Possession - Chapter 4.1

Video 4.1a

• Forces are vectors.  Forces are added by vector addition.

• parallelogram of forces: find resultant which is the diagonal of a parallelogram

• tip-to-tail method: graphical method to add forces (and other vectors)

• algebaric method: using geometry mathematics, calculate the resultant of forces (or other vectors)

Video 4.1b

• resolving components of a force:
  usually we resolve a force into two perpendicular components, namely the horizontal and vertical components

• the compoents are ... F cos (theta) and F sin (theta) - Memorise and learn to apply!!

• For a slope, component of weight downhill is mg sin (theta) - IMPORTNAT !

• In the normal direction to a slope, the net force is zero!

• Along the slope, there may be net force that produces an acceleration

• For friction compensated slope, net force = 0, so mg sin (theta) = friction

Video 4.1c

• Algebaric method:
  (1) Resolve all forces into horizontal and vertical components
  (2) Find the sum of forces in the two directions
  (3) Find the resultant by pythagoras theorem

• Experiment 4a verifies the graphical and algebaric methods 

Possession - Chapter 4.2

Video 4.2a

• For a helicopter, the uplifting force (thrust) and the weight act on it.

• draw free body diagram to identify all the forces on an object

• You may set up equation using F = ma according to the free body diagram.

• three forces that balance each other (i.e. net force = 0), then the three forces form a triangle.

• when there are more than three forces ... what do you think?

Video 4.2b

• for a sledge, the vertical net force = 0 while the horizontal net force produces an acceleration

• for a man pushing a cargo up a slope, the cargo accelerates is PUSH - mg sin (theta) - friction > 0

• for a helicopter,
  > the vertical net force is zero if the helicopter travels horizontally, it acceleratates horizontal
  > it travels to top left if the thrust is larger
  > it travels to bottom left if the thrust is smaller

• When a trolley moves on a slope,
  > for the uphill motion, BOTH mgsin (theta) and friction directs downhill
  > for the downhill motion, mg sin (theta) directs downhill, friction directs uphill 

REVISION VIDEO - Chapter 4 (6:27) - 249

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