past hsc questions - help needed (1 Viewer)

[Petinga]

1. A fan that ventilates an underground mine is run by a very large dc motor. This motor is connected in series with a resisitor to protect the windings of the coil.
When the motor is started up, the resisotr is adhusted to a large resistance but slowly lowered as mtoor increases operating speed. Explain why none needed when running at high speed but lots for starting?

2. A sports magazine commenting on thje atheletic ability of Micheal Jordan, the famous basketball player said:
"Being an athlete takes more brains than brawn. it takes time and effort. it takes endurance and commitiment. it takes an athelete who can saty in the air for 2.5 seconds while shooting a goal; an athlete who knows which laws of phsycis keeps him there."
Assess the informatrion preesented, using appropriter calculacultions to support your argument.

if posssible jus anwser in pt form. thanx

 

[myeewyee]

In regards to the 2nd one, here is my answer, summarised: (incidently I just did this question about 2 hours ago!)


- the information is grossly exaggerated and presents feats that exceed the limitations of the human body. consider his motion as a parabola, after 2.5s:
dy = ut + 1/2at^2
0 = 2.5xu - 4.9x2.5^2
u = 12.25ms-1

- this is faster than the average velocity of elite sprinters (~10ms-1). furthermore, at this velocity, the max height of the jump would be:
dy = 12.25x1.25 - 4.9x1.25^2
= 7.66m
- again this is an unrealistic value, considering the man would have to jump three times his height.
- thus, the magazine presents greatly exaggerated information which is not corroborated by the laws of physics

 

[Gowr]

In response to your first question...

When the motor starts spinning back EMF is minimal (The change in flux is less if the speed of rotation is less, for obvious reasons), hence if there is little resistance there will be a very high current in the motor, which could cause it to overheat. When the motor has spun for a while back EMF is increased, and as this decreases the potential difference in the coil it decreases the current in the coil. Therefore, less resistance can be used, as the current will not be so high.