# Homework Help: An elevator Physics Accelerating Frame

1. Jun 10, 2012

### kerimabdullah

An elevator slows down with a 6 m/s2 acceleration and an object m=3 kg is freely released as shown in figure There is no friction between object an semispherical shape.
The object reaches L point what is the force which the object exerts L point?
A)120 N
B)240 N
C)360 N
D)480 N
E)600 N

For elevator
a=6 m/s2
V2=Vo2-2ah
V=Vo-at
h=(a/2)t2

For object
inside the elevator gravitational constant g'=16m/s2 because
N-mg=ma
N=m(g+a)=mg'

I cant solve I need help

#### Attached Files:

• ###### asansoring .png
File size:
14.6 KB
Views:
144
2. Jun 10, 2012

### tiny-tim

welcome to pf!

hi kerimabdullah! welcome to pf!

(and if the elevator is going up but slowing down, isn't it 4 not 16?)

3. Jun 11, 2012

### kerimabdullah

solution how to? g'=16m/s2 is correct
N-mg=ma+(m/r)Vfinal^2
Energy equation? m(g+a)Integral(rdr)? Because The height is changeabele for every second This is Turkish Physics Olympiad Question. I couldnt solve
Or (1/2)mV^2 +m(g+a)Integral(rdr)= (1/2)mVfinal^2

4. Jun 11, 2012

### tiny-tim

5. Jun 11, 2012

### Staff: Mentor

This is a very interesting and challenging problem. In addition, as Tiny Tim pointed out, there is some ambiguity as to whether the elevator is moving upward initially, and slowing down, or whether the elevator is moving downward initially, and slowing down. So I guess it's best to work the problem both ways, and then to see whether either solution matches the choices offered.

Let's first concentrate on the motion of the elevator. Let an arbitrary origin be located on the centerline of the elevator shaft, at some point well below the elevator's starting location, and let yo be the y coordinate of the top of the elevator at time t = 0. If the elevator is moving upward at t = 0 with an upward velocity of vo, and is slowing down with a downward acceleration of 6m/s2, what is its vertical coordinate y at time t? If the elevator is moving downward at t = 0 with a downward velocity of vo, and is slowing down with an upward acceleration of 6 m/s2, what is its vertical coordinate y at time t? If iy represents a unit vector pointing in the upward y-direction, express the position vector from the arbitrary origin to the top center of the elevator in terms of y and iy for each of these two cases.

Chet

6. Jun 11, 2012

### kerimabdullah

7. Jun 11, 2012

### kerimabdullah

Lets suppose the elevator moves with constant velocity downward and a man press the button to stop elevator then elevator makes slowing down motion downward with a=6m/s2 and angle and height for the object changes with the time.

8. Jun 11, 2012

### azizlwl

If accelerating upward 6m/s2

v2=2aL
a=6+10
ac=32m/s2

F=m(32+6+10)=3(48)N
None matches.

9. Jun 11, 2012

### kerimabdullah

ELevator slows down ward with a constant acceleration of 6 m/s2 or (a = 6 N/kg )

10. Jun 11, 2012

### kerimabdullah

Turkish Physics Olympic Questions (The original question 13) I am waiting solutions since 2 months but they didnt publish. I cant sleep nights because I didnt solve problem 13
www. fen.bilkent.edu.tr /~eurasia_pacific/ITAP_FOO/exams/xcj1p8id2cvtzu75ztrrk383b9hffup2wzucy3gq529vm7rbr8t12su3r29wnpp9eac33jkzbklnh2py19qurbn83mvg2bg6bj05.pdf

11. Jun 12, 2012

### Staff: Mentor

If the mass were 30 kg, rather than 3 kg, then one of the answers would make sense. If the elevator were initially moving upward and decelerating at a rate of 6 m/s2 and the mass were 30 kg, then the contact force would be 360 N. According to my analysis of this problem, the contact force on the mass at point L is given by:

N = 3m (g + a)

where a is the upward acceleration of the elevator. If a = -6 m/s2, and m = 30 kg, then N = 360 N.

The main part of this problem is deriving the above equation. See if you can confirm my result.

Chet

Last edited: Jun 12, 2012