Can someone answer and explain it please. Here are the options:
- Moving downwards and decelerating.
- Moving downwards with a constant velocity.
- Moving upwards and decelerating.
- Moving upwards with a constant velocity.
closed as off-topic by John Rennie, Brandon Enright, jinawee, WetSavannaAnimal aka Rod Vance, Kyle Kanos Mar 18 '14 at 13:01
This question appears to be off-topic. The users who voted to close gave this specific reason:
I'm sure this has been answered before, but I can't see it in a search for "lift".
Anyhow the forum rules say we can't answer homework questions, but just discuss the concepts involved. You know from Newton's first law, F = ma, that if there is a force on a body it will accelerate, and it will accelerate in the direction of the force. So the question is, is there a net force on the person in the lift, and in which direction is the force?
The force downwards is just due to gravity, and the force upwards is the force being applied to the person by the floor of the lift. The scales are measuring the force being applied to the person by the floor of the lift. That should be all you need to answer the question.
This is in addition to JohnRennie's excellent answer.
Another trick that may help you visualise this is the psuedoforce. The psuedoforce enables you to imagine stuff in an accelerated frame easily. It's the "pressing" you feel in a plane during takeoff, etc. It's also the "pressing" you feel in an ascending lift.
I realised that I've already written a sizeable chunk on psuedoforces, so I'll just blatantly copy it from this answer of mine.:
Depending on the instructions of your professor, this problem can be solved two different ways: in the inertial and the non-intertial frame of reference.
In the inertial frame of reference, you are watching the lift, the scale and the person within it, from outside. You see that the all three are traveling with a constant acceleration up or down. Just two forces are acting on the person, the force of gravity and the force of the scale. You do the 2nd Newton law. Important: The scale is nothing but a simple force gauge showing the actual force with which it supports the person (of course this is transformed on the scale into the mass under the assumption of the gravity acceleration).
In the non-inertial frame of reference, you are standing in the lift, so the scale and the person seem to be at rest. However, now three forces are acting on the person, force of gravity, force of the scale and fictitious or pseudo force. Fictitious force equals mass of the person multiplied by the acceleration of the lift and is acting in the opposite direction of the actual acceleration of the lift. So in this case you do the 1st Newton law.
In both frames of references, you should get the same result. Moreover, when lift is not accelerating, both frames of reference are inertial and you should use even the same calculation procedure (the first one).
Let the lift in free fall from high (with the teacher inside to better understand the problem). The weigth is 0 (no units needed, because 0 is shared in the same way in all scales) because he is in free fall (and because he is not very very very very very tall I will assume the acceleration in his head equals the one at the feet level , uniform field approximation in a radial non uniform field ).
Now the lift smash in the concrete floor. The weight will increase under the decceleration, during the impact, in such a way that the bones eventually break.