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!

Pulley Problem: Speed after falling a known distance

  1. Nov 11, 2013 #1
    A 1.2·kg block is tied to a string that is wrapped around the rim of a pulley of radius 7·cm. The block is released from rest.

    Assuming the pulley is a uniform disk with a mass of 0.37·kg, find the speed of the block after it has fallen through a height of 0.43·m.




    2. F=mrω^2 and possibly F=mg and v=r*omega



    3. I wasn't sure how to do this problem at all. I looked around online for help and got a little bit but I'm still unsure. I used F=mg to determine the F then used that to determine ω^2. Is that correct? Obviously there would be more to the problem but this is as far as I got and I'm not even sure if it's correct.

    So I got F=11.76 by multiplying 1.2kg * 9.8m/s^2 which I then put in the equation for 11.76=(.37kg)(.07m)w^2 and then got 21.3 for omega. This seems high and is probably due to the conversion from CM to M which I'm unsure is correct. Assuming it is I moved on and put that omega value 21.3 into v=wr for v=21.3*.07 and got 1.49 which is incorrect.
     
    Last edited: Nov 11, 2013
  2. jcsd
  3. Nov 11, 2013 #2

    adjacent

    User Avatar
    Gold Member

    That depends on the friction of the pulley.
     
  4. Nov 11, 2013 #3
    I believe it's assumed frictionless because friction isn't mentioned
     
  5. Nov 11, 2013 #4

    adjacent

    User Avatar
    Gold Member

    Conversion from meters to cm is wrong.(thats all I know) 7/1000 is 0.007m
     
  6. Nov 11, 2013 #5
    Isn't a centimeter 1/100th of meter?
     
  7. Nov 11, 2013 #6

    adjacent

    User Avatar
    Gold Member

    Ah.Sorry,I was mistaken.You are right.
     
  8. Nov 12, 2013 #7

    adjacent

    User Avatar
    Gold Member

    It seems you didn't read this part(find the speed of the block after it has fallen through a height of 0.43·m. Hope it helps.
     
  9. Nov 12, 2013 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You need to consider the pulley and the mass separately. What they have in common are the tension in the string and the acceleration. Draw a FBD for each and write out the equations (ƩF=ma etc.)
     
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: Pulley Problem: Speed after falling a known distance
  1. Distance/speed problem (Replies: 2)

Loading...