Transcript chap 6 A - Rush Arts Physics

1) Go over tests
2) Science and math in the US
DO NOW:
1) I will hand back Black
History Month reflections
2) You will write a definition
for “momentum” based on
your present knowledge
3) We will watch an X-Men clip!
Juggernaut from
X-Men 3: The Last Stand
Chapter 6: Momentum and Collisions
Section 1: Momentum and Impulse
Essential Questions:
1) Why must trucks start braking sooner
than cars? (according to the
momentum-impulse theorem instead
of the work-kinetic energy theorem)
2) Why do firemen use a mattress to save
people jumping from a height?
Notes: Momentum
~ definition: the product of an object’s
mass and velocity
~ symbol: p
~ SI unit: kg(m/s)
~ equation: p = mv
~ a vector (because velocity is a vector)
Notes: Impulse
~ definition: the change in momentum
~ symbol: ∆p
~ SI unit: kg(m/s) or N(s)
~ equation: ∆p = F∆t
~ equation comes from…
F = ma → F = m∆v/∆t → F = ∆p/∆t
Notes: Momentum-Impulse Theorem
~ ∆p = F∆t = mvf – mvi
~ the greater the momentum of an object,
the greater the time required to stop it
(when force is constant) ∆p↑ = F ∆t↑
~ the greater the time interval to stop an
object, the lesser the impact force (when
momentum is constant) ∆p = F ↓ ∆t↑
Homework
Page 204 #1,4
Page 201 #1,3
Use the Sample Problems on the
same pages if you need help
DO NOW:
1) I will check homework
2) You will do #1 on pg 199
AGENDA:
1) Go over page 199
2) Review impulse
3) Go over homework
4) Answer the first essential question
Chapter 6: Momentum and Collisions
Section 1: Momentum and Impulse
Essential Question:
1) According to the momentum-impulse
theorem why must trucks start
braking sooner than cars?
DO NOW:
Finish challenging problem from last class
OR
1)What is the relationship between mass and
stopping time?
2)What is the relationship between speed and
stopping time?
3)What is the relationship between momentum
and stopping time?
Chapter 6: Momentum and Collisions
Section 1: Momentum and Impulse
Essential Question:
2) Why do firemen use a mattress to save
people jumping from a height?
Homework
Bring in materials
from home for your
“Save the Egg!” project
DO NOW: help others if you solved this problem
During last year’s NASCAR Sprint Cup, two racecars passed
the finish line at the exact same time and they were both
traveling an average speed of 202 mph (1 mi = 1,609 m). As
they crossed the finish line, they both started braking with a
force of 33,000 N because they wanted to check the replay
video to determine who won. The mass of car #1 was 1.25
times greater than the mass of car #2. How long did it take
each car to stop if the mass of car #2 was 704 kg? What
conclusion can you make about the relationship between
mass and stopping time?
Review of the Project:
Pretend that we are an engineering firm and have been
asked to recreate saving devices for firefighters. Your job is
to design and build a structure that will save somebody (an
egg) falling from a high building (the school lobby).
You must work within these perimeters:
1)Use at least three separate materials
2)The structure must be set up prior to testing it (ie: you
must be able to transport your structure from my room to
the lobby)
3)The structure must require some construction (ie: you
cannot just throw a blanket and cotton balls into a box)
Physics Concept:
The greater the time over
which a collision occurs, the
smaller the force of impact.
Thus, to minimize the force
on an object, the time must
be increased.
Exhibit A
Fans of boxing frequently observe this same principle of minimizing
the effect of a force by extending the time of collision. When a boxer
recognizes that he will be hit in the head by his opponent, the boxer
often relaxes his neck and allows his head to move backwards upon
impact. In the boxing world, this is known as riding the punch. A
boxer rides the punch in order to extend the time of impact of the
glove with their head. Extending the time results in decreasing the
force and thus minimizing the effect of the force in the collision.
Merely increasing the collision time by a factor of ten would result in
a tenfold decrease in the force.
DO NOW: help others if you solved this problem
Two farmers were bored so they decided to race their tractors
down a cornfield. It just so happens that both farmers were
90.7 kg and their John Deere tractors both had a mass of 1,751
kg. Tractor #1 had a speed of 28 mph (1 mi = 1,609 m) and
tractor #2 had a speed of 32 mph just before reaching the end
of the field. How long did it take each tractor to stop in time
to avoid hitting the cows at the end of the field if they both
applied a braking force of 8,888 N? What conclusion can you
make about the relationship between speed and stopping
time?
Exhibit B
Air bags are used in automobiles because they are able to minimize the effect of
the force on an object involved in a collision. Air bags accomplish this by
extending the time required to stop the momentum of the driver and passenger.
When encountering a car collision, the driver and passenger tend to keep
moving in accord with Newton's first law. Their motion carries them towards a
windshield that results in a large force exerted over a short time in order to stop
their momentum. If instead of hitting the windshield, the driver and passenger
hit an air bag, then the time duration of the impact is increased. When hitting an
object with some give such as an air bag, the time duration might be increased by
a factor of 100 thus decreasing the force by a factor of 100.
DO NOW: help others if you solved this problem
Two tractor-trailers are traveling 55.0 mph (1 mi = 1,609 m) on
I-95 S. Truck #1 is half the mass of truck #2 because it is not
carrying any cargo. As they go around the curve near
Cottman Ave, they don’t expect to see traffic at a halt because
of a recent accident. To avoid creating another accident, they
both slam on their brakes with a force of 25,000 N. How long
will it take each truck to stop before hitting the car in front of
them if the mass of truck #1 is 2,500 kg? What conclusion
can you make about the relationship between momentum
and stopping time?
Exhibit C
Nylon ropes are used in the sport of rock-climbing for the same reason. Rock
climbers attach themselves to the steep cliffs by means of nylon ropes. If a rock
climber should lose her grip on the rock, she will begin to fall. In such a situation,
her momentum will ultimately be halted by means of the rope, thus preventing a
disastrous fall to the ground below. The ropes are made of nylon or similar material
because of its ability to stretch. If the rope is capable of stretching upon being
pulled taut by the falling climber's mass, then it will apply a force upon the climber
over a longer time period. Extending the time over which the climber's momentum
is broken results in reducing the force exerted on the falling climber. For certain,
the rock climber can appreciate minimizing the effect of the force through the use
of a longer time of impact.
AGENDA:
1)TEST YOUR EGG SAVER!
2)Vote for the “Save the Egg”
awards
3)Discuss the lab report
4)Clean Up!
Homework
Work on your
“Save the Egg!”
lab report
DO NOW: help others if you solved this problem
During a street race in Fast and Furious, two guys race in the
middle of the night to prove their manliness. It just so
happens that both guys are 100.0 kg and their cars are both
1,000.0 kg. Car #1 has an average speed of 110 mph (1 mi =
1,609 m) and car #2 has an average speed of 100.0 mph. In the
middle of the race, they see a group of cop cars approaching
them so they slam on their brakes with a force of 4,000.0 N.
How long does it take each car to come to a stop? What
conclusion can you make about the relationship between
momentum and stopping time?
Exhibit D
In racket and bat sports, hitters are often encouraged to follow-through when striking a
ball. High-speed films of the collisions between bats/rackets and balls have shown that the
act of following through serves to increase the time over which a collision occurs. This
increase in time must result in a change in some other variable in the impulse-momentum
change theorem. Surprisingly, the variable that is dependent upon the time in such a
situation is not the force. The force in hitting is dependent upon how hard the hitter
swings the bat or racket, not the time of impact. Instead, the follow-through increases the
time of collision and subsequently contributes to an increase in the velocity change of the
ball. By following through, a hitter can hit the ball in such a way that it leaves the bat or
racket with more velocity (i.e., the ball is moving faster). In tennis, baseball, racket ball,
etc., giving the ball a high velocity often leads to greater success.
SAVE
THE
EGG
AWARDS