Quizzes > Engp Q5 02

UNIVERSITY OF SOUTH AUSTRALIA Applied Physics - School of Electrical & Information Engineering ENGINEERING PHYSICS QUIZ 5 [ week 11 ] Time: 20 minutes

Name .....................................................................................

QUESTIONS ( 8 MARKS )

Marks:

/15

ANSWERS

1. Consider a point on an electric motor armature as it turns about its axis, neither speeding up nor slowing down. The angular momentum, L, of the armature … A. is zero. B. increases linearly with time. C. is dependant on the shape of the armature. D. all of the above.

2. The prefix mega represents … A. B.

10-6, 10-3,

C. D.

10+3, 10+6.

3. A Tae Kwon Do black belt executes a swift knife hand chop and breaks 2 cement roofing tiles. The magnitude is the same for the … A. force on both the tiles and the black belts hand, B. impulse on both the tiles and the hand, C. time of impact on both the tiles and the hand, D. all of the above.

4. The center of mass is not located within the material of .. A. B. C. D.

a doughnut, a Cooper pair (2 coupled electrons which provide super-conducting current), both of these. No, the center of mass has to be inside the mass.

5. Pat is a health conscious engineering student who works out on exercise machines at the gym 3 nights a week.

During exercise on one particular machine Pat exerts a force that allows a mass m to be lowered a vertical distance h at a constant velocity. How much work does Pat do in this process? A. zero, B. -mgh, C. +mgh, D. since the weight acts downwards and the movement is also downwards, Pat’s work is +2mgh.

6. Which statement below is FALSE? A. B. C. D.

L = I x ω, FB = qv x B, r x p = -(p x r), τ = r x F.

7. Consider looking at the face of a wall clock which has a steadily moving second hand. The tip of the second hand has v that changes continually and ω that is … A. zero, B. pointing towards you, C. pointing away from you, D. also changing continually.

8. If the polar ice caps melted because of global warming, the resulting water would spread over the entire earth. This new mass distribution (mass is the same) would tend to make the length of a day … A. longer, B. shorter, C. stay the same,

PROBLEMS ( 11 Marks possible ) Question 1 (4 marks) As part of the fabrication process to produce many electronic devices, ion implantation is used to change the electrical properties of the semiconductor material in a particular region. This process involves accelerating ions (charged particles) to a particular velocity, directing the beam of ions onto the material and upon impact they travel some distance into the material having a number of collisions with atoms before coming to rest. What happens to the momentum of the ion? Is momentum conserved? Explain. ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..……………………………………………………………………………

Problem 1 (6 marks) An Olympic ice skater in a spin has a moment of inertia I = 3 kg.m2 with arms outstretched and I = 2 kg.m2 with them tucked in at her sides. She starts the spin at 2 revolutions per second with arms outstretched and then pulls them in. (i) What is her final angular speed in radians per second? [3 marks]

L = Iω

………………………………………..…………………………………………………………………………… v = rω ………………………………………..……………………………………………………………………………

ω f = ω +α ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… (ii) What is her initial and final kinetic energy? [2 marks] ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… (iii) Was work done? Justify your answer. If work was done, who did work on who? [2 marks] ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..…………………………………………………………………………… ………………………………………..……………………………………………………………………………

t