Transcript Momentum (Chap. 6)

Chapter 6
Momentum
Object A hits Object B. What happens?
28-Mar-16
Physics 1 (Garcia) SJSU
Forks
In 7th Century royal courts of the Middle East
began to use forks at the table for dining
but was not until the 16th Century that
forks were common in Europe.
Early table forks were modeled after kitchen
forks but in the late 17th Century forks with
four curved tines appear.
The additional tines made diners less likely to
drop food, and the curved tines served as
a scoop so people did not have to
constantly switch to a spoon while eating.
By the early 19th Century, multi-tined forks
were also used in England and slowly
began to spread to America.
28-Mar-16
Physics 1 (Garcia) SJSU
Force, Momentum, Energy
With Newton’s Laws, we can understand
motion just using forces. Can also eat food
just using knives.
Easier to understand motion by introducing
concepts of momentum and energy.
Think of them as the fork and spoon of
mechanics.
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Physics 1 (Garcia) SJSU
Momentum
Momentum of an object is,
(Momentum) =
(Mass) X (Velocity)
Examples of objects with
large momentums are
supertanker (large mass)
and bullet (large velocity).
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Physics 1 (Garcia) SJSU
Check Yourself
A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h..
Which vehicle, the car or the truck, has greater momentum?
What would the car’s speed have to be for the momentums to match?
Aren’t you forgetting something?
How does that matter?
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Physics 1 (Garcia) SJSU
Momentum and Force
To stop an object with a large momentum
requires either:
• Large force (stopping the object quickly).
• Small force applied for a long time.
Notice that changing object’s momentum
depends on force and time interval.
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Impulse
Define impulse acting on an object as,
(Impulse) =
(Force on object) X (Time interval)
Objects have momentum.
Impulse acts on an object.
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Physics 1 (Garcia) SJSU
Impulse & Momentum
Impulse is related to momentum by,
(Change in momentum) = (Impulse)
or
(Mass) X (Change in velocity) =
(Force) X (Time interval)
This relation comes from Newton’s 2nd law.
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Physics 1 (Garcia) SJSU
Demo: Egg Throw
Throw a raw egg
as fast as
possible at a
plastic sheet
that’s held
loosely.
X
X
X
X
X (Hold here)
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Physics 1 (Garcia) SJSU
Check Yourself
Throw egg at sheet or wall with
same speed. Which case has:
Greater change of velocity?
Greater change of momentum?
LONG TIME
small force
Largest impulse on the egg?
Largest time of impact?
Largest force on the egg?
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Physics 1 (Garcia) SJSU
short time
LARGE FORCE
Demo: Vampire Stake
Safest when slow moving stake is placed on
a soft, fleshy spot on the chest.
(force) x (TIME)
(FORCE) x (time)
X X
Ouch!
Not safe if stake
strikes hard skull
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Physics 1 (Garcia) SJSU
Check Yourself
A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h..
The force of impact is greatest on which vehicle, the car or the truck?
The impulse is greatest on which vehicle, the car or the truck?
Change of momentum greatest?
Change of velocity greatest?
Driver injury greatest?
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Automobile Safety
Maximizing the time of impact on the driver minimizes
the force of impact. This principle used in design of:
Seatbelts
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Air Bags
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Crumple
Zones
Collisions
When two objects collide, impulse is equal and
opposite for the two objects.
Before collision
IMPULSE
IMPULSE
Impact
After collision
Each object has equal and opposite change in momentum.
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Physics 1 (Garcia) SJSU
Conservation of Momentum
Since change of momentum in a collision is equal
and opposite, the momentum gained by one
object is the amount lost by the other.
Momentum
Object A
Before Collision
A
+
Momentum
Object B
Before Collision
=
Momentum
Object A
After Collision
A
B
+
Momentum
Object B
After Collision
B
Actual amount of momentum exchanged depends on the details of
the collision, such as whether or not collision is elastic.
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Physics 1 (Garcia) SJSU
Demo: Elastic Collisions
Objects of equal mass exchange momentum
on elastic collisions.
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Demo: Newton’s Balls
Steel balls collide elastically, exchanging
momentum on collision.
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Physics 1 (Garcia) SJSU
Demo: Don’t Scratch
To sink a billiard ball that is very close to the
pocket without having the cue ball go in as well
(“scratching”) strike the cue ball hard so it makes
a crisp, elastic collision.
As with Newton’s balls, cue
ball will stop after giving all
its momentum to the other
ball in the collision.
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Physics 1 (Garcia) SJSU
Demo: Blaster Balls
When masses unequal, momentum change can be large.
Speed of
ping-pong ball
is 3x larger
(Slingshot effect)
Ping
pong
ball
Golf
ball
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Demo: Inelastic Collisions
Objects stick together after colliding.
A
B
A
B
A
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B
Note: Use this concept in
lab experiment entitled
“Projectiles”
Check Yourself
Large (4 kg) fish swims at 3 m/s towards a small
(2 kg) fish and swallows it for lunch.
Total momentum before lunch?
Total momentum after lunch?
Velocity of the large fish (with small fish inside)?
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Physics 1 (Garcia) SJSU
Recoil
Momentum conservation also explains recoil
(MASS) x (velocity)
(mass) x (VELOCITY)
Recoil effect is like an inelastic collision in reverse.
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Physics 1 (Garcia) SJSU
Complicated Collisions
Collisions at an angle (not head-on) are
more complicated. Learn by playing pool.
8
8
8
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Physics 1 (Garcia) SJSU