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!

Newton's laws questions

  1. Jan 15, 2004 #1
    Right well there is this question which I am stuck on and could do with some help with! Thanks!

    A runner decelerates as he runs into a windy exposed area, from a speed of 4 m/s to 2 m/s in 8 seconds. His mass is 60 kg. Assuming he also experiences a frictional force of 3 N, find the size of the force exerted by the wind on the runner.

    I have found the resultant force to be 15N but to calculate the wind I'm sure it cannot just be 12N because the force pushing the man forwards has to be taken into account as well. I'm all rather confused about it!

    There is also this question which I am a bit stuck with:

    A stone of mass 50g is dropped into some liquid and falls vertically through it with an acceleration of 5.8 m/s^2. Find the force of resistance acting on the stone.

    Many thanks!
  2. jcsd
  3. Jan 15, 2004 #2


    User Avatar
    Science Advisor

    What force "pushing him forward"?

    The one ambiguous point in this problem is the statement
    "Assuming he also experiences a frictional force of 3 Newtons" Since there is also a reference to the wind force, I take that that is the friction of the ground on the runner. It is, I agree, ambiguous whether this force is positive or negative (whether the runner is trying to accelerat or not). Since the runner slows down, I think it is most reasonable to assert that this is also negative.

    Slowing from 4 m/s to 2 m/s in 8 seconds means an acceleration of
    -2/8= -0.25 m/s2. F= ma so F= (60)(-0.25)= -15 N as you calculated. Assuming that the friction force is negative, the force of the wind is -12N.

    For the second problem, a stone of mass 50g (so 0.05 kg) would have acceleration -9.8 if it were falling freely. Since it is falling with acceleration -5.8 m/s2, there must be a retarding force of (0.05)(-5.8-(-9.8))= 0.05(4.1)= 0.205 N. (positive since it is upward.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Newton's laws questions