Conservation of Angular
Momentum, Change in Angular
Momentum and, Angular Impulse
October
Conservation of Angular Momentum
 Lmom is a conserved quantity/remains constant in the absence of an external force (i.e.
torque)
 Since Lmom is a constant for an isolated system, and because the Lmom formula can be
rearranged to give v = Lmom /(mr), the velocity v and the separation r are inversely correlated.
 thus, conservation of angular momentum demands that a decrease in the separation r be
accompanied by an increase in the velocity v, and vice versa.
 We observed this during our Ironman demo:
mrBIGvsmall
Another example
=
mrsmallvBIG
Change in Angular Momentum
 How do we deal with a change in angular momentum (∆ Lmom or ∆ Iω)?
 The same way we did with a change in linear momentum
 Recall, for linear momentum:
Change in linear momentum = ∆P= m∆v
 Same for change in angular momentum except symbols change:
Change in Angular momentum = ∆ Lmom = I ∆ ω
where ∆ ω = ω2- ω1
 Let’s try Example 2 part “a” from pg.56
Angular Impulse

Recall from linear impulse that J=F∆t which means that
a linear impulse is the result of a force applied in a certain amount of time
ex. Hydraulic spreader
(HUGE jaw force applied to outer edges of car door
for 1min results in HUGE IMPULSE to pry open door)



Angular Impulse is what causes objects to start or stop rotating
Like linear impulse it still happens in a certain amount of time EXCEPT, since
rotation, the force required to start that rotation is called TORQUE
TORQUE is a force applied from a certain distance from the center of object such
that;
Torque(T) = Fxd where F(N or lb), d (m or ft), T (N.m or lb.ft)
Working with Angular Impulse


Angular Impulse causes rotation when torque (F x d) is applied to an object in a
certain period of time.
Screw begins to turn due to angular impulse applied from wrench
Ang.Imp. = (Fxd)∆t
where Ang.Imp. has units (N.m).s or (lb.ft).s

we could re-write equation as Ang.Imp.= T∆t

And of course don’t forget:
Impulse (in this case angular impulse) is what causes change in momentum so:
T∆t = I∆ω

Let’s try part “b” of Example 2 pg.56
Classwork/Homework (whatever is not
finished in class must be finished for hmwk)
Read pg.44-50
Complete Qd-g from Let’s Review units on
pg.55
Complete Student Exercises(#1-11) on
pg.51. You must write out the full question
for Q1-10 and use GFS for #11.
Answer Qs #4-6 on pg.57 of workbook