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Flashcards · 7408 · Topic 4
Mechanics and materials flashcards
19 cards on Topic 4, subtopics 3.4.1 to 3.4.2. Pick a subtopic or study them all.
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3.4.1 Force, energy and momentum
- Resolving the weight mg of an object on a slope at angle θ
- mg sin θ along (down) the slope; mg cos θ perpendicular to (into) the surface.
- Moment, couple and the principle of moments
- Moment = force × perpendicular distance from the pivot to the line of action. Couple: equal, opposite, coplanar forces; moment = one force × separation of their lines of action. In equilibrium, sum of clockwise moments = sum of anticlockwise moments about any point.
- When can the suvat equations be used?
- Only for uniform (constant) acceleration, so not, for example, when air resistance changes the acceleration.
- Required practical 3: finding g by free fall from a graph
- Plot h against t² for a ball released from rest. Gradient = g/2, so g = 2 × gradient.
- Projectile method
- Split the launch velocity into horizontal (u cos θ) and vertical (u sin θ) parts. Horizontal: constant velocity. Vertical: acceleration −g, use suvat. Time links the two.
- Newton's third law, and why weight and normal reaction are not a pair
- Equal and opposite forces of the same type acting on different bodies. Weight and normal reaction act on the same body and are different types of force.
- Impulse
- FΔt = Δ(mv), the change in momentum. Equal to the area under a force–time graph. Unit N s.
- Elastic vs inelastic collision
- Momentum is conserved in both. Elastic: kinetic energy also conserved. Inelastic: kinetic energy not conserved.
- Why does a longer contact time reduce the force (crumple zones, packaging)?
- F = Δ(mv)/Δt. For the same change in momentum, a longer time gives a smaller force.
- Work done by a force at angle θ to the motion, and power at constant velocity
- W = Fs cos θ. P = Fv.
- When does a falling object or a vehicle reach terminal (top) speed?
- When the resistive forces (drag and friction) equal the weight or driving force, so the resultant force is zero and the acceleration is zero.
3.4.2 Materials
- Limit of proportionality
- The end of the straight-line (Hooke's law) region.
- Elastic limit
- Beyond it, a permanent extension remains after unloading.
- Plastic vs brittle behaviour
- Plastic: large permanent extension for little extra force (copper). Brittle: fractures with almost no plastic deformation (glass).
- Tensile stress, tensile strain and the Young modulus
- Stress = F/A (Pa). Strain = ΔL/L (no unit). Young modulus E = stress/strain = FL/(AΔL), the gradient of the linear part of the stress–strain graph.
- Elastic strain energy
- ½FΔL = area under the force–extension graph. In the Hooke's law region it equals ½kΔL².
- Required practical 4 (Young modulus): which measurement dominates the uncertainty, and why?
- The wire's diameter, because the area depends on d². Measure it with a micrometer at several places and take the mean.
- Loading beyond the elastic limit then unloading: what does the graph show?
- The unloading line is parallel to the loading line. The area between them is the energy used to deform the material permanently.
- Stiffness k vs Young modulus E
- k is a property of one object (depends on its shape and size); E is a property of the material.