1. Make-up Labs
Arrange with Viktor in BSB-117
Physics 1D03 - Lecture 31

2. Angular Momentum II General motion of a rigid body
Collisions involving rotation
Text Section
Physics 1D03 - Lecture 31

3. Angular momentum” is the rotational analogue of linear momentum.
Recall:
Angular momentum” is the rotational analogue of linear momentum.
m I
v w
F t
p L (“angular momentum”)
L = Iw
|L| = mrvt
= mvr sin f
Physics 1D03 - Lecture 31

4. Question
Two astronauts are held together by a long rope and rotate about their common center of mass. One has twice the mass of other. One astronaut gathers in 1/3 of the rope separating them.
Which of the following remain constant?
Kinetic energy
Angular velocity
Angular momentum
Tension in the rope
Physics 1D03 - Lecture 31

5. In general, for a moving, rotating rigid body,
Angular momentum of a particle:
of a rotating rigid body:
L = I w.
In general, for a moving, rotating rigid body,
The first term is called the “orbital” angular momentum and the second term is the “spin” angular momentum. Example: angular momentum of a planet about the sun.
Physics 1D03 - Lecture 31

6. Example:
The earth (m = 6.0 x 1024 kg, R = 6400 km) moves at speed v = 30 km/s in an orbit of radius r = 150 x 106 km around the sun. It also spins on its axis once per day (ω = 7.3 x 10-5 rad/s).
The angular momentum of the earth relative to the centre of the sun is L = mvr + ICM ω. The “orbital” part is calculated as if the earth were a particle orbiting the sun; then we add a the angular momentum or the spinning earth relative to its own centre of mass.
Physics 1D03 - Lecture 31

7. Collisions: Collisions can conserve angular momentum as well as linear momentum.
Total linear momentum is conserved if there is no external force during the collision (or if the external forces are small compared to the forces the colliding bodies exert on each other).
Total angular momentum is conserved if there is no external torque during the collision (or if the external torques are small).
Angular momentum may be calculated about any axis. Usually it is convenient to use an axis through the centre of mass, unless one of the colliding objects actually rotates about some other fixed axis.
Physics 1D03 - Lecture 31

8. Question: Which of the following describe the motion
A metre stick (mass M, length L= 1m, moment of inertia I ) is suspended from one end by a frictionless pivot at P. A ball of mass m, velocity v0, strikes the other end of the (stationary) stick at right angles, and stops (final velocity of the ball is zero). P
Question: Which of the following describe the motion
of the stick after the collision? (Answer True, False, or Maybe for each one.)
ICM w = mv0 L/2
IP w = mv0 L
MvCM = mv0
½ IP w 2= ½ mv02 v0
Physics 1D03 - Lecture 31

9. in the same direction as v0 in a direction opposite to v0
Quiz P
A stick (uniform thin rod) is lying on the ice. A hockey puck hits the stick, at right angles, and the stick starts to slide. Point P is on the end farthest from where the puck hits. Immediately after the collision, the end P will start to move: CM
in the same direction as v0
in a direction opposite to v0
at an angle (not 0o or 180o) to v0
It depends where the puck hits v0
Physics 1D03 - Lecture 31

10. I=1.33kg m2. Assuming an elastic collision, find the
Example:
A 2.0kg disk moving at 3.0m/s hits a 1.0kg stick lying flat on a frictionless surface. The moment of inertia of the stick is
I=1.33kg m2. Assuming an elastic collision, find the
speeds of the disk and stick after the
collision and the rotational speed of
the stick. v0 2m
Physics 1D03 - Lecture 31

12. Example: Sticky clay of mass m and velocity v hits a cylinder of mass M and radius R. Find the angular speed of the system after the collision. Is energy conserved?
Physics 1D03 - Lecture 31

13. Summary In general, for a rigid body,
In collisions, angular momentum will be conserved it there is no external torque.
Physics 1D03 - Lecture 31