Phys101 Term:123 Online HW-Ch10-Lec02

KFUPM-Physics Department 1

Q1: Calculate the net torque (magnitude and direction) on the beam in Figure about an axis through O perpendicular to the page.?

A) 30 N.m counterclockwise B) 30 N.m clockwise C) 36 N.m counterclockwise D) 36 N.m clockwise E) 20 N.m clockwise

Ans:

A τ0 = 25 cos30° ∗ 2 − 10 sin20° = +3.0 N. m

Q2: The thin uniform rod in Figure has length 2.0 m and can pivot about a horizontal,

frictionless pin through one end. It is released from rest at angle 40o above the horizontal. Use the principle of conservation of energy to determine the angular speed of the rod as it passes through the horizontal position?

Ans:

ki + Ui = kf + Uf

0 + mgh =1 2

Iw2 + 0

I = Icom + ML2 = 12

ML2 + ML2 =13

ML2

h =L2

sinθ

L2

h

⇒ mgL2

sinθ =12

�13

ML2�w2 ⇒ w = �3g sinθL

= 3.1 rad

Phys101 Term:123 Online HW-Ch10-Lec02

KFUPM-Physics Department 2

Q3:

A 32.0 kg wheel, essentially a thin hoop with radius 1.2 m, is rotating at 298 rev/min. It must be brought to a stop in 15.0 s. How much power required to stop it?

Ans: I = MR2 = (32)(1.2)2 = 46.1 kg. m2

w = ∆k = kf − ki = 0 −12

Iw2 = −12

(46.1) �298 × 2π

60�2

= −2.24 × 104J

P = |w|∆t

= 1.49 × 103 watt