Gravity

[Primarygun]

What is the net force for a man (1kg)in a lift falling with a speed of 12m/s2?

[MATH] \Sigma F_y=F_n-mg [/MATH]???

Is the gravitational force considered as external force?

[Primarygun]

Maybe I change my question.

Is gravitional force considered as external force when a man in a lift falling down?

[Ben_Phys618]

Is that meant to be a lift falling with acceleration of 12 ms/s2? coz that is decidedly more dimensionally accurate.

 

Gravity acts downwards with an acceleration of 9.8 m/s2. Your lift is accelerating in the same direction at 12m/s2, so the man accelerates upward at 2.2m/s2, and if he has mass of 1kg, the net force he feels is 2.2 newtons vertically upwards

[Leison]

Maybe I change my question.

Is gravitional force considered as external force when a man in a lift falling down?

 

i think gravity is always external.

[Ben_Phys618]

Actually I think I should amend that answer. Its been too long since doing those nice small problems, that I'd forgotten the naunces of them. The man still accelerates upward at 2.2 m/s2 (if your frame of reference is the lift) but he feels no force from it. The lift would fall out from under his feet, and he woyuld begin falling, but only at 9.8 m/s2 (which means he wil eventually hit his head on the ceiling of the lift - that would be amusing, somebody should perform this as an experiment lol) so the only force he feels is F = mg => F = 1 x 9.8 => F = 9.8 Newtons vertically downwards.

 

And no, I don't think gravity is always an external force. It depends what you consider as your system. If you take your system as being simply a man (100kg) standing on the Earth, with the Earth as your frame of reference. The two forces acting are gravity downwards, balanced by the normal reaction force from the earth. The system is at rest. If the man jumps at an initial velocity of 10 ms-1, his momentum will be 1000 sN upwards, and the earth moves downwards at a velocity of (1000/[mass of earth]) which is a very small number, but it stil moves. Momentum is conserved, whichb means for that system, Gravity is acting as an internal force, doesn't it?

[Primarygun]

Actually I think I should amend that answer. Its been too long since doing those nice small problems' date=' that I'd forgotten the naunces of them. The man still accelerates upward at 2.2 m/s2 (if your frame of reference is the lift) but he feels no force from it. The lift would fall out from under his feet, and he woyuld begin falling, but only at 9.8 m/s2 (which means he wil eventually hit his head on the ceiling of the lift - that would be amusing, somebody should perform this as an experiment lol) so the only force he feels is F = mg => F = 1 x 9.8 => F = 9.8 Newtons vertically downwards.

 

And no, I don't think gravity is always an external force. It depends what you consider as your system. If you take your system as being simply a man (100kg) standing on the Earth, with the Earth as your frame of reference. The two forces acting are gravity downwards, balanced by the normal reaction force from the earth. The system is at rest. If the man jumps at an initial velocity of 10 ms-1, his momentum will be 1000 sN upwards, and the earth moves downwards at a velocity of (1000/[mass of earth']) which is a very small number, but it stil moves. Momentum is conserved, whichb means for that system, Gravity is acting as an internal force, doesn't it?

 

I think the equation of mine shows the [MATH] \Sigma F_y [/MATH] is only concerning about the forces created by man-kind.

The gravitional force is considered as net force but not in this man-kind.

May you give me the equation that you calculate the apparent weight for the man in the lift?

[Ben_Phys618]

well, weight is a measure of the force acting on a mass in a gravitational field, and is equal to mass x gravitational acceleration, or in your case, 1 kg. x 9.8 m/s2. = 9.8 newtons

 

If you are only concerned with his apparent weight relative to the lift, and therefore why he appears to rise off the floor of the lift, then replace gravitational acceleration with just acceleration, which in relation to the lift was 2.2 m/s2 upwards, therefore his weight is -2.2 newtons