1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A worker lifts a 20.0-kg bucket of concrete from the ground

  1. Nov 29, 2014 #1
    1. The problem statement, all variables and given/known data
    1. A worker lifts a 20.0-kg bucket of concrete from the ground up to the top of a 20.0-m-tall building. The bucket is initially at rest, but is traveling at 4.0 m/s when it reaches the top of the building. What is the minimum amount of work that the worker did in lifting the bucket?

    2. Relevant equations
    F= ma
    W= Fd
    F=KE at the tope of the building

    3. The attempt at a solution
    Since bucket has KE at the top of the building, would I be finding the KE using KE=(1/2)mv^2 equation and then plugging that final number into my W=Fd equation where d would be 20.0 m?

    Help please!
     
  2. jcsd
  3. Nov 29, 2014 #2

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    How were you "plugging it in?"
     
  4. Nov 29, 2014 #3
    I solved for F in my equation of W=Fd by using the kinetic energy formula. I did (1/2)*(20)*(4^2) and got 160J. I plugged that into the W=Fd as my F. So then I solved doing F=160J and d=20m but I got 3200 N*m instead and that's not the right answer. The answer is 4.08kN*m apparently
     
  5. Nov 29, 2014 #4

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    How much work do you do getting the bucket to the top of the building?
     
  6. Nov 29, 2014 #5
    Would that be the 3200?
     
  7. Nov 29, 2014 #6
    I'm confused with that question actually
     
  8. Nov 30, 2014 #7

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Let's start at the beginning. What is the minimum force you must apply to lift the bucket?
     
  9. Nov 30, 2014 #8

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    You are overlooking the change in potential energy of the bucket between the ground and the top of the building.

    You also don't know how the bucket acquired the velocity of 4 m/s (whether it was constant the whole time the bucket was rising or if it was in the last second before the bucket reached the top of the building. The best you can do is calculate the KE of the bucket and add it to the change in potential energy.
     
  10. Nov 30, 2014 #9

    So is this what you're saying I should do?

    Find the PE=mgh=(20)(9.8)(20)=784 J
    And find the KE=(1/2)(20kg)(4.0^2)=3200 J
    And then add PE+KE so that my final answer is 784+3200=3894?
    Because the final answer is supposed to be 4.08kN*m
     
  11. Nov 30, 2014 #10
    Excuse me, KE at top of a Building? isnt that suppose to be Potential Energy??
     
  12. Nov 30, 2014 #11
    So what would I do to get my final answer?

    Would I solve for my PE with the given numbers from the problem?

    I'm feel like I'm getting nowhere
     
  13. Nov 30, 2014 #12

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You have the method. Now, you need to double check your arithmetic. Once you've done that, you can point at the source of the 4.08 kNm final answer and laugh loudly.
     
  14. Nov 30, 2014 #13
    I can truly be such an idiot sometimes. Thanks I think I need some sleep
     
  15. Nov 30, 2014 #14

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    You might want to check your arithmetic on the calculation of PE again. IDK how you can multiply 400 * 20 and come out with 784.
    Ditto the calculation of the KE. IDK how you get 3200 J for this either.

    And what do you think a Joule is equal to? You should also check its definition as well.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: A worker lifts a 20.0-kg bucket of concrete from the ground
Loading...