Lift and screw impact velocity

[robax25]

Homework Statement

Consider the elevator and screw in Example 2-17. Assume the velocity of
the elevator is 16 mls upward when the screw separates from the ceiling.
(a) How far does the elevator rise while the screw is falling? How far does
the screw fall? . I have only problem second part of the question. Lift height 3m.

Homework Equations

t=0.659s a (lift)=4m/s²,g(screw)=9.81m/s²
x=volt+0.5at²

The Attempt at a Solution

X lift=11.41m

 

[haruspex]

You don't state the internal height of the lift cage.
I see no attempt at the second part.

 

[berkeman]

Homework Statement

Consider the elevator and screw in Example 2-17. Assume the velocity of
the elevator is 16 mls upward when the screw separates from the ceiling.
(a) How far does the elevator rise while the screw is falling? How far does
the screw fall? . I have only problem second part of the question.

Homework Equations

t=0.659s a (lift)=4m/s²,g(screw)=9.81m/s²
x=volt+0.5at²

The Attempt at a Solution

X lift=11.41m

Is there a figure that is supposed to go with this question (or Example 2-17)? Can you use the Upload button to attach it please?

 

[robax25]

I am done almost.However, I have a little bit confusion. It is relative velocity question. If I consider that displacement of screw and lift differently and I find displacement of the screw -2.13m and I do deduction between initial height due to velocity and I get right answer. Book is right in one sense if I consider that the total displacement of the lift 11.41 and I just do deduction from the height of lift. I get the answer. My question why do I not consider them separately?

 

[haruspex]

I am done almost.However, I have a little bit confusion. It is relative velocity question. If I consider that displacement of screw and lift differently and I find displacement of the screw -2.13m and I do deduction between initial height due to velocity and I get right answer. Book is right in one sense if I consider that the total displacement of the lift 11.41 and I just do deduction from the height of lift. I get the answer. My question why do I not consider them separately?

Please state the whole problem and show all your working.

 

[robax25]

Actually I have solved the problem using sketch.

 

[haruspex]

Actually I have solved the problem using sketch.

Does that mean you are no longer confused?

 

[robax25]

yes, because it is only reference frame and displacement problem. If you consider that lift and screw in your Initial position where screw velocity and lift velocity are same I mean vo=0; but displacement of floor x=0 and displacement of screw =3m. displacement changes due to velocity for screw and floor.However, screw is falling when lift is accelerating. They are impacting when the displacement of floor is 11.41m but displacement of screw is 8.41m. It is really hard to understand what is happening there.

 

[haruspex]

yes, because it is only reference frame and displacement problem. If you consider that lift and screw in your Initial position where screw velocity and lift velocity are same I mean vo=0; but displacement of floor x=0 and displacement of screw =3m. displacement changes due to velocity for screw and floor.However, screw is falling when lift is accelerating. They are impacting when the displacement of floor is 11.41m but displacement of screw is 8.41m. It is really hard to understand what is happening there.

You have to decide whether your reference frame continues up at the initial velocity of the lift or accelerates with the lift.
If the latter then you have to introduce a virtual force to account for the fact that the lift is not accelerating (in this frame) despite the imbalanced forces on it.

 

[robax25]

If my reference frame Initial position of the lift, then, it is really easy to solve.I mean floor and also Initial Postion of the screw which is located 3m high from the floor.Your position is changing due to velocity and acceleration. Floor and screw have same velocity once and screw is falling down when the lift is accelerating. It happens also in real life. I mean when you feel jark, or sudden change of motion, you feel negative force. That's way we are going foward and backward when we are standing on a bus.

 

[haruspex]

I mean when you feel jark, or sudden change of motion, you feel negative force.

"Change of motion" is a bit vague. Jerk is the rate of change of acceleration. When a bus comes to a stop, the acceleration suddenly changes from small, negative, to zero. This abruptly changes your force balance, and is why you tend to fall over if standing and bracing yourself against the negative acceleration.

I'm still unclear whether you have a remaining issue. I asked if you were no longer confused. You replied "yes" but finished with

It is really hard to understand what is happening there.

If I interpret your answers correctly, you are confident working with the initial position of the lift as the reference frame and get the right answer. In what frame, if any, are you confused? Initial velocity of lift? Acceleration of lift?

 

[robax25]

You are right and understand my Situation. In university exam, they give very hard and tough question. I am currently studying mechanical Engineering in Germany. Most of the question, they ask very difficult. It sometimes happens that I can not even imagine what they ask such kind of non-sense question.There is no limitation asking question. you can only pass if you understand the real phenomena of the problem. that's why I am finishing my word using the sentence.