1. Limited time only! Sign up for a free 30min personal 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!

Buoyant Force of a block in water

  1. Apr 5, 2015 #1
    A flask of water rests on a scale that reads100 N. Then, a small block of unknown material is held completely submerged in the water. The block does not touch any part of the flask, and the person holding the block will not tell you whether the block is being pulled up (keeping it from falling further) or pushed down (keeping it from bobbing back up).

    What is the measurement on the scale now?

    a) Greater than 100 N
    b) Less than 100 N
    c) Equal to 100 N
    d) We cannot tell without more information

    The correct answer is a. Though I don't know how they reached this conclusion
    Could someone please help me with this? :)
  2. jcsd
  3. Apr 5, 2015 #2


    User Avatar
    2017 Award

    Staff: Mentor

    What is the force the person has to apply for different cases (like no mass, something with the same density of water, something heavier)? The force of the scale plus the force by the person together have to keep the whole system balanced, so knowing one allows to find the other one.
    You'll see that all those cases lead to answer (a).
  4. Apr 5, 2015 #3
    To expand on what mfb said, draw a free body diagram for each of the three cases that mfb has identified. The importance of drawing free body diagrams cannot be overemphasized.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted