In part c I considered the forces acting on box C. I found net force from F=ma and then subtracted friction force and since thats the 'driving' force on box C then Force of B on C must be equal to it?
However I always get 'em wrong
Look at all three blocks as a whole to find the acceleration of the system: m=21kg, F(applied)=140N, friction=42N, acceleration=14/3
Force of B on C, set up an equation for block C individually:
Force of B on A, look at Box A by itself, you've got the push force, force of B on A and the frictional forces:
And one more sorry - For Q7 they times the torque supplied by the pole by 1.5. I thought you times it by 3 since its 3m from the pivot point?
Why is that?
Torque is calculated as distance multiplied by perpendicular component of force, the 100N of weight is not perpendicular to the pole PQ, so we multiply the 100N of weight by the component of the pole which is perpendicular to the weight force which is 1.5m (using a bit of trigonometry).
What are some devices that can act as modulators and demodulators, and what can act as a carrier wave?
Modulators: you're mainly looking for transducers (electro-optic) which convert electrical signals to some physical signal, where their output can be adjusted based on an input signal. So things like an LED (can't think of any others right now).
Demodulators: take the opposite, so opto-electric devices such as LDR's and photodiodes. They will basically take the variations in the signal and convert it back into an electrical signal.
Carrier Waves: VCAA usually uses light waves in their scenarios, or even the brightness of an LED.
EDIT: Also, how do you go about finding the rebound Force of a wall that is holding a cantilever as well as the angle of said force?
Well, if you've got a rope or something at an angle holding up the cantilever, the force from the wall will just ensure the whole system is equilibrium. So find the value of tension in a cable/rope using torque calculations (to oppose the vertical weight force of the cantilever), then look at translational equilibrium - if the system isn't in translational equilibrium then the wall is likely applying a force to the cantilever in some direction.
So in this example, we'd find the tension in the cable such that it opposes the weight of the beam. But the tension in the cable would be made up of a vertical component which opposes the weight of the beam, and a horizontal component (because the tension is at an angle). So the wall (point A) will provide a horizontal force to work against the tension so the whole thing stays in equilibrium.