Phys 101 written Problems #7 Due: Friday, Nov 3, 2017 at 4 PM. Put finished assi

Question

Phys 101 written Problems #7 Due: Friday, Nov 3, 2017 at 4 PM. Put finished assignment in P101 drop box on 9000 level of Physics (put into the box that has your tutorial section number as a label). Please note to get full credit for any written homework assignment you must 1) Put the assignment name, your full name, your student number, your tutorial section number (see Canvas for tutorial sections), and TA first name (-1 mark if not done correctly): PHYS 101-Homework #1, Susie Smith, 123456789, D102, Tim 2) Show all of your work, clearly and legibly. Use diagrams if necessary. 3) Try to work out answers symbolically. Only put #s in at the end. 4) Put a box around your answers 5) Don't forget units. 6) If you use more than one page, staple all of the pages together (-1 mark for unstapled assignments) An ultracentrifuge is spinning at 15000 rpm in the counter-clockwise direction. It is then turned off and takes 15 mins to stop spinning. It has a diameter of 26 cm. a) What is the initial angular velocity of the disc in rad/s?(1 mark) b) What is the angular acceleration that it experiences as it slows down assuming that it is constant over this period? (2 marks) c) How many revolutions does it go through in coming to a stop? (2 marks) d) What is the magnitude and direction of the centripetal acceleration of the black point located at the outer edge as shown? (2 marks) e) What is the magnitude and direction of the tangential acceleration for the black point shown? (2 marks)

Explanation / Answer

(a) Intial angular velocity, w0 = 15000 rpm = (15000*2*3.141) / 60 = 1570.5 rad/s

(b) time t = 15 min = 15x 60 = 900 s

use the expression -

w = w0 + alfa*t

=> 0 = 1570.5 + alfa*900

=> alfa = -1570.5/900 = -1.745 rad/s^2

(c) theta = w0^2 / (2*alfa) = 706725 radians.

So, no. of revolutions = 706725 / (2*3.141) = 112500 rev.

(d) centripetal acceleration = alfa x r = 1.745 x 0.13 = 0.227 m/s^2

And its direction is towards the center.

(e) Tangential acceleration = 1.745 m/s^2.

Its direction is perpendicular to the centripetal acceleration.

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