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!

What is strain?

  1. Mar 13, 2009 #1
    In class we discussed a force exerted on a soft ball against a rigid surface. We were told that the strain (of the order: we are doing dimensional analysis) h/a where h is the height difference from the undeformed ball, and a is the radius of the circle intersection of the ball with the surface. However, on wikipedia it says that strain is the "measure of how much a given displacement differs locally from a rigid-body displacement." Wouldn't this be h/R (Radius of the undeformed ball.) Can someone give a more clear definition of strain?
    Thank you in advance.
  2. jcsd
  3. Mar 13, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Be careful using Wiki ....its explanations tend to be quite complex. But the ball/rigid surface example of strain is also not a good start point for understanding strain, because it involves 3 dimensional deformations. Instead, consider strain as the amount of deformation per unit length. Take a rubber band about 10 cm long and stretch it 2 cm so that it is now 12 cm long. The deformation, or stretch, is 2 cm, and the strain is 2 cm/10 cm = 0.2, a dimensionless quantity.
  4. Mar 13, 2009 #3
    how does one go about finding the strain for more complex objects? My professor pretty much just told us it was h/a with little explanation.

    For example in 2 dimensions compressing a soft triangular sheet against a rigid surface (on its point). Isn't the strain again just Δheight/height? Why is it different in different shapes?
    For 2D and 3D is it ΔArea/Area and ΔV/V?
    Last edited: Mar 13, 2009
  5. Mar 14, 2009 #4
    The triangle is more complex than a rubber band which has strain in one direction only (of course in the real world it is a bit more complicated than that). Not only does it get shorter, but it will also get wider. So you have two different strains in two different dimensions.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook