what would happen to a car if its reaction force was less than the weight
If a structure on which an object rests is not capable of sustaining the weight, the object would break through.
(using squats for the sake of explanation) The lifter exerts an action force on the weight, which is in the opposite direction to gravity. The lifters shoulders also experience the reaction force (from the weight - otherwise, the weight would pass through the lifter or vice versa). The feet of the lift also experience a reaction force, which, combined with the shoulder-reaction force, is equal to the action force (Newton's Third Law). The feet also experience friction on the floor surface in opposite directions (which cancel out, so the lifter is stationary on the ground) :)
Thr body's mass would not change, but its weight would suddenly disappear.
(using squats for the sake of explanation) The lifter exerts an action force on the weight, which is in the opposite direction to gravity. The lifters shoulders also experience the reaction force (from the weight - otherwise, the weight would pass through the lifter or vice versa). The feet of the lift also experience a reaction force, which, combined with the shoulder-reaction force, is equal to the action force (Newton's Third Law). The feet also experience friction on the floor surface in opposite directions (which cancel out, so the lifter is stationary on the ground) :)
nothing would happen...
If a structure on which an object rests is not capable of sustaining the weight, the object would break through.
The details depend on what strange physics you make up, to replace the action-reaction force pairs.
The object would float in a given liquid.
(using squats for the sake of explanation) The lifter exerts an action force on the weight, which is in the opposite direction to gravity. The lifters shoulders also experience the reaction force (from the weight - otherwise, the weight would pass through the lifter or vice versa). The feet of the lift also experience a reaction force, which, combined with the shoulder-reaction force, is equal to the action force (Newton's Third Law). The feet also experience friction on the floor surface in opposite directions (which cancel out, so the lifter is stationary on the ground) :)
Thr body's mass would not change, but its weight would suddenly disappear.
(using squats for the sake of explanation) The lifter exerts an action force on the weight, which is in the opposite direction to gravity. The lifters shoulders also experience the reaction force (from the weight - otherwise, the weight would pass through the lifter or vice versa). The feet of the lift also experience a reaction force, which, combined with the shoulder-reaction force, is equal to the action force (Newton's Third Law). The feet also experience friction on the floor surface in opposite directions (which cancel out, so the lifter is stationary on the ground) :)
(using squats for the sake of explanation) The lifter exerts an action force on the weight, which is in the opposite direction to gravity. The lifters shoulders also experience the reaction force (from the weight - otherwise, the weight would pass through the lifter or vice versa). The feet of the lift also experience a reaction force, which, combined with the shoulder-reaction force, is equal to the action force (Newton's Third Law). The feet also experience friction on the floor surface in opposite directions (which cancel out, so the lifter is stationary on the ground) :)
That would be quite a radical change; many other laws of physics we are accustomed to would then no longer be valid, either.
Not all forces can cause an acceleration. In a perfect world, newton's law of F = M*A holds true, but in our world, it is easy to have a force without a resulting acceleration. If you were to poke a 100lb weight, would it move? probably not, because friction would cancel out the force on the weight. since you receive the third law reaction force, any acceleration that would hypothetically happen to the Earth is also canceled out
every thing would weigh half its normal weight
Nothing would happen to mass, but as weight is technically a force due to gravity, based on mass, the weight would be doubled, but again mass would remain the same.
The reaction would shift to balance the change