Objectives Describe the deformation of a solid in response to a tension or compression.
What’s the point? How do solids react when deformed?
Structure of Solids Atoms and molecules connected by chemical bonds Considerable force needed to deform compressiontension
Structure of Solids Atoms are always “attracting each other when they are a little distance apart, but repelling upon being squeezed into one another” distance force 0 equil apart toward apartequil
Structure of Solids Atoms are always “attracting each other when they are a little distance apart, but repelling upon being squeezed into one another” distance force 0 equil apart toward
Force and Distance distance force 0 equil apart toward
Elasticity of Solids Small deformations are proportional to force small stretchlarger stretch Hooke’s Law: ut tensio, sic vis (as the pull, so the stretch) Robert Hooke, 1635–1703
Hooke’s Law Graph Force exerted by the spring Displacement from equilibrium position 0 0 slope < 0 forward backward forwardbackward
Hooke’s Law Formula F = force exerted by the spring k = spring constant; units: N/m; k > 0 x = displacement from equilibrium position negative sign: force opposes distortion F = –kx
Poll Question forward backward Displacement Spring’s Force What direction of force is needed to hold the object (against the spring) at its plotted displacement? A.Forward (right). B.Backward (left). C.No force (zero). D.Can’t tell. forwardbackward
Group Work A spring stretches 4 cm when a load of 10 N is suspended from it. How much will the combined springs stretch if another identical spring also supports the load as in a and b? Hint: what is the load on each spring? Another hint: draw force diagrams for each load. 0 N 10 N 0 N
Work to Deform a Spring To pull a distance x from equilibrium x kx force displacement kx 2 1 2 = slope = k area = W Work =F·x ; 1 2 F = kx Work = kx·x 1 2
Potential Energy of a Spring The potential energy of a stretched or compressed spring is equal to the work needed to stretch or compress it from its rest length. PE = 1/2 kx 2 The PE is positive for both positive and negative x.
Group Poll Question Two springs are gradually stretched to the same final tension. One spring is twice as stiff as the other: k 2 = 2k 1. Which spring has the most work done on it? A.The stiffer spring (k = 2k 1 ). B.The softer spring (k = k 1 ). C.Equal for both.
Reading for Next Time Vibrations Big ideas: –Interplay between Hooke’s force law and Newton’s laws of motion –New vocabulary that will also apply to waves