Transcript Conservation of Momentum

 car

crashes
https://www.youtube.com/watch?v=4eCJ3rJ-wP0
EQ: How is momentum conserved when
there is interaction between two objects in
a closed system?
Medicine ball collisions
https://www.youtube.com/watch?v=W9EqU1_DXUw
 Think
Newton’s 3rd Law in a
collision
 The
size of the force on the first
object equals the size of the force
on the second object. They are
just opposite in direction
 Forces
always come in pairs equal and opposite actionreaction force pairs.
 Newton’s
3rd Law of Motion
 Collision
of unequal masses
 not
observing unequal forces
upon the ball and club head, but
rather unequal accelerations
 Collision
of equal masses
 each
object experiences the same
acceleration.
 Forces
motion
cause acceleration, not

For a collision occurring
between object 1 and
object 2 in an isolated
system, the total
momentum of the two
objects before the collision
is equal to the total
momentum of the two
objects after the collision.
That is, the momentum lost
by object 1 is equal to the
momentum gained by
object 2.

This means that the
momentum doesn’t change.
 Isolated
System
An isolated system is a system that is
free from the influence of a net
external force that alters the
momentum of the system
 No system is completely isolated,
there will always be a resistance force
of some kind robbing the system of its
momentum (air, friction)

Momentum is conserved in any collision, elastic and
inelastic.
Momentum is conserved in recoil or propulsion
Pi = Pf
m1v1i  m2 v2i  m1v1 f  m2 v2 f
Kangaroos are good runners that can sustain
a speed of 56 km/h (15.5 m/s). Suppose a
kangaroo is sitting on a log that is floating in a
lake. When the kangaroo gets scared, she
jumps off the log with a velocity of 15 m/s
toward the bank. The log moves with a
velocity of 3.8 m/s away from the bank. If the
mass of the log is 250 kg, what is the mass of
the kangaroo?
 Conservation
of momentum states that the total
momentum of the two objects before the
collision is equal to the total momentum of the
two objects after the collision. How is this
accomplished? Identify who experiences the
greater force, impulse, momentum change,
acceleration?

Elastic collision: Particles bounce off each other without loss of
energy.

Perfectly inelastic collision: After colliding, particles stick
together and move with a common velocity after the collision.
There is a loss of Kinetic Energy (sound, heat, deformation).

Inelastic collision: Particles collide with some loss of energy, but
don’t stick together.
Hockey collisions:
http://science360.gov/obj/video/1a6a8602-2e8b-4eab93e2-b6d5a9e697ed/force-impulse-collisions
11
Momentum is conserved


pi  p f



m1v1i  m2 v2i  (m1  m2 )v f
KE is transformed to other
types of energy = Eloss
K i  K f  Eloss
1
1
1
2
2
2
m1v1i  m2v2i  (m1  m2 )v f  Eloss
2
2
2
12
An arrow is fired into a small target
at rest on a frictionless surface.
The arrow’s mass is 20.0 g and the
target’s mass is 2.50 kg. If the
speed of the arrow and target
combined is 0.67 m/s, what is the
arrow’s initial speed?
An Elastic
 collision
 is a collision in which the
m1v1i  m2 v2i  m1v1 f  m2 v2 f
objects don’t stick together so the total
Energy and
is conserved
momentum
the total: kinetic energy
1
1
1
2
2
2
2
remain1 constant.
m1v1i  m2 v2i  m1v1 f  m2 v2 f
Momentum is conserved:
2
2
2
2
17
In the game of marbles, a shooter is a large marble about
0.02 m in diameter that is used to knock smaller marbles
out of the ring. Suppose a shooter with a speed of 0.80 m/s
hits a 4.8x10-3 kg marble that is at rest in the ring. The
shooter continues forward with a speed of 0.51 m/s while
the smaller marble moves forward with a speed of 1.33
m/s. What is the mass of the shooter?
o
A large truck and a Volkswagen have a head-on collision. Which vehicle experiences the
greatest force of impact? (Remember Newton’s 3rd Law)
o
A large truck and a Volkswagen have a head-on collision. Which vehicle experiences the
greatest impulse? (Remember Newton’s 3rd Law)
o
A large truck and a Volkswagen have a head-on collision. Which vehicle experiences the
greatest acceleration? (Remember Newton’s 3rd Law)
o
Two objects stick together and move with a common velocity after colliding. Identify the
type of collision
https://www.flickr.com/photos/physicsclassroom/galleries/72157625280491634/
In a two body collision in an isolated system
a.
b.
c.
d.
Kinetic energy is always conserved
Momentum is always conserved
Neither momentum not kinetic energy is
conserved
Both momentum and kinetic energy are always
conserved