Lec 10: Solids Molecules and Molecular Bonds Solids: Density Springs: Hooke’s Law Ropes and Rods: Elasticity

Molecular Bonds Ionic Bonds: NaCl Covalent Bonds:

Van der Waals Bonds Van der Waals Interaction between the electric dipole of atoms or molecules. DARPA (2014) Geckskin: 100-kg person climbed 8-m glass wall

Hydrogen Bonds Ionic Bond Polar Covalent Covalent Bond Polar Covalent Neither completely ionic, nor completely covalent.

Hydrogen Bonds Ionic Bond Polar Covalent Covalent Bond

Density Amount of mass per unit volume of a material. Unit of density is kg/m3 or gm/cm3. Example: Density of water is 1000 kg/m3, or 1 g/cm3. mass volume Density =

Springs: Hooke’s Law Question: If you wish to triple the stretching of a spring, how much more force do you need to apply?

Elastic Properties of Materials Question: If you double the thickness how much force do you need to stretch it the same? Question: If you double the length how much force do you need to stretch it the same?

Puzzles P1. If you apply a force of 100 N to a rope and you stretch it by 1 cm, by how much would it be stretched by a 200 N force (everything else being equal) P2. A spring with a stiffness (spring constant) of 500 N/m needs to be stretched by 1 cm. How much force do you need to apply to it? P3. A rope pulled by a force is stretched by 1 mm. By how much will it be stretched by the same force if the rope were 4x longer and 2x wider? P4. A 10-cm-long spring extends to 12 cm when a 1-kg load is suspended from it. What would be its length if a 3-kg load were suspended from it? Ans. 16 cm