F=ma? Newton Was Wrong!

Physics teachers tell us force equals mass times acceleration, but, as
any rocket scientist would tell you, this is false.

Force is actually the change in momentum over time. Since momentum (p) is
mass times velocity, force is actually:

F = dp/dt = d(m*v)/dt = m*dv/dt + v*dm/dt = ma + v*dm/dt

In other words, force is mass times acceleration PLUS velocity times
change in mass per unit time.

For items with fixed mass, dm/dt=0, and this reduces to Newton’s equation.

For items with changing mass (like rockets, which expel mass to gain
velocity), Newton was wrong.

[Of course, Einstein also showed F=ma isn’t 100% accurate because of relativistic effects]

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4 Responses to F=ma? Newton Was Wrong!

  1. Big Steve says:

    Einstein knew he was wrong and said so. When he did, he was wrong. Newton was not wrong. Teachers will present things that are considered incorrect, but neither teachers nor the vaunted rocket scientists know why certain things are “wrong,” but the present accepted mainstream theories indicate so. No one knows why certain tried and true laws are correct in one set of circumstances and incorrect in another set, but since that appears to be the case, they (incorrectly) state that the laws change according to the circumstances.

    • barrycarter says:

      I’m not sure I follow this, but I’m approving it on the off chance someone else does. My main questions would be:

      – When did Einstein say he was wrong?

      – It’s generally accepted that Newton’s Laws of Motions are only an approximation that apply at low velocities. Do you disagree?

      Einstein’s laws encompass and expand Newton’s Laws, so I don’t think anyone is saying the laws change according to the circumstances.

  2. haris mahmud says:

    if we consider a football and hit it on the by some tangent line then it will move in parabolic path why this is so,,,,newtons says that the direction where force is applied accelration is produced in same direction

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