Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Newton's law

  1. Sep 20, 2006 #1
    Hi I have a problem and a doubt regarding newton's laws of motion.
    Consider three blocks of 5,2,5kg respectively placed one over the other and dropped from the top of a tower. (the 5 kg if at the top, 2kg in the middle and the 5kg at the bottom).

    i) Find the acceleration of each block.

    My calculations give me 'g'. Wont the acceleration be same for all three blocks?

    ii) Find the contact forces between each block.

    I got 0 as the answer for all the contact forces. I am not sure whether it is right though.

    2) The second part of the question is: Consider the same details. Now, A 10N force is applied (directed upwards) on the 5kg block(which is at the bottom) from downwards.

    i) Find 'a' between the blocks.
    Here is my work:
    5g - f2 = 5a
    2g + f2 - f1 = 2a
    5g + f1 - 10 = 5a

    solving i got a = 55/6

    ii) find the contact forces between the block.
    f2 = 25/6 ; f1=575/6

    Can someone please verify what I have done?Thanks!
  2. jcsd
  3. Sep 21, 2006 #2
    one more doubt. if I use pseudo forces to calculate the acceleration of something, am I calculating its acceleration with respect to the non-intertial frame or the acceleration with respect to the ground frame(stationary frame)? :confused:

    Please help
  4. Sep 21, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    You are correct. All three blocks accelerate with gravity (g), and since all are accelerating at the same rate, there is no net force between them. If however, there was air resistance on the lead block then the other two would be pushing against the lead block, and the last block would push against the middle block.

    I'll get back to you later on the other problem. :smile:
  5. Sep 22, 2006 #4
    ok thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook