4. Mechanics + Materials

Cards (64)

  • Scalar quantity: A quantity that has only magnitude
  • Vector quantity: A quantity that has magnitude as well as direction
  • Acceleration is a vector quantity
  • Mass is a scalar quantity
  • Difference between mass and weight:
    • Mass is scalar and not dependent on gravity
    • Weight is a vector and depends on gravitational field strength (W = mg)
  • If an object is in equilibrium:
    • Sum of anti-clockwise moments = sum of clockwise moments (principle of moments)
    • Object is not accelerating, so is either stationary or moving at a constant velocity
  • How forces acting on an object can be shown to be in equilibrium:
    • Adding horizontal and vertical components of forces, showing they equal zero
    • For 3 forces, draw a scale diagram forming a closed triangle if the object is in equilibrium
  • Moment: A turning force, force multiplied by perpendicular distance from point to line of action of the force
  • Couple: A pair of equal and opposite coplanar forces
  • Centre of mass: Point through which all mass of an object acts, for a uniform object, the centre of mass is at the centre of the object
  • Velocity: Change in displacement per unit of time, instantaneous velocity found by measuring gradient of tangent to a displacement-time graph
  • Area under a velocity-time and acceleration-time graph:
    • Displacement travelled and velocity respectively
  • As speed increases, air resistance increases (proportional to the square of the speed)
  • Horizontal velocity of a ball projected off a castle at 6m/s remains the same until it hits the ground
  • SUVAT equations reflect that all objects fall at the same rate because mass is not included in the equations
  • In projectile motion, vertical acceleration is equal to gravitational field strength (g)
  • Terminal velocity: When forces acting on a falling object become balanced, acceleration becomes zero and object moves at maximum velocity
  • Friction: Resistance to motion between an object and a surface or fluid, force that acts opposite to movement
  • Newton's third law states 'every action force has an equal and opposite reaction force'
  • Newton's second law: F = ma, where F is force applied and a is acceleration
  • Newton's first law: An object stays moving at a constant velocity until a force acts upon it
  • Difference between elastic and inelastic collisions:
    • Elastic: Kinetic energy before = kinetic energy afterwards
    • Inelastic: Kinetic energy at the end is not equal to the kinetic energy at the start
  • Equation to calculate momentum: Momentum = mass × velocity
  • Linear momentum is always conserved, not only in elastic collisions
  • Rate of change of momentum is described as force
  • Impulse: Change in momentum, F∆t = ∆mv
  • Area under a force-time graph represents impulse, the change in momentum
  • Fs cos( 𝜽 ) = The work done / the energy transferred
  • Rate of work done is equal to power
  • Efficiency = Useful output power / input power
  • Principle of conservation of energy: Energy cannot be created or destroyed, only transferred into other forms, total energy in a closed system remains constant
  • Lift: Upward force acting on objects in a fluid, caused by object changing direction of fluid flow, acts perpendicular to fluid flow
  • Hooke’s law:
    • Extension (∆L) is directly proportional to force applied (F), given that the environmental conditions are kept constant
    • F= k∆L
    • k is the stiffness constant in Nm^-1
  • Equation to calculate density:
    • Density = Mass / Volume
    • Density units: kgm^-3
    • Mass: kg
    • Volume: m^3
  • Tensile stress:
    • The force applied per unit cross-sectional area
    • Stress = force / CSA
    • Stress units: Nm^-2
    • Force units: N
    • Cross-sectional area units: m^2
  • Tensile strain:
    • A measure of how the material stretches: the extension (ΔL) divided by the original length (L), strain has no units
    • Strain = ΔL / L
  • Difference between elastic and plastic deformation:
    • Elastic deformation: when the force is removed the object will return to its original shape
    • Plastic deformation: after the load is removed the object will not return to its original shape
  • Breaking stress:
    • The minimum stress needed to break a material
  • Brittle material:
    • Doesn’t deform plastically but breaks when the stress reaches a certain value
  • Elastic limit:
    • The force above which the material will be plastically deformed (permanently stretched)