AQA A Level Physics 3.4 Mechanics and Materials Exam Questions

About AQA A Level Physics 3.4 Mechanics and Materials

Mechanics and Materials is AQA section 3.4 and is examined on Paper 1. Section 3.4.1 covers scalars and vectors, moments and equilibrium, motion along a straight line, projectile motion, Newton's laws, momentum and impulse, work, energy, power and conservation of energy. Section 3.4.2 covers bulk properties of solids: density, Hooke's law, elastic and plastic behaviour, stress, strain, the Young modulus and energy stored in a stretched material. Mechanics is used throughout the course, especially in fields, SHM and the Engineering Physics option. Examiners like resolving forces, SUVAT and projectile calculations, momentum conservation and the Young modulus required practical.

What you need to know

• 3.4.1 Vectors and moments Resolve forces into perpendicular components and combine vectors by scale drawing or calculation. Moment = force x perpendicular distance; in equilibrium the principle of moments applies and the resultant force is zero. Know couples and centre of mass.
• 3.4.1 Motion and projectiles Use the SUVAT equations for uniform acceleration. Gradient of displacement-time gives velocity; gradient of velocity-time gives acceleration and its area gives displacement. In projectiles treat horizontal (constant velocity) and vertical (acceleration g) motion independently; know the qualitative effect of air resistance.
• 3.4.1 Newton's laws F = ma for constant mass, and the third law pairs act on different objects and are the same type of force. Terminal speed is reached when drag equals the driving force or weight.
• 3.4.1 Momentum Momentum p = mv is conserved in all collisions if no external force acts. F = change in momentum / time, and impulse is the area under a force-time graph. Elastic collisions conserve kinetic energy, inelastic ones do not.
• 3.4.1 Work, energy and power Work = F s cos theta, P = W/t = Fv, efficiency = useful output / input. Use mg change in h and 1/2 mv^2 for energy conservation problems.
• 3.4.2 Materials Density rho = m/V. Hooke's law F = k delta L up to the limit of proportionality; energy stored = 1/2 F delta L, the area under a force-extension graph. Stress = F/A, strain = delta L/L, Young modulus = stress/strain. Know brittle, ductile, plastic behaviour and loading and unloading curves.

Common mistakes in 3.4 Mechanics and Materials

• Using the full speed in both directions of a projectile. Split the initial velocity into u cos theta horizontally and u sin theta vertically.
• Saying the forces in a Newton's third law pair cancel. They act on different bodies so they cannot cancel.
• Using diameter instead of radius when calculating cross-sectional area for stress. A = pi d^2/4, and convert mm to m first.
• Confusing limit of proportionality with elastic limit. Beyond the limit of proportionality the graph curves, but the material may still return to its original length until the elastic limit.
• Assuming kinetic energy is conserved in every collision. Only momentum is always conserved; kinetic energy is conserved only in elastic collisions.

Try three 3.4 Mechanics and Materials exam-style questions