30. The rate of change of the angular momentum is
d
dt
=
τ
1
+
τ
2
= 2.0ˆi
− 4.0ˆj N·m .
Consequently, the vector d
/dt has a magnitude
2.0
2
+ (
−4.0)
2
= 4.5 N
· m andis at an angle θ (in the
xy plane, or a plane parallel to it) measuredfrom the positive x axis, where θ = tan
−1
−4.0
2.0
=
−63
◦
,
the negative sign indicating that the angle is measured clockwise as viewed “from above” (by a person
on the +z axis).