Notes and practice - Shoreline School District

More Momentum
SWHS AP Physics
What is conserved?
Imagine observing the following phenomena occurring with a two-object system. Two carts (the system)
move on a dynamics track. Complete the table that follows for each experiment.
Experiment
a.
Cart A (2 kg) moving right at
constant 1.0 m/s speed hits
identical cart B (2 kg) that is
stationary. Cart A stops and cart
B starts moving rightward at 1.0
m/s.
b.
Cart A loaded with blocks (total
mass of the cart is now 4 kg)
moving right at 1.0 m/s hits
stationary cart B (2 kg). After the
collision, both carts move to the
right, Cart A at 0.5 m/s and Cart
B at 1.0 m/s.
c.
Cart A (2 kg) with modeling clay
attached to the front moves right
at 1.0 m/s. Identical cart B (2 kg)
moves left at constant speed 1.0
m/s. The carts collide, stick
together thanks to the clay and
stop.
Sketch the process before the
collision and after the collision.
Determine if anything is the same
before and after the collision.
(Hint: Think of mass, speed,
velocity, acceleration, or some
combinations of these quantities)
After you have come up with a physical quantity that is the same before and after each collision, decide
whether this quantity remains constant in all of the experiments. Explain below
Momentum Conservation
An “isolated” system:
Impulse-Momentum Bar Chart
A “non-isolated” system:
A (toy) rifle:
Q1: What is the “recoil” velocity of rifle?
Q2: What impulse does the bullet impart to the rifle?
Q3: If the “explosion” lasts for 0.1 sec, what force does the bullet impart on the rifle?
In-elastic Collisions:
Rocket Science:
Q: How can a rocket “go” in space if it has nothing to push against?
A:
A rocket in deep space at rest (relative to say, a nearby planet):
Now…let’s consider this as a non-isolated system (only the rocket):
In-Class Practice - Momentum Conservation and Inelastic Collisions
1.
Blocks 1 and 2 rest on a horizontal frictionless surface. A compressed spring of negligible mass
separates the blocks. Block 1 has twice the mass of block 2. When the spring is released, the blocks
are pushed apart. Answer the questions below concerning this process.
1
2
a.
Compare the momentum magnitudes of block 1 and block 2 after the spring is released.
b.
Compare the combined momentum of both blocks before the spring is released and the
combined momentum after the spring is released.
c.
Compare the speed of block 1 to that of block 2 after the spring is released.
d.
Represent the process using an impulse-momentum bar chart for a system consisting of the
spring and two blocks.
2.
A 0.02 kg bullet travelling horizontally at 250 m/s embeds in a 1.0 kg block of wood resting on a
table. Determine the speed of the bullet and wood block together immediately after the bullet
embeds in the block.
3
Two carts traveling in opposite directions are about to collide. The carts are all identical in size and
shape, but they carry different loads and are traveling at different speeds. The carts stick together
after the collision. There is no friction between the carts and the ground.
A
B
20 cm/s
500 g
25 cm/s
C
20 cm/s
400 g
10 cm/s
400 g
800 g
D
20 cm/s
500 g
20 cm/s
500 g
20 cm/s
800 g
40 cm/s
400 g
Rank the speed of the two-cart systems after the collision.
OR
1
Greatest
4.
2
3
4
Least
All
the same
All
zero
Cannot
determine
In Case A, a metal bullet penetrates a wooden block. In Case B, a rubber bullet with the same initial
speed and mass bounces off of an identical wooden block.
Case A
Before
After
Case B
Before
After
Will the speed of the wooden block after the collision be (a) greater in case A, (b) greater in case B,
or (c) the same in both cases? Explain.