Page 122 - Physics Coursebook 2015 (A level)

Cambridge International AS Level Physics
Summary
■■ Density is defined as the mass per unit volume of a substance:
density = mass volume
■■ Pressure is defined as the normal force acting per unit cross-sectional area:
pressure = force area
■■ Pressure in a fluid increases with depth: p=ρgh
■■ Hooke’s law states that the extension of a material is directly proportional to the applied force. For a spring or a wire, F = kx, where k is the force constant. The force constant has units of N m−1.
■■ Stress is defined as:
■■ Strain is defined as: strain = extension
original length or ε=Lx
■■ To describe the behaviour of a material under tensile and compressive forces, we have to draw a
graph of stress against strain. The gradient of the initial linear section of the graph is equal to the Young modulus. The Young modulus is an indication of the stiffness of the material.
stress =
σ = AF
force cross-sectional area
■■ The area under a force–extension graph is equal to the work done by the force.
■■ For a spring or a wire obeying Hooke’s law, the elastic potential energy E is given by:
E = 12 k x F x = 12 k x 2
■■ The Young modulus E is given by: stress σ
E = strain = ε
■■ The unit of the Young modulus is pascal (Pa) or N m−2.
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