Chapter 4 - Forces in Action

Force, Mass and Weight
1. Mass - Physical property depending on the amount of matter an object has
2. A net force (N) acting on the object makes it accelerate in the direction of the force
3. Mass is constant, but weight can vary. Weight is also a force
Centre of Mass
1. C.O.M - A point through which any externally applied force produces straight line motion but no rotation
2. The total weight of an object, acts through a point called the centre of gravity, coinciding with the C.O.M
3. To find the C.O.G, hang the object freely, e.g with a clamp stand. Using a plumb line, draw lines vertically down from multiple points, until they overlap at the C.O.G
Free Body Diagrams
1. Each force vector is represented by a labelled arrow, which is drawn to the same scale
  1. Component of the weight down the slope is responsible for acceleration down the slope
    1. Normal contact force also = mgcosΘ
      1. For a trolley on a ramp, using a=f/m, where f=mgsinΘ, acceleration is independent of the mass (as m cancels out)
Drag and terminal velocity
1. An object moving through a fluid (Air or liquid) experiences a Drag force, a frictional force opposing the motion of the object
  1. The magnitude depends on speed, shape, texture of the object and the medium
  2. Greater Cross sectional Area results in greater drag
    1. Drag is directly proportional to speed squared
  3. At the instant he falls, no air resistance. Force = W, [a=g]
    1. Speed and therefore Drag increases, meaning net force decreases [a=(mg-D)/m]
      1. At T.V, Drag is equal and opposite to weight. No Acc, constant speed. [a=0]
Moments and Equilibrium
1. The turning effect of a force about some axis or pivot
  1. Moment = Force x Perpendicular distance of the line of action of force from the axis or pivot
    1. Moment = Fx (Units Nm)
      1. It must be perpendicular distance
    2. Principle of moments = For a body in rotational equilibrium, the sum of anticlockwise moments about any point is equal to the sum of clockwise moments about the same point
Couples and Torques
1. A couple is a pair of equal and opposite forces acting on a body but not in a straight line
  1. The moment of a couple is known as a torque
    1. Torque of a couple = One of the forces X perpendicular seperation between the forces = fd
Triangle of forces
1. Arrows are drawn end to end, to represent the three forces
  1. The triangle is closed because the net force is zero and so the object is in equilibrium
Density and pressure
1. Density is the amount of mass per unit volume
  1. For density you need to know the mass and volume. This can be measured, e.g with a ruler or measuring cylinder for liquids, along with a measuring scale. For irregular solids, volume is worked out through displacement
2. Pressure is the normal force per unit area (Units Nm^-2, or Pascals (pa))
P=Hpg and Archimedes principle
1. Fluid pressure does not depend on C.S.A, and it also shows that presure is directly proportional to depth
2. Upthrust is the upward buoyant force exerted on a body immersed in fluid, due to differences in pressure on the top and bottom of an object
  1. Archimedes principle - The upthrust exerted on a body immersed in a fluid, either fully or partially submerged, os equal to the weight of the fluid that the body displaces
    1. For a floating object, Upthrust = Weight of the object. If it sinks, Weight>Upthrust

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Chapter 4 - Forces in Action

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