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But then what does inertia mean at all!? Cant we calculate how much it resists the applied force? I mean considering an imaginary plane with no friction but gravity as of Earth and an enormous mass of square shape is standing on the plane at equilibrium(say rest) like a 1000kg. So if we have to make it start moving what force should be applied to make it move? or is it any force no matter how small will set it into motion with a corresponding final velocity? and another doubt is force itself.. if we are pushing against a wall for example.. what would be the force applied? in the eqn F=ma what would be the mass and acceleration taken as? and will it be of the body applying force (us) or will it be the body receiving force(wall)? I have really confused myself today. Sorry if its really silly. I just wish to understand it clear and complete. Thank You.

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  • $\begingroup$ Inertia means that it is much harder to get a cart of 1000kg moving than it is to get one of 100kg moving. Force is that which gets them moving. Newton's second law simply puts these two naive observations on a precise numerical footing. $\endgroup$ – CuriousOne Feb 1 '16 at 16:38
  • $\begingroup$ I think of inertia as some property of a body which "opposes" changes in the body's motion. Mass is a measure of the amount of inertia a body has. $\endgroup$ – Farcher Feb 1 '16 at 16:42
  • $\begingroup$ It might help to think of inertia not in terms of forces and accelerations, but in term of momentum and velocities. Momentum is understandable because it is invariant (conservation of momentum) and velocities are measurable. Inertia is the thing that converts velocity into momentum. A change in velocity requires a change in momentum, which by definition we call a force (Newton's seecond law). $\endgroup$ – ja72 Aug 19 at 4:47
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Inertia is the term we use to say that an object can't change its velocity without a force acting on it. This does mean that a force always changes the velocity of an object (if net force is not zero). The wall scenario is a bit different because the wall will feel the force and will try to change its motion accordingly but in this case, inertia has less to do with the fact that the wall barely moves. Here there are simply other forces like the connection between ground and wall that balance your push sufficiently to keep the wall stable.

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