Unit 3 Blog Reflection
This unit brought an even clearer
understanding for me that physics is EVERYWHERE. Over Thanksgiving Break, I was
watching a show on Netflix called Revenge. In the beginning of an episode the
narrator said “Every action has an equal and opposite reaction.” Immediately I
thought of Newton’s Third Law.
Newton’s Third Law states that
every action has an equal and opposite reaction. Newton’s second law can be
explained in the formula (a=Fnet/m)
which explains that accleration is directly proportional to
force and indirectly proportional to mass. Newton’s Third Law can use this same
formula but you can write it in a different way: (F=ma).
An
example where Newton’s Third Law is put into play can be seen when a large
truck and a small car have a head on collision. The small car experiences the
greater force because even though the truck and car exert the same force, the
truck has a greater mass therefore the car will have a greater acceleration. We
know this because of Newton’s third law, which states that every action has an
equal, and opposite reaction. This means the mass and acceleration must equal each
other out so that they will have the same force. This can be described in the
formulas below:
F=ma
Car
F (10N) =ma
Truck
F (10N)=ma
Continuing
with Newton’s Third Law, we practiced action reaction pairs. If Margaret Anne
pushes Naeem, Naeem pushes Margaret Anne. The key part about these reaction
pairs is that the verb is the same (equal force) and the reaction is opposite.
Another example is if an apple falls out of a tree. If the action force on the
apple is the force of gravity on the apple, the reaction to that force would be
apple pulls on earth. Of course, there are much more complicated examples as
well. Imagine a book at rest on a table. One of the forces on the book is the
support force and another force is the force of gravity on the book. So the
action reaction pairs would be; Earth pulls apple downward and apple pulls
earth upward. Table (support force) pushes apple upward and apple pushes table
downward. So then, we moved on to an example that might help us understand more
everyday things. For example, if forces are always equal and opposite then how
does a horse pull a buggy forward?
This image is an attempt to show the different forces going
on to cause the horse to pull the buggy forward. The reason the horse pulls a
buggy is because of a few things. We know the horse exerts the same force on
the buggy that the buggy exerts on the horse because of Newton’s Third Law which
states that every action has an equal and opposite reaction. But the reason the
horse and the buggy move forward is because the horse pushes on the ground with
a greater force than the buggy pushes on the ground. A key part of this drawing
is that the pink arrows are larger than the grey arrows to show that the horse
has a greater force than the buggy. So here are the action reaction pairs; the
grey would be buggy pushes on earth forward and earth pushes on buggy backward.
The orange would be, the horse pulls buggy forward and the buggy pulls the
horse backward. Lastly, the pink would be the horse pushes on the earth forward
and the earth pushes on the horse backward.
Then
we moved on to vectors, which seemed really terrifying at first but turned out
to be a lot easier and a lot more fun than I was expecting. A key example we
used was a box on a ramp. Why does it go down the ramp? We figured out the
reasoning by drawing vectors for the image.
You begin this drawing by drawing your fweight, which is the
light blue line. You then draw the navy blue line above it and make sure it’s
the same length as the fweight. This will allow you to draw the guide, which is
the black line parallel to the ramp. From there, you draw your support force
(fsupport), which is the red line, which must be perpendicular when it
intersects the guide. Then you must draw lines that are equal in size and
parallel to the fsupport and fweight. This creates the guide to draw the vector
or the fnet, which goes diagonally through and must be parallel to the ramp.
The fnet shows the direction in which the box is going, which is downhill.
This is another example of vectors. This shows which side of
the rope will be more likely to break (the ropes are the black lines connected
to the ball or circle. I started figuring this example out the same as the
previous example; I drew the fweight and then I drew a line equal its length
right above it (these are the blue lines). The blue line that is drawn above
the fweight helps to draw the parallel lines. These green lines were drawn to
be parallel to each of the ropes starting at the tip of the blue line. The pink
lines were drawn to determine the outcome of the problem. The rope on the left
will be more likely to break because it has greater tension. We know this
because the vertical pink line is greater than the one on the right.
Then
we moved on to the universal gravitational force formula. This formula is: F=G(m1m2/d^2) I saw this equation and
immediately became discouraged. I thought this unit was going to turn into a
complicated math unit that I wouldn’t understand and wouldn’t be able to relate
it to my everyday life. However, my outcome was much different than my
expectations. One of the first questions we were asked in this unit was “Where
do you weigh more, at the ocean or on the top of Mt. Everest? Why?” My
immediate response was that my weight would not change because when you tell
someone like your doctor your weight you don’t say “Well I weigh 237 lbs. at
the ocean but 234 lbs. at the top of Mount Everest.” Anyways I was proved wrong
because I learned that I weigh more at the ocean because my gravitational pull
is a lower elevation therefore it will be stronger. The distance is key. At the
top of Mt. Everest I weight much less because I am at a much higher elevation
therefore my gravitational pull will not be as strong. The longer the distance the less force
there is. The shorter the distance the bigger the force. Also, I was reminded that although my
weight would decrease my mass would remain the same. An astronaut weighs less
in space because he or she is farther away from the earth therefore their
gravitational pull is not as strong, however their mass remains the same.
To
put this equation into use we practiced a lot of problems. One of these
problems asked us to find the gravitational force between the earth and the
sun. Presuming that G=7X10^-11
Nm^2/C^2 , the distance between the earth and the sun is about 2X10^11 and the
mass of the earth is 6X10^24kg and the sun’s mass is 2X10^30. All I had to do
was plug these numbers into the formula and cancel out the exponents to solve
for the answer which is 21X10^21N.
Learning
about the earth’s gravitational pull brought us to the concept of tides. The
force between the earth and the moon is greater than the force between the sun
and the earth because the moon is CLOSER. Therefore, the moon affects these
tides. Tides are caused by the difference in force felt by the opposite sides
of the earth. Whichever side is closer to the moon will feel the greater force.
There are two high tides and two low tides each day (4 tides per day total).
High to low tide has a time span of 6 hours and from high tide to high tide and
low tide to low tide there is a time span of twelve hours. There is not a specific
time each day at which these tides occur. It’s constantly changing because the
moon is constantly orbiting the earth. It takes about 27 days for the moon to
complete a full orbit. There is a tidal bulge that forms around the earth.
There are two tides we learned about called neap tides and spring tides. During
spring tides the moon is either a full moon or a new moon and the tides are at
its highest highs and lowest lows. Hurricane Sandy was so destructive because
it came at a full moon therefore it was a spring tide so the tides were at its
most extreme. If Hurricane Sandy had come during a neap tide or a half moon,
the damage would have been much less severe. This is because during neap tides,
the high tides aren’t as high as usual and they aren’t as low as usual. Here
are images of each to show the position of the earth, moon, and sun during each
of these tides.
This is an image of spring tides. There are two moons in
this picture two show that the moon can either be to the left or right of the earth
during spring tides. Also notice the tidal bulge is directed towards wherever
the moon is. The high tides will
be where you see the tidal bulge and the low tides will be above and below the
earth. 
This is an image of neap tides. Again there are TWO moons in
this picture to show that the moon can either be directly above or below the
earth during neap tides. Also notice the tidal bulge is directed towards
wherever the moon is. The high tides
will be where you see the tidal bulge and the low tides will be to the left and
right of the earth. (High tides and low tides occur on opposite sides of the
earth.)
An
important note to keep in mind when thinking about tides is that lakes don’t
experience tides. Lakes don’t experience tides like the ocean does because the
mass of the lake is not nearly big enough to be affected by the pull of the
moon. Next,
we moved on to momentum and impulse. Momentum can be defined as inertia in
motion or the product of the mass of an object and its velocity. Momentum= mass
x velocity or Momentum=mv. To simplify this formula further, we use P to
represent momentum so the official formula used to find the momentum of an
object is P=mv. Impulse
is the change in force. Impulse=
quantity force x time interval. Impulse= Ft. We use J to represent impulse. So
the official formula is J=Ft.
Impulse changes momentum. Therefore impulse= the change in momentum. Ft=
change (mv). You can increase momentum by increasing the mass or increasing its
speed. In other words to increase (change) impulse you can either increase the
force or change the time interval. This was seen when we did the egg toss in
class. Naeem and Ethan won because they decrease the impact in which the egg
landed. They changed its momentum by increasing its time (which decreased its
force).
P=mv
P=mv
P=mv
We must also take into consideration the conservation of
momentum. This by definition states that in the absence of a new external
force, the momentum of an object or system of objects is unchanged. The formula
for this is mv(before)=mv(after). Therefore the change in P will always equal
Pfinal-Pinitial.
Change
in P= Pfinal- Pintial
Change
in P=mvfinal-mvinitial
Take airbags into consideration. Why do airbags keep us
safe? Air bags keep us safe because they slow down the force by increasing the
time. The change in momentum is always the same with the dash or the airbag. P=mv.
Change in P= Pfinal-Pinitial. If change in P is the always the same so is the
impulse whether you hit the dash or the airbag. J=Change in P. However, the
airbag increases the time of the impulse so the force on you is less. Small
force=less injury. J=F(change)t. No airbag J=F(change)t
With airbag J=f(Change)time.
Impulses
are greater when an object bounces off an object. Impulse required to bring an
object to a stop and then “throw it back again” is a greater impulse than the
impulse required to just bring an object to a stop. This also ties into the
conservation of momentum. Newton’s second law states that net force must be
applied for acceleration. If you want to change momentum you must exert
impulse. Only an impulse external to a system can change the momentum of the
system. Otherwise the change in momentum will be the same before and after. For
example, car bumpers are made of plastic and no longer made of rubber, which
was popular for awhile, because the rubber bounces therefore a greater force
would be exerted upon the car because according to Newton’s Third Law every
action must have an equal and opposite reaction whereas plastic doesn’t bounce
therefore the force exerted will be smaller. The plastic crumples which
increases time however the impulse is the same.
Ja=-Jb
Change in Pa= -Change in Pb
Change in Pa+Change in Pb=0
Conservation of momentum says that impulse causes the
momentum forces to be equal and opposite. Impulse causes the change in
momentum.
Change in P=J
J=F(change)t
Ptotal before= Ptotal after
Ma+Va+Mb+Vb(before)=(Ma+Mb) Vab (after)
Momentum is only conserved with a system. The total momentum before and after are
always the same but individually the momentums can change.
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