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Why Oh Why (1 Viewer)
- Thread starter jebus0069
- Start date
Very cute.
OK the person who is lifting themselves up must exert a force down through their feet in order to lift upwards. However, the force of the person pushing down and the force of them pushing up will be equal (Newton's 3rd Law - for every action there is an equal and opposite reaction). There is another force pulling down on this person - gravity.
If Pull up = P, then Push down = P
Therefore, vector addition of forces (with up being the positive direction and down being negative):
Fresultant = P - P - mg
= - mg (or mg DOWN)
Therefore, resultant force is downwards in the order of the weight of the person, and so a person cannot pull themselves up by their own shoestrings.
:rofl:
OK the person who is lifting themselves up must exert a force down through their feet in order to lift upwards. However, the force of the person pushing down and the force of them pushing up will be equal (Newton's 3rd Law - for every action there is an equal and opposite reaction). There is another force pulling down on this person - gravity.
If Pull up = P, then Push down = P
Therefore, vector addition of forces (with up being the positive direction and down being negative):
Fresultant = P - P - mg
= - mg (or mg DOWN)
Therefore, resultant force is downwards in the order of the weight of the person, and so a person cannot pull themselves up by their own shoestrings.
:rofl:
Well, if we must add in Newton's first law, then someone cannot change their own motion and will continue in a state of no motion unless acted upon by a force.
So we have the forces on the person:
Pull up by shoe laces, push down on floor, gravity down and also the push up of the floor on the person which is equal and opposite to the remainder force down by the person. Hence there is zero net force and the person will not move.
F<sub>resultant</sub> = P - P - mg + F<sub>floor</sub> = 0 = ma
Therefore a = 0
So we have the forces on the person:
Pull up by shoe laces, push down on floor, gravity down and also the push up of the floor on the person which is equal and opposite to the remainder force down by the person. Hence there is zero net force and the person will not move.
F<sub>resultant</sub> = P - P - mg + F<sub>floor</sub> = 0 = ma
Therefore a = 0