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!

Constant velocity and an object at rest

  1. Jan 26, 2014 #1
    1. The problem statement, all variables and given/known data

    True or False

    The stresses on an object traveling at constant velocity are the same as the stresses on an object.at rest.

    2. Relevant equations

    F = m*a

    3. The attempt at a solution

    For both an object at constant velocity and an object at rest the acceleration is zero therefore by newtons second law the force should be zero a stress is a force therefore I concluded that the stresses on an object at constant velocity and an object at rest is zero which is the same so I say the answer is true. Is this correct? any help would be appreciated.
     
  2. jcsd
  3. Jan 26, 2014 #2
    Can someone please validate the solution I proposed to this question.
     
  4. Jan 26, 2014 #3

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Tiven
    This question is a bit ambiguous. Newtons First Law states in effect that the net force acting on an object at rest or in motion at constant velocity is zero. But the problem asks about stresses, which are internal to the system and which are not zero, whether the object is at rest or in motion at constant velocity. If the object at constant velocity is subject to the same system of forces as at rest, the vector sum of which is zero, then the stresses, which are non-zero, are the same in both cases.
     
  5. Jan 26, 2014 #4

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Correct.

    However you might want to state what assumptions you have made in giving your answer. Clearly the forces/stresses acting on a car aren't the same when it's moving or stationary.
     
  6. Jan 26, 2014 #5
    There is no option to provide assumptions made on this question it is strictly true or false so
    Could u clarify the last sentence In which u said the forces on a car is.not.the same when it is at rest and when it is moving at.constant.velocity. and make reference to the term ' stresses' when doing so. This would aid in clearing up any ambiguity of.whether false is an option .
     
  7. Jan 26, 2014 #6
    Though it is ambiguous what would be ur response regarding this question?
     
  8. Jan 26, 2014 #7

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    I would answer true.

    One example would be air resistance (eg drag). That would create stresses in various things. Most obvious would be the windscreen or other surfaces at the front, but also in the tyres (because the car has to overcome drag by "pushing the ground backwards").

    So in answering true I would like to add an assumption that the object is moving in free space and not subject to, for example, air resistance.
     
  9. Jan 26, 2014 #8

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    My first inclination would be to answer TRUE, but the way the problem is worded, the answer is SOMETIMES. Which apparently is not an option. So I assume that the problem implies that the object is subject to the same set of forces in both cases.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted