force pairs in all inertial frames. Forces can vary by frame, but in every inertial frame the force exerted by and upon you must be equal. – Nuclear Hoagie Jul 10 '23 at 12:29
-
@Nuclear No, you are wrong. If I go to moon then from the frame of reference of me its the moon that is moving towards me. Since I dont move at all no force is applied by me. Also, no force is applied on me. No force is applied on me because I dont move, no part of me, no particle in me move. Action / Reaction pair has nothing to do with it. Consider this, if you move towards sun do you apply force on sun? No. You dont push sun to move to sun. You push your feet to ground, your rocket exert force on air or propellant. Sun is not affected by the force. – Atif Aug 04 '23 at 09:52
-
@Atif The notion that you can't apply or feel forces in your own frame of reference is directly contradicted by everyday experience - from what other frame of reference do you experience the universe? – Nuclear Hoagie Aug 04 '23 at 12:32
1 Answers
0
"If the forces were equal, wouldn't I still be moving?"
No: the upward contact force on YOU from the Earth continues to act (with changing magnitude) until you are at rest, in equilibrium, at which stage the upward contact force on you is equal to the Earth's downward gravitational pull on you. (This is the equilibrium condition and nothing to do with Newton's third law.)
The downward contact force that you exert on the Earth is always equal and opposite to the contact force that the Earth exerts on you; these forces are Newton's third law partners in the same interaction. But the force that you exert on the Earth is irrelevant to your motion! It's the forces that act on YOU that affect your motion.
Philip Wood
- 35,641
-
The upward normal contact force isn't a third law pair with any gravitational force - the normal forces pair, as do the gravitational forces. The second paragraph is incorrect, the upward force from the ground is basically unrelated to the force of gravity. If you land on concrete, the normal force will have a high magnitude for a short duration and will be much greater than the force of gravity, but if you land in a pile of feathers it may be a much lower force for a longer period of time. – Nuclear Hoagie Jul 06 '23 at 20:06
-
You haven't read my middle paragraph carefully enough. Of course I'm not claiming that the gravitational force is a Newton Third Law partner to the contact force. I simply said that when you are at rest (I should have added 'in equilibrium') they are equal and opposite – which they are when you are in equilibrium standing on the ground. If they weren't equal and opposite you wouldn't be in equilibrium! – Philip Wood Jul 06 '23 at 20:10
-
It's worded a bit confusingly- you describe the contact force as having varying magnitude which itself indicates that we are not yet in equilibrium, and a varying force obviously cannot always be equal to the gravitational force of constant magnitude. The way it's worded sounds like the contact and gravitational forces are equal in general during the entire time they act, not only after coming to rest. – Nuclear Hoagie Jul 06 '23 at 20:16
-
I think that you missed the phrase "until you are at rest". It's only after this phrase that I talk about the gravitational force being equal and opposite to the contact force. – Philip Wood Jul 06 '23 at 20:24
-
"The upward force continues to act until you are at rest, with the upward contact force on you equal to the downward gravitational pull". You need something like "at which point" instead of "with", as written this does not unambiguously imply equality only after coming to rest. – Nuclear Hoagie Jul 06 '23 at 20:31
-