Forces in elevator

[Martoman]

A box is on the floor of a lift accelerating upwards at 2.5. The mass of the box is 10kg. Find the normal, the reaction force of the floor on the box.

All that I want to know is why its R-10g instead of 10g-R isn't the weight force greater than the R force?

[Mao]

You are accelerating upwards kiddo, R > mg

[Martoman]

but when im in an elevator i feel heavyer no? My weight force is increased?

[Whatlol]

but when im in an elevator i feel heavyer no? My weight force is increased?

The normal force is the source of that apparent weight.

[Martoman]

What so its pushing up making me feel like something is pushing me down? Sorry this stuff doesn't come that naturally to me.

[Whatlol]

What so its pushing up making me feel like something is pushing me down? Sorry this stuff doesn't come that naturally to me.

Essentially, the floor is exerting a larger force than the gravitational force making you feel as though you are heavier than normal.

[moekamo]

you feel your weight through the normal force exerted by the floor, so to feel heavier there is a larger normal force, likewise if you are free falling, there is no normal force so you feel weightless.

[tram]

wait........... i'm really confused now.... like martoman, physics isn;t exactly my forte and i'm kidna struggin with the entire chapter

the way i see it, is ok, say you are just standing on the ground, there is your weight force and there is the normal reaction acting you you, both equal mg N.

When you are in a lift, becasue you are accelerating upwards, the upwards force must be less than the downwards force.

Thus either the downwards weigh force must decrease, or the normal reaction must increase. Because your weight force can't change (you can't change your mass), thus the normal reaction must decrese.....is that right??? :S

[Whatlol]

wait........... i'm really confused now.... like martoman, physics isn;t exactly my forte and i'm kidna struggin with the entire chapter

the way i see it, is ok, say you are just standing on the ground, there is your weight force and there is the normal reaction acting you you, both equal mg N.

When you are in a lift, becasue you are accelerating upwards, the upwards force must be less than the downwards force.

Thus either the downwards weigh force must decrease, or the normal reaction must increase. Because your weight force can't change (you can't change your mass), thus the normal reaction must decrese.....is that right??? :S

If the downwards force is greater than upwards force, the elevator would be going downwards ( so that is incorrect)
When the elevator accelerates, the net force on the box is upwards.

Basically this is how to look at it, the net force on the box is equal to ma which equals 25 N up
The net force must equal the normal force plus the gravitational force.

So Fn+ Fg = 25N
Lets take down to be negative, and up positive
.: Fn - 100 = 25
Fn = 25 +100

[fady_22]

wait........... i'm really confused now.... like martoman, physics isn;t exactly my forte and i'm kidna struggin with the entire chapter

the way i see it, is ok, say you are just standing on the ground, there is your weight force and there is the normal reaction acting you you, both equal mg N.

When you are in a lift, becasue you are accelerating upwards, the upwards force must be less than the downwards force.

Thus either the downwards weigh force must decrease, or the normal reaction must increase. Because your weight force can't change (you can't change your mass), thus the normal reaction must decrese.....is that right??? :S

Yup.
Just look at it this way; the direction where there is the most force acting (which is the direction of the net force) is the direction of acceleration.
So, if accelerating upwards, the normal force is greater, so that the net force is in the same direction as the acceleration, and if accelerating downward, then the normal force is less than the weight force, again, so that the net force is in the direction of the acceleration.

[tram]

When you are in a lift, becasue you are accelerating upwards, the upwards force must be less than the downwards force.

my bad, i MEANT to say the upawrds force must be MORE than the downads force >.<

SO from what gather from whatlol's anwser, we assume that the weight force has remained constant and that the normal focre has just increased..... is that right??

soz to be all noob >.<



EDIT: i just realsie that the quesiton has already been anwsered and the anwser is yes >.<

[98.40_for_sure]

Yup.
Just look at it this way; the direction where there is the most force acting (which is the direction of the net force) is the direction of acceleration.
So, if accelerating upwards, the normal force is greater, so that the net force is in the same direction as the acceleration, and if accelerating downward, then the normal force is less than the weight force, again, so that the net force is in the direction of the acceleration.

I believe that is incorrect, direction of net force is NOT equal to direction of acceleration.
A particle can be moving ->
but accelerating <-
thus the net force is ->, however it is just slowing down

[tram]

there's a difference between the direction of a force acting upon a particle and the particle's moving direction..........

I'm pretty sure the definion of f=ma would mean that the direction of net force would be in the same direction of the acceleration.....

again.....correct me if i wrong.....mechanics is not my strongest topic.....

[Whatlol]

umm if a particle is moving ---> that just indicates velocity, the net force will be in the direction of its acceleration.

[tram]

umm if a particle is moving ---> that just indicates velocity, the net force will be in the direction of its acceleration.

*phew* so i'm not completely clueless with mechanics....or if i am at least i'm not alone