Transcript Document

What is MOMENTUM?
Units: kg m/s
Newton’s 2nd Law written in terms of
change in momentum
What is IMPULSE?
Units: N s
Force
area = I
Force on a car
versus time
for a typical
car collision.
Time
BOXING AND BRAIN INJURY
Bare fists were used in the past, now fighters wear padded
gloves. Why?
Other examples with the same application:
Car bumpers, Air bags, Seat belt
From Newton’s 2nd Law:
Impulse-Momentum Theorem
Quick Quiz
A car and a large truck traveling at the same speed
collide head-on and stick together. Which vehicle
undergoes the larger change in the magnitude of its
momentum?
1. the car
2. the truck
3. the change in the magnitude of momentum is
the same for both
4. impossible to determine without more
information
Impulse-Momentum Theorem
Force when the
bodies are in contact
Velocities before and
after the contact (no
contact is happening)
Quick Quiz
(A)
m
3.0v
3m
m
2m
m
x0 = 0
2.5v
(B)
(C)
The same force F is
applied to different
bodies originally at rest
for different amounts of
time. The diagrams
below show the results
(after the force ceased).
1.0v
(D)
1.5v
(E)
Rank the IMPULSE for
each diagram from the
greatest IMPULSE to
the least IMPULSE
Collision of astronauts
During a collision
(only while the objects are in touch)
the objects receive an IMPULSE (I)
I  F  t
Units of Impulse
I = [ N . s]
I1
(3)
I2
Impulse on astronaut 2
Impulse on astronaut 1
IMPULSE HAPPENS DURING THE COLLISION
Bouncing Ball
Conservation of momentum
Two skaters of different masses prepare to
push off against one another. Which one will
gain the larger velocity?
a)
b)
c)
The more massive one
The less massive one
They will each have equal but
opposite velocities.
The net external force acting on the
system is zero, so conservation of
momentum applies.
Before the push-off, the total initial
momentum is zero.
The total momentum after the pushoff should also be zero.
Collisions
Glancing collisions
Two lumps of clay of equal mass are traveling
at right angles with equal speeds as shown,
when they collide and stick together. Is it
possible that their final velocity vector is in
the direction shown?
a)
b)
c)
yes
no
unable to tell
from this graph
No. The final momentum will
be in a direction making a 45o
degree angle with respect to
each of the initial momentum
vectors.
Two cars of equal mass Collide at right angles
to one another in an intersection. Their
direction of motion after the collision is as
shown. Which car had the greater velocity
before the collision?
a)
b)
c)
d)
Car A
Car B
Their velocities were
equal in magnitude.
It is impossible to tell
from this graph.
Since the angle with respect to
the original direction of A is
smaller than 45º, car A must
have had a larger momentum
and thus was traveling faster.