Transcript Momentum

I. Momentum
A.
B.
Product of an objects mass and velocity.
ρ = mv
1.
C.
Momentum is a vector.
1.
D.
Units for momentum are
The direction of momentum is the same as the
direction of the velocity.
A 1000-kg elephant and a 0.01-kg ant are
traveling down Church Street with the same
momentum. If the elephant is traveling at 2
m/s, how fast is the ant traveling. (200000
m/s)
II. Impulse
A.
B.
C.
The product of the net force and the
time interval over which it acts
Impulse = FNET Δt
Units for impulse are Ns
1.
2.
D.
Note that the units for impulse are the same as
the units for momentum.
.
Impulse Is equal to the area under the
curve of a force-time graph.
III. Impulse-Momentum
Theorem
A.
Impulse is equal to the change in
momentum for an object.
1.
2.
This is a restatement of Newton’s
2nd Law
By increasing the time of a collision
you can decrease the amount of
force and achieve the same result.
IV. Impulse-Momentum
Problem
A.
Example: A 0.144 kg baseball is
pitched horizontally at +38 m/s. After
it is hit by a bat, it moves horizontally
at –38 m/s.
1.
2.
3.
What impulse did the ball receive?
If the bat were in contact with the ball for
0.0008 s, what force did the bat exert on
the ball?
Find the acceleration of the ball during its
contact with the bat.
Impulse – Momentum
Practice Problems
B.
C.
D.
Jim strikes a 0.058-kg golf ball with a force of
272 N and gives it a velocity of 62.0 m/s.
How long was the club in contact with the
ball? (0.013 s)
A force of 186 N acts on a 7.3-kg bowling ball
for 0.40 s. What is the change in velocity for
the bowling ball? (10.2 m/s)
A 0.24-kg volleyball approaches Taylor with a
velocity of -3.8 m/s. Taylor bumps the ball
giving it a velocity of 2.4 m/s. What average
force did she apply if the interaction time
between her hands and the ball is 0.0254 s.
(58.6 N)
Egg Drop
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Save the egg with impulse.
V. Systems
A.
A system is a defined collection of
objects.
1.
2.
3.
Your system is what you are
working with in a problem.
A system is closed if objects neither
enter or leave it.
A system is isolated if there is not
net external force.
VI. Internal and External
Force
A.
B.
An internal force is within your
system.
An external force is not in your
system. When the net external
force on your system is zero, the
system is said to be isolated.
VII. Conservation of
Momentum
A.
B.
C.
Momentum in a closed, isolated
system does not change.
Momentum within the system is
transferred to different objects
within the system.
In the example of a kicked soccer
ball, the momentum of the
kickers foot is transferred to the
ball.
Conservation of Momentum
(cont.)
D.
E.


F.
Another way of stating the Law of
Conservation of Momentum is to say that
that momentum before a collision is equal to
the momentum after a collision.
The change in momentum for the system is
zero.
http://www.physicsclassroom.com/mmedia/momen
tum/momentumTOC.html
http://library.thinkquest.org/27948/momentum.htm
l
http://library.thinkquest.org/27948/momentum.htm
l
VIII. Conservation of Momentum
Practice Problems
1.
A 550-kg car traveling at 24.0 m/s collides head-on
with a 680-kg pick-up truck. Both vehicles come to
a complete stop on impact.
A. What is the change in momentum of the car?
(-13200 kgm/s)
B. What is the change in momentum of the truck?
(13200 kgm/s)
C. What was the velocity of the truck before the
collision? (-19.4 m/s)
IX. Elastic and Inelastic
collisions
A.
In an elastic collision, the objects
that collide do not stick together.
1.
B.
Example: Getting hit in the side of
the head with a ball.
In an inelastic collision, the
objects that collide stick together.
Example: Catching a ball.
http://library.thinkquest.org/27948/c
ollision.html
1.
http://www.physicsclassroom.com/cl
ass/momentum/u4l1b.cfm

http://www.youtube.com/watch?v=eQ3RfKPP
O2o&feature=related&safety_mode=true&per
sist_safety_mode=1
X. Elastic and Inelastic
Example Problems
A.
A 0.105-kg hockey puck moving
at 48 m/s is caught by a 75-kg
goalie at rest. With what speed
does the goalie slide on the ice?
B.
Two opposing hockey player , one of
mass 82-kg skating north at 6.0 m/s
and the other of mass 70 kg skating
south at 3.0 m/s collide and get
tangled. In what direction and with
what velocity do they move after the
collision? ( 1.86 m/s North)
Elastic and Inelastic Example
Problem
C.
A 35.0-g bullet moving at 475
m/s strikes a 5.0-kg stationary
block. If the bullet leaves the
block traveling at 275 m/s, how
fast is the block moving after the
collision?
XI. Conservation of
Momentum
A.
Momentum is also conserved if
you are dealing in two
dimensions.
1.
You have to look at the x-direction
separate from the y-direction, just
as we did in the last unit.
XII. More Practice Problems
1. A 6500-kg freight car traveling at 2.5 m/s collides with a
8000-kg stationary freight car in an inelastic collision. How
fast is each car moving after the collision? (1.1 m/s, 1.1 m/s)
2. Two lab carts are pushed together with a spring mechanism
compressed between them. Upon release, the 5.0-kg cart
repels one way with a velocity of 0.12 m/s while the 2.0-kg cart
goes in the opposite direction. What velocity does it have?
(0.30 m/s)
`
3. Tim, m=42.0 kg, is riding a skateboard, m=2.0 kg, traveling at
1.2 m/s. Tim jumps off the skateboard and the skateboard
stops dead in its tracks. In what direction and with what
velocity did he jump? (1.26 m/s in the same direction he was
riding)