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HONORS PHYSICS DAY 28

Here we go!
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Today:
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HW Quiz on the last HW’s
We do great stuff with MOMENTUM
We learn about IMPULSE
We predict before we do math
We learn about graphs
HW QUESTIONS?
Answers up. Questions?
 Reminder: HW is for practice to make sure you
are where you should be. Seek me for help.
 We are about to go fast, so if they are not making
sense, see me. If you are behind, there is time to
catch up.

Get out the 3 HW’s for me.
 HW 4.3
 HW 4.2
 Hon Day in Class

HW QUIZ

Only the 3 HW’s, no formula sheet or calulators.
 Reminder:
 Watch
this video of two cars
colliding: There is a slight difference
in impact speed between them.
 Get
ready to answer some questions
about it.
 In
both scenarios, did the hit car receive
the same amount of momentum as the
other scenario?
 A)
Yes, Newtons 3rd law says that the
forces are the same.
 B)
Yes, these are isolated systems
 C)
No, both cars impacted at different
speeds, and moved different amounts
 D)
No, it was not an isolated system
 Is
the car-car impact an isolated system?
 A)
Yes, there was no outside force on the
cars
 B)
Yes, the cars are the objects before and
after the impact
 C)
No, the surface alone had an outside
force on the cars as friction.
 D)
No, the car-car system had outside
forces and changed mass
 How
did the change of momentum of the
hitting car compare to the change in
momentum of the hit car?
 A)
The same during the moment of
impact, both in magnitude and direction.
 B)
The same during the moment of
impact, in magnitude but not in direction.
 C)
Different due to it not being isolated.
 How
did the FORCE from one car to the
other compare, along with the TIME they
impacted?
 A)
The amount of FORCE and TIME are the
same during the impact.
 B)
The amount of FORCE is the same, but
the TIME of impact is different
 C)
The amount of FORCE is different, but the
TIME of impact is the same.
 D)
The amount of FORCE and TIME
different during the impact.
 What
if this was a big SUV vs. a smart
car. How would the FORCE from one to
the other compare along with the TIME
they impacted?
 A)
It would change because of the mass
difference between the two things.
 B)
It would not change even though the
masses of the two things are different.
OUR EXAMPLE
 Car
A has a mass of 1500kg and
travels to the right at a speed of
20m/s.
Car B travels left with a speed of
10m/s.
After the vehicles collide, they stick
together and move left with a common
velocity of 5m/s.
LOGIC
Which
car has more momentum,
A or B?
A) A! It has more velocity than B
B) B! After the collision they are
both going left, so B had more
C) Don’t know until we do math
 How
much did Car A and Car B’s
velocity change by?
 A)
A changed by -15, B changed by 5
 B) A changed by -15, B changed by -5
 C) A changed by -25, B changed by
15
 D) A changed by -25, B changed by
-15
 From 20 Right to 5 left is a change of
25!
LOGIC
So
if A changed by -25 and B
changed by 5, how much bigger
is B compared to A?
B
is 5 times as big, since it
changed by 5 times less!
OUR EXAMPLE


Car A has a mass of 1500kg and travels to the
right at a speed of 20m/s. Car B travels left with
a speed of 10m/s. After the vehicles collide, they
stick together and move left with a common
velocity of 5m/s.
1. Calculate the mass of Car B
Use your tables, do it on the blank slide to the side

Idea: Compare the momentums before and after
LETS GO BACK, AND DETERMINE THE
CHANGE OF MOMENTUM FOR EACH
 The
CHANGE of MOMENTUM for a
single thing is called the IMPULSE.
 In
this system, both objects had the
same IMPULSE, but in different
directions.
 So
for the cars, lets say this occurred
over a .5 second interval.
 We
can use the CHANGE of
MOMENTUM to find the FORCE
with a time.
 On
the next slide, lets do some math
Front Car
Getting a from Vi, Vf and t
Back Car
Getting a from Vi, Vf and t
Using Imbalance to get F
Using Imbalance to get F

 The
CHANGE of MOMENTUM for a
single thing is called the IMPULSE.
 IMPULSE:
 It
J
can be found through two ways:
J
Front Car
Using the Impulse:

Back Car
Using the Impulse:
 So
for the cars, lets say this occurred
over a 2 second interval.
 On
the next slide, lets do some math
Front Car
Using the Impulse:

Back Car
Using the Impulse:
 What
is NOT set is the amount of
FORCE it takes.
 If
you slow down over a long time:
 The amount of force needed is
SMALL.
 If
you change over a quick time:
 The amount of force needed is
LARGE

Showing this through bar graphs:
 How
can you tell this is an isolated system?
Lets turn it into a non-isolated one
Idea
In Words
Impulse
The change of
Symbol is Δp momentum of
or J
a SINGLE
object
In Equation
Units
Δp or J =
Kg*m/s
Pfinal-Pinitial
or a N*s
OR
Δp or J =
Fave*t
How its used
During a change
of motion, a force
for a certain time
causes that
change in motion
 How
does the impulse of the person
compare to the impulse of the box?
 A)
The impulse of the person is
larger than the impulse of the box
 B) The impulse of the person is equal
to the impulse of the box
 C) The impulse of the person is less
than the impulse of the box
 How
does the impulse of the person
compare to the impulse of the box?
 A)
The impulse of the person is
larger than the impulse of the box
 B) The impulse of the person is equal
to the impulse of the box
 C) The impulse of the person is less
than the impulse of the box
 How
fast is the person going after
the collision? Which idea is correct
 A)
The persons change of velocity is
equal to the box’s change of velocity
 B) The persons gain in velocity is 1/3
the box’s change of velocity
 C) The persons gain in velocity is 2/3
the box’s change of velocity
 How
fast is the person going after
the collision? Which idea is correct
 A)
The persons change of velocity is
equal to the box’s change of velocity
 B) The persons gain in velocity is 2/3
the box’s change of velocity
 C) The persons gain in velocity is 1/3
the box’s change of velocity
WHAT IS WRONG HERE?
60kg
7m/s
20kg
0m/s
420 kgm/s
0 kgm/s
60kg
V m/s
20kg
5 m/s
60*V kgm/s
100 kgm/s
CORRECT AND MAKE IT WORK

1)
WHAT IS HIS VELOCITY AFTER THROWING
THE OBJECT??
(A)8
m/s
(B)9 m/s
(C)10 m/s
(D)11 m/s
(E)12 m/s
WHAT IS HIS VELOCITY AFTER THROWING
THE OBJECT??
(A)8
m/s
(B)9 m/s
(C)10 m/s
(D)11 m/s
(E)12 m/s