0

I know Newton's laws especially the 1st and 2nd laws of motion:

1st law: object in motion stays in motion unless it is influenced by an outside force.

2nd law: F=ma

So Gravity is not a force, then what is the "a" causing the apple to fall on his head? doesn't it seems that both laws contradict each other?

For those confused how there's a contradiction, here goes: Since we know gravitational force is not an actual force but it acts on object aka apple and changes it's speed over time but we still apply F=ma or in this case F=mg where g is 9.81m/s/s.

Based on Newton's 1st law we know that apple should stay in motion and not under any influence of outside force since we just ruled out gravity as an actual force then why is there an acceleration which implies 2nd law must be true? See the contradiction here.

user6760
  • 12,980
  • an object not subject to an outside force moves in a geodesic. small update to the rules based on GR – shai horowitz Dec 18 '21 at 01:18
  • 1
    possible duplicate: https://physics.stackexchange.com/q/219306/50583 – ACuriousMind Dec 18 '21 at 02:08
  • Don’t we say gravity = 9.8m/s^2 & force by gravity = mg ? – S.M.T Dec 18 '21 at 08:50
  • @S.M.T: a=g=9.81m/s^2 so F=ma=mg for those who is confused ;D – user6760 Dec 18 '21 at 09:12
  • @user6760 I think I’m getting your Q. Your text isn’t too clear exactly. You mean to say if gravity is an acceleration , then come it’s pulling us since it’s not a force. Therefore , how does an apple even move if gravity is not a force. It’s a good point. Think about it , if a car moves on with uniform velocity = 60m/s & no acc on ground & hits you. You would say u didn’t feel hit because F = 0. It’s a good Q to ponder upon. – S.M.T Dec 18 '21 at 09:21
  • @S.M.T: thank you. – user6760 Dec 18 '21 at 09:23
  • @user6760 Did you get sth though , like it’s not necessary to use the term ‘force’ means moving only. – S.M.T Dec 18 '21 at 09:24
  • @user6760 take magnets. They keep on attracting all the time without any influence of force on them. I think it’s similar to that for earth too. – S.M.T Dec 18 '21 at 09:26
  • @S.M.T: I edited to clarify the contradiction hope it helps ;D – user6760 Dec 18 '21 at 10:17
  • First of all, please don't try to mix Newtonian physics with Einstein's physics. – batchcoding____s Dec 18 '21 at 11:53

3 Answers3

3

If you step on a the gas pedal in a car, you can feel the acceleration pressing you into the seat, and an accelerometer will register it. So let's consider the case of Newton and the apple:

(1) Does Newton feel a force? A: yes, he can feel the Earth pressing on his feet (or bottom if he is sitting down). If he holds an accelerometer, it will register 9.8 meters per second squared of acceleration.

(2) Does the falling apple feel a force? A: no, it feels nothing. A falling accelerometer reads 0.

So, in the equation F = ma, which of the two is accelerating? Which one has the force acting on them? A: it's Newton who is accelerating, and who is having force applied to him (the contact force with the Earth is pushing him up).

We like to think of the Earth as an inertial frame that is at rest, but in relativity it is not: the surface of the Earth is experiencing constant acceleration. This seems like a contradiction, because it isn't going anywhere, but as an analogy consider a rock tied to a string being swung in a circle: it too isn't going anywhere (or at least, not very far at all) and yet it is constantly accelerating.

Eric Smith
  • 9,064
1

If gravity isn't a force, then none of the other forces are forces either.

Gravity isn't taken to be a force because it is seen as the expression of curvature of spacetime and this has an influence on the density of the energy-stress tensor field.

However, the electromagnetic, the weak and strong force classically are expressions of curvature of a fibre bundle over spacetime. Hence by the above argument we shouldn't take these as forces either.

This means revisting what we mean by a force and I think Aristotle's definition is salient here. He calls a force any agent that cause change. As the forces above do cause change, they are forces under this definition.

Mozibur Ullah
  • 12,994
0

From what I read there seems to be two differences between the conclusions of Einstein and Newton. Energy effects gravity and responds to it (and if you add energy to a system, there is a corresponding increase in the inertial mass). And, gravity effects the rate of flow of time. Bottom line: If you stay on the surface of the earth, there is no significant difference between a gravitational field and a curvature of space-time. (A rose by any name.)

R.W. Bird
  • 12,139