Mechanics and Materials

• In this video we're going to be doing some revision about eight QA mechanics and materials
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• Scalar quantities


  Quantities that only have a size or magnitude
• Vectors


  Quantities that have both a size (magnitude) and a direction
• Examples of scalar quantities


  • Distance
  • Speed
  • Mass
• Examples of vector quantities

  • Displacement
  • Velocity
  • Acceleration
  • Weight
  • Force
• Displacement


  The distance between the start and end position, including direction
• Velocity


  The rate of change of displacement
• Acceleration


  The rate of change of velocity
• Mass


  The amount of matter an object contains
• Weight


  The force exerted by gravity on an object with mass
• Adding vectors


  1. Draw scale diagram

  2. Measure angle with protractor

  3. Find resultant
• Resolving a vector


  1. Find horizontal component (F_x = F cos(θ))

  2. Find vertical component (F_y = F sin(θ))
• If an object is in equilibrium, the sum of the forces is zero and the sum of the moments is zero
• Moment of a force

  Force x perpendicular distance from point to line of action of force
• Moment of a couple

  Force x distance between parallel forces
• The principle of moments states that in equilibrium, the total clockwise moments = total anticlockwise moments
• Center of mass


  The point at which an object's mass appears to act
• If the center of mass is within the base, the object is stable
• Speed = distance / time, velocity = displacement / time
• Center of mass


  • Acts at the center for uniform shapes
  • If center of mass is within the base, the object is quite stable
• The center of mass of a ruler with different masses hung on it is at the midpoint
• Velocity (V)


  The rate of change of displacement (s) over time (t)
• Average velocity


  Depends on the total displacement over the total time
• Instantaneous velocity


  The value at a particular time
• Acceleration (a)


  The rate of change of velocity over time
• Displacement-time graph


  1. Gradient = velocity

  2. Positive and negative displacement

  3. Gradient changes show changes in velocity
• Velocity-time graph


  1. Gradient = acceleration

  2. Constant, increasing, decreasing, and negative velocity
• Acceleration-time graph


  Constant, positive, and negative acceleration
• Average velocity


  Equals (initial velocity + final velocity) / 2
• SUVAT equations


  Relate displacement (S), initial velocity (U), final velocity (V), acceleration (a), and time (t)
• Solving motion problems


  Write down SUVAT

  2. Identify known and unknown values

  3. Select appropriate SUVAT equation

  4. Rearrange and solve
• Projectile motion


  • Horizontal and vertical motion are independent

  Acceleration in horizontal direction is 0

  Acceleration in vertical direction is -g (gravity)
• Gravitational acceleration (g) = 9.81 m/s^2
• Terminal speed


  Speed at which the drag force equals the weight force, resulting in 0 acceleration
• Newton's Laws


  • No net force -> constant velocity

  2. Net force = mass x acceleration

  3. Action = -reaction
• Momentum (p)


  Mass x velocity
• Momentum is conserved in collisions and explosions
• We have to talk about an event so this could be an explosion where initially things were both at rest and they move away in different directions or we could talk about a collision where things come together and then it might kind of stick together or they might move off again in different directions