OxfordAQA International A Level Physics 3.2 Mechanics and Materials Exam Questions

About OxfordAQA International A Level Physics 3.2 Mechanics and Materials

Mechanics and materials is the largest section of Unit 1 (PH01) on the OxfordAQA International AS course. It starts with scalars, vectors and moments, moves through motion in a straight line, projectiles, Newton's laws and momentum, then covers work, energy and power and finishes with the bulk properties of solids and the Young modulus. This content is the foundation for circular motion, simple harmonic motion and fields in the A2 year. Examiners regularly set multi-step calculations using the uniform acceleration equations, conservation of momentum problems, interpretation of velocity-time and force-extension graphs, and questions on the required practicals for measuring g and the Young modulus.

What you need to know

• Vectors and moments Resolve a force F into F cos theta and F sin theta. For equilibrium the resultant force is zero (a closed vector triangle) and the sum of clockwise moments equals the sum of anticlockwise moments. Moment = force x perpendicular distance from the pivot; a couple is a pair of equal, opposite, parallel forces with torque = force x perpendicular separation.
• Motion along a straight line Velocity is the gradient of a displacement-time graph; acceleration is the gradient of a velocity-time graph and displacement is the area under it. Use the suvat equations, v = u + at, s = (u + v)t / 2, s = ut + 1/2 at^2 and v^2 = u^2 + 2as, only for uniform acceleration.
• Projectiles and terminal speed Horizontal and vertical motion are independent: horizontal velocity is constant if air resistance is ignored, while vertical motion has acceleration g. Drag increases with speed, so a falling object reaches terminal speed when drag plus upthrust equals weight.
• Newton's laws and momentum F = ma for constant mass; more generally F = rate of change of momentum, F = delta(mv) / delta t. Impulse = F delta t = change in momentum = area under a force-time graph. Momentum is conserved in all collisions; kinetic energy is conserved only in elastic collisions.
• Work, energy and power W = Fs cos theta, P = W / t = Fv, efficiency = useful output / total input. Use Ep = mg delta h and Ek = 1/2 mv^2, remembering energy lost doing work against resistive forces.
• Materials Density rho = m / V. Hooke's law F = k delta L up to the limit of proportionality. Tensile stress = F / A, tensile strain = delta L / L, Young modulus E = stress / strain. Elastic strain energy = 1/2 F delta L = area under the force-extension graph. Know the difference between elastic, plastic and brittle behaviour, and the required practical for finding E of a wire.

Common mistakes in 3.2 Mechanics and Materials

• Using the suvat equations when acceleration is not constant. Check that the acceleration is uniform before using them.
• Treating the horizontal velocity of a projectile as changing. Without air resistance only the vertical component changes.
• Stating that momentum is lost in an inelastic collision. Momentum is always conserved; it is kinetic energy that is not conserved.
• Using the full length of a beam instead of the perpendicular distance from the pivot when calculating a moment.
• Calculating strain with the extended length rather than the original length, or forgetting that strain has no units.

Try three 3.2 Mechanics and Materials exam-style questions