Lift Questions

[azn_kiwi91]

K i came across this in some exam paper

A child travelling in a lift that is accelerating downwards at 2.4m/s2 has an apparent weight of 30 kg wt. When the lift is stationary, the child's weight in kg would be closest to....

I wasn't too sure what it meant by apparent weight. Anyone care to explain?

[vce08]

That would be the normal reaction force.
Thats why you do physics noob.

[azn_kiwi91]

That would be the normal reaction force.
Thats why you do physics noob.

So apparent weight is the normal reaction force? But I thought weight was the force due to gravity. The reaction force is going up.

[shinny]

It's the 'apparent' weight because that's the force they're actually exerting on the floor since some of their real weight is lost due to the downwards acceleration. Then using action/reaction, the force the kid exerts on the floor is equal to the normal reaction force.

[Captain]

apparent weight of 30 kg

Weight is Newtons, not kg's.

[shinny]

Isn't that why he said kg wt?

[Captain]

Isn't that why he said kg wt?

What the hell is "kg wt" ?

[Collin Li]

1 kg wt = 9.8 Newtons

[shinny]

1 kg-wt=1g Newtons=9.8 Newtons

It's just a more convenient unit to use at times to avoid having to write g after every weight force you give. I've seen it show up a few times in spesh papers.

[Pandemonium]

i think it's a good habit to use newtons instead of using kgwt notation because you can't use kgwt in dynamics. unfortunately.

[Captain]

Good to know, never seen that notation before.