Work Energy Theorem Question I cant do

In summary: Originally posted by Little Dump I got all of them except b nowis my formula for b right?cuz i get the wrong answer when i use itIt's hard to say without seeing your work and the actual numbers you're using. But make sure you're consistent with your units and that you're using the correct formula for work (W=F*d*cos(theta)). Also, don't forget to include the weight of the elevator in your calculation for the work done by gravity.
  • #1
Little Dump
19
0
I have no clue how to do b, c and especially d.

Thanks

The cable of the 1,800 kg elevator cab in Fig. 8-51 snaps when the cab is at rest at the first floor, where the cab bottom is a distance d = 3.9 m above a cushioning spring whose spring constant is k = 0.14 MN/m. A safety device clamps the cab against guide rails so that a constant frictional force of 3.6 kN opposes the cab's motion. (a) Find the speed of the cab just before it hits the spring. (b) Find the maximum distance x that the spring is compressed (the frictional force still acts during this compression). (c) Find the distance (above the point of maximum compression) that the cab will bounce back up the shaft. (d) Using conservation of energy, find the approximate total distance that the cab will move before coming to rest. (Assume that the frictional force on the cab is negligible when the cab is stationary.)
 
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  • #2
Since this is homework, I will only give some hints, you're on your own for the rest ...

Originally posted by Little Dump
(b) Find the maximum distance x that the spring is compressed (the frictional force still acts during this compression).

Write down a work equation. The spring does some work (change in potential energy), the frictional force does some work (force through a displacement), this total work can be related to the kinetic energy by the work-energy theorem. You can solve for the compression of the spring.


(c) Find the distance (above the point of maximum compression) that the cab will bounce back up the shaft.

You can solve it in a manner analogous to (b).


(d) Using conservation of energy, find the approximate total distance that the cab will move before coming to rest. (Assume that the frictional force on the cab is negligible when the cab is stationary.)

To come to rest, it has to shed all of its kinetic energy. The only way to dissipate that energy is through friction. If you assume all the energy loss (work) is frictional, you can solve for the total displacement.
 
  • #3
ok i got b and c

thanks

but I am still not getting d

any chance you can give me an equation or something?

more hints?

thanks a bunch
 
  • #4
Post what you've got, LD!

Forum rules, and all...
 
  • #5
for b i got the following


1800(9.81)(3.9) - 3600(3.9+x) = 1/2k(x)^2

im not quite sure if that is completely correct. I think their might need to be an x in the first term so its

1800(9.81)(3.9+x)

and for c

1/2k(x)^2 - 3600h = (1800)(9.81)h

not sure if that is right either but i think it is, just plug in x from b

and for d...

i still got nothing

someone help please!
 
  • #6
Originally posted by Little Dump

and for d...

i still got nothing
You have to have something! Even if you don't have any clue where to go, you should just write things down, even random things, to get you started. Try to think of how you can you what you know to find what you want. Specifically, try to find a formula that contains your unknown. If you know how to find all the other variables, then you're done.
For example, I randomly wrote down Wnet-nonconserv=[del]Emechanical. Of course, the nonconservative work is the work done by _____. The mechanical energy is given by _____.
 
  • #7
Ok, here's another hint: the fact that the elevator is bouncing up and down the whole time doesn't matter. The fact that there is a spring at all, doesn't matter. This is simply a problem concerning a body whose kinetic energy is continually being dissipated by a force of constant magnitude; whether the elevator is bouncing, spinning around, or doing a little dance doesn't change this fact. Think about how you'd find the total distance moved until it comes to rest if it were, say, a block sliding across a table.
 
  • #8
I got all of them except b now

is my formula for b right?

cuz i get the wrong answer
 

FAQ: Work Energy Theorem Question I cant do

1. What is the Work Energy Theorem?

The Work Energy Theorem is a fundamental principle in physics that states that the work done on an object is equal to the change in its kinetic energy. It is also known as the Kinetic Energy Theorem or the Work Kinetic Energy Theorem.

2. How is the Work Energy Theorem expressed mathematically?

The Work Energy Theorem can be expressed mathematically as W = ΔKE = ½mv2 - ½mu2, where W is the work done, ΔKE is the change in kinetic energy, m is the mass of the object, and u and v are the initial and final velocities, respectively.

3. What are the assumptions made in the Work Energy Theorem?

The Work Energy Theorem assumes that there are no external forces acting on the object and that the object is moving in a straight line with a constant acceleration. It also assumes that the mass of the object remains constant and that there is no energy lost due to friction or other dissipative forces.

4. How is the Work Energy Theorem used in real-world applications?

The Work Energy Theorem is used in various real-world applications, such as calculating the amount of work needed to move an object, determining the speed of an object after a certain distance, and understanding the energy transfer in different systems, such as a roller coaster or a swinging pendulum.

5. Can the Work Energy Theorem be applied to all types of motion?

No, the Work Energy Theorem can only be applied to systems where work is being done by a constant force. It cannot be applied to systems with changing forces, such as objects moving in circular motion or objects experiencing non-constant acceleration.

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