In this unit we learned all about
Newton's third law. Newton's third law states that every action has an equal
and opposite reaction. For example, person pushes wall, wall pushes person. You
are probably wondering how anyone ever moves anywhere if this is true. Well,
take a horse and buggy for example. Horse pulls buggy, buggy pulls house. Buggy
pushes ground down, ground pushes buggy up. This is the important part; horse
pushes ground backwards, ground pushes horse forwards. That is why people walk
and cars move and also why if you put a magnet in front of a magnetic car, it
will not move because they will pull on each other and there is not another
force causing them to move. Something that surprised me when learning this is
we discovered that when an 18 wheeler and a small prius crash into each other,
they both exert the same force because of Newtons third law. The only reason
that a small car is more damaged is because it has a smaller force therefore a
bigger acceleration, which connects to Newtons second law of acceleration
equals net force divided by mass. Another thing we learned about is vectors,
which relates to Newton's third law. When someone sleds down a mountain, where
does their direction come from? Well, first of all, because of Newton's third law,
sled pushes ground down, ground pushes sled up. This force up is equal to the
weight of the sled/person, and is called the support force. Gravity, however
causes a force to pull the sled directly downward, instead of diagonally
downward to the mountain. If you take the support force line and the gravity
line and make a square, you can connect the corners to find the actual
velocity. This can be applied to which way a boat is going to go across a river
with a current, pushing a couch with two people, and an object hanging in mid
air connected with one string.
The next thing we learned about
was the universal gravitational formula, which is a formula that can tell us
lots of things but mostly it informs us about tides. The formula is force
equals gravity (which equals around 7 times ten to the negative 11) times mass one multiplied by mass two
all divided by the distance between the two squared. Though this sounds
complicated, it is easy once you do it. The easiest way to think about this is
when you relate it to tides. For example, when the moon is on the left side of
the earth, the distance to the right side of the earth is very large, therefore
the force on the ocean on that side of the earth is very small, since force and
distance are inversely proportional in this equation. Obviously, this means
that the tides on the right side of the earth are going to be low, because they
have less force. This also means that the left side of the earth is a small
distance away from the sun, meaning it will have a great force on it. This is
why one side of the earth experiences high tides while the other experiences
low tides. Now the moon is not always in the same place, so there are two
different types of tides. When the moon is either above or below earth, the
tides are called neap tides. These tides are higher or lower than normal tides
because of the distance it is from the moon. The tides when the moon is on the
right or left side of the earth are called spring tides. This is when there is
a full or new moon, and the tides are average. Because the moon has a cycle of
twenty seven days, the tides are different each day every month, therefore you
can not predict them.
The next thing we learned about
after the universal gravitational formula and tides, was momentum and impulse.
The formula for momentum is mass multiplied by velocity. We also learned about
something called the conservation of momentum, which relates to another formula
we learned about called the change in momentum. The formula for change in
momentum is p(the symbol for momentum)final minus p initial. This is basically
the same thing as the formula for conservation of momentum, however the
conservation of momentum breaks down the change in momentum a little more. The
conservation of momentum formula is mass one times velocity one plus mass two
times velocity two equals mass one plus mass two times velocity one/two (which
is the final velocity). Or, MaVa + MbVb = (Ma + Mb) Vab. This can be used to
find any one of the variables in the equation, but it is mostly used to find
the velocity after the collision. We learned about this through a lab where we
took two carts and had them almost touching and then tapped a button which made
them be pushes apart and comparing the momentum before to the momentum after.
In the same lab we used those carts and had one moving one crash into a non
moving one and found the differences in momentum. We learned that the
conservation of momentum is directly related to newtons third law. We learned
this through formulas. This is what we found:
Fa = -Fb (newtons third law, equal
amount of force)
Fa∆t = -Fb∆t (acting for the same
time)
Ja = -Jb (impulse, which
I'll explain in a second)
∆Pa= -∆Pb (conservation of
momentum)
This is how one equals the other.
The last thing we learned about
was impulse. The formula for impulse was j(the symbol for impulse) equals force
times change in time. As you saw earlier, this can be related to momentum. The
relationship between momentum and impulse is the reason why things like airbags
and the mats on gym floors keep us safe. I will explain this through the mats on
the gym floors example. The floors for gymnast are covered in pads because the
gymnasts are going from moving to not moving. Which means they are going from
having momentum to not having any momentum (referring to p=mv and ∆p=pfinal -
pinitial). Because of this, the change in momentum wil always be the same no
matter how long it takes them to stop. Change in momentum is equal to impulse
(J = ∆p), therefore the impulse will also be the same. Because the impulse is
force times change in time, the pads allow longer time to stop. The more time
it takes to stop, the less force and therefore the less injury. In other
words;
J=f∆t
Or, if the mats were no there and
it barely took any time to stop;
J=f∆t
What I have diffcult about all
that we have studied is that it is hard to keep all the facts straight in my
head. There are lots of formulas along with lots of confusing information that
have to do with the same things, and they all originate from Newton’s third
law. I have found it challenging to keep them all straight.
I overcame these difficulties,
however, by relating each one to the different examples we learned in class.
Once you relate it to an example in real life that we learned about in class,
it is much easier to keep it all straight.
My effort in this unit has been up
and down. Though I want to get a good grade, I often got frustrated with the
difficult concepts that we were learning and at times I sort of gave up.
However, the more close attention I paid to my homework and the harder I
studied for quizzes the more that I found that I could do it and know exactly
what was going on with a little big of hard work. My homework was pretty hard
effort but my blog postings and class effort could have been stronger, which I
have been trying to improve towards the end of the unit. I could have been a
lot more persistent and creative, and my self-confidence was definitely not
high. However, I thought my problem solving skills in this unit were
particularly good, considering the past view have been very bad. I think my
group members and I communicated very well however I could have had more
patience in understand the problems.
My goal for the next unit is to
always be persistent in believing that I will understand a topic no matter how
difficult. I plan to do this by going in for extra help if I need it and always
completing my homework with my full effort.
Besides the connections we made in
class to everyday life, there are tons of things that this unit can be related
to. An egg toss with conservation of momentum and impulse, car crashes,
catching and throwing balls, and many other things. The main thing I think
about when I think about this unit is throwing my phone from across the room.
When I want to leave my dorm room and I need to put my phone down, I almost
always throw it on my bed. I try to lean over to get as close as I can, and I
pull my arm down so that the curve of my phone is higher. Now, I know why
because of change in momentum, the universal distance formula, and impulse.
