Angular/ Rotational Momentum

Angular/Rotational Momentum is the rotational inertia multiplied by the rotational velocity. Rotational inertia is an object's resistance to spinning around an axis. Rotational velocity is the number of rotations per unit of time. If the guy who flew off of the merry-go-round, had been closer to the axis of rotation (the center of the merry-go-round), it would have been harder for him to fly off of it, due to the tangential speed. Tangential speed increases the further away one is from the axis of rotation, therefore, if the guy had been closer to the center, he would not have spun as quickly, possible saving himself from flying off. The Distribution of mass is important because it affects the rotational inertia. The closer to the axis of rotation the mass is, the quicker the object will spin. The further away the mass is from the axis of rotation, the slower the object will spin. The Distribution of mass changing will cause the rotational inertia to change, which causes the rotational velocity to change.

Unit 3 Reflection

This unit, I learned about…

A.    Newton’s 3^rd Law and  Action/Reaction Pairs

• Newton’s 3^rd Law of Motion: “every action has an equal and opposite reaction.”
• Action Reaction Pair: Hammer pushes nail down, Nail pushes hammer up

B.        Tug of war/horse and buggy

• The Horse exerts a greater force on the ground than the buggy exerts on the ground, and therefore, the system is able to move.

C.        Forces in perpendicular directions

1. Draw Fgravity/Fweight straight down
2. Draw Fsupport Perpendicular to the GROUND
3. Draw Fnet

If done correctly, Fnet will be parallel to the ground and pointing downhill.

"Therefore, the box accelerates downhill."

D.        Gravity and Tides

Spring: Higher Highs
             Lower Lows
Neap:   Lower Highs
             Higher Lows

Gravity:

F=(Gm₁m₂)/d²
F=1/d²
x2 distance   = 1/4 original force
x3 distance   = 1/9 original force
x4 distance   = 1/16 original force
x1/2 distance= x2 original force
x1/3 distance= x9 original force
x1/4 distance= x16 original force

E.        Momentum – and Impulse momentum relationship

P=mv

Change in Momentum= P final- P initial

Change in Momentum= mv final-mv initial

Change in Momentum= Impulse

Impulse=(Force)(change in time)

J=Newtons
P=Kgm/s

Correct way to solve egg toss problem:

P=mv
Change in Momentum= P final- P initial

Regardless of how the egg is stopped, it will go from moving to not moving, therefore the change in momentum is the same, regardless of how it is stopped.

Change in Momentum= Impulse

Since the change in momentum is the same regardless of how the egg is stopped, the impulse is also the same.

J=Ft

The Winners increased the time it took to stop the egg, thus because the impulse is constant we can predict that the force will be less on the egg. A small force is less likely to break.

F.         Conservation of Momentum

****Individual objects can change their momentum, but systems cannot*****

Ptotal before=Ptotal after

Together in the Beginning:

m_av_a+m_bv_b = m_av_a+m_bv_b

Together in the Ending:

m_av_a+m_bv_b=m_a+bv_ab

Cart Problems:

****REMEMBER THAT IF THE CARTS ARE MOVING IN OPPOSITE DIRECTIONS, TO MAKE ONE OF THE VELOCITIES NEGATIVE***********************************************

Extra stuff:

Lab equation Steps:

1. Write equation of a line (y=mx+b)
2. Fill in y and x units (Ptotal before=slope(Vab))
3. Slope=___
4. Compare the theoretical to the actual slope
5. if close, yo confirm the law

Bouncing:

Bouncing creates more force. x2 impulse because 1 for stopping and 1 for pushing off. This is why police vests absorb bullets instead of bouncing them off.

Reflection:

What I have found difficult is remembering the simple things.

I have (hopefully) overcome these difficulties by writing myself disclaimers and making things I find tricky very noticeable.

I don't feel like me creativity, persistence, use of creativity, self-confidence in physics, skills in solving difficult problems, communicating both by my spoken and written words, collaborating with my group members, has changed since the first unit. I still feel confident, though the quizzes after the break took their toll on me a bit, but I think I am  back. It was just hard remembering all the things we had learned before the break, and then having no review time for what I, and the majority of the class, had forgotten. I think that I need to find better ways to not forget things. 

Tides Video Source

I found this video by Hewitt to be very helpful. Since he is the one who wrote the textbook we use, this information can be seen as accurate. Hewitt explains that the elongation of the earth is dependent of the lunar pulls on it. He also explains that the sun plays an affect of the tides. When the sun, moon, and the earth are aligned, there are high tides, called spring tides (although they have no correlation with the spring season). Spring tides will occur at the time of a full or a new moon. When the moon is positioned at 90 degrees between the sun and the earth, neap tides will occur.

Unit 2 Reflection

Topics Covered This Unit

Free Falling (straight down)         Free Falling (throwing things up at an angle)

Free Falling (at an angle)                    Free Falling (throwing things straight up)

 Newton’s Second Law                        Falling with Air Resistance (Skydiving)

Important Relationships

“Acceleration is directly related to force and is inversely proportional to mass.”

a=F/m

Important Equations

d=(1/2)gt²        a²+b²=c²

v=gt                     v=d/t

What Equations go with What

	Vertical	Horizontal
How Far	d=(1/2)gt2	v=d/t
How Fast	v=gt	v=d/t

Newton’s First Law

a=F/m              w=mg              a=1/m              a~F

“Acceleration is directly related to force and is inversely proportional to mass.”

To increase acceleration, increase the force, or decrease the mass. To decrease the acceleration, decrease the force, or increase the mass

Translating it to graph:

y=mx+b à acceleration=Fnet (1/m)            slope= fnet

Falling with Ari Resistance (Skydiving)

	Increasers of Air Resistance
1.)	Increase of Surface Area
2.)	Increase of Speed

When a person falls through the air, their acceleration decreases, their velocity increases and their Fnet decreases

acceleration=(Fweight-Fair)/(mass)

When a person opens their parachute, their acceleration changes direction (because Fair is larger than Fweight), their velocity stays same direction but slows down (because person is still falling), and their Fnet changes direction (upwards) because Fair is larger than Fweight.

Why does a lead ball hit the ground before a ping pong ball when dropped from a building and not when falling from a table?

This is because from the building, there would be enough time for the two balls to reach terminal velocity. The steel ball will go faster because it has a greater weight than the ping pong ball. This makes the lead ball have to compensate by going faster, which increases it’s Fair.

How do the velocities,  acceleration, and net-forces compare when a skydiver is skydiving without the parachute open, and after the parachute is open (both times in terminal velocity)?

The only thing that is different between the two is that the velocity is slower. Netforce is the same, and acceleration is the same because the weight of the diver does not change, meaning their F-weight does not change, meaning that if the diver is to reach Terminal Velocity, it must retain the same F-air, meaning the net-force is the same.

During Terminal Velocity…

Acceleration is 0m/s²                                   Velocity is constant

Netforce is 0N        Diver is at their fastest point possible

Free Falling (Straight Down)

THE ONLY FORCE IS GRAVITY

Weight does not matter

“When an object falls due to the effect of gravity ONLY”

How Far	d=(1/2)gt2
How Fast	v=gt

acceleration=gravity

Free Falling (at an angle)

The only thing that determines time in the air is vertical height

	Vertical	Horizontal
How Far	d=(1/2)gt2	v=d/t
How Fast	v=gt	v=d/t

Falls in a parabolic curve

Free Falling (throwing things up at an angle)

a²+b²=c² will help you find actual velocity

These are special triangles that will appear on a test. The Hypotenuse is the actual velocity (c)

	Vertical	Horizontal
How Far	d=(1/2)gt2	v=d/t
How Fast	v=gt	v=d/t

Acceleration at the top of an object’s path will be 10m/s² because of gravity

Velocity at the top of an object’s path will be the horizontal velocity because vertical velocity=0

Free Falling (throwing things straight up)

Always measure things from rest, meaning measure time and distance from the top of an object’s path, down.

Velocity at the top of an object’s path will be 0m/s

Acceleration at the top of an object’s path will be 10m/s² because gravity does not stop working.

Free Falling

This source is helpful because it talks about a misconception of free falling; that heavier objects fall faster than the lighter ones. This clip gives examples of why this is not true, by telling us that Galileo discovered that all objects fall at the same speed towards the earth, and the "lighter bodies fall at a slower rate due to the resistance of the air." This video also shows this through showing an experiment conducted by Robert Boyd, where he took out the air of a bottle, with a feather and a penny inside. When flipped over, the feather and the penny fell at the same time, thus proving that air resistance is the reason things fall faster than others.

Newton's 2nd Law

In this video, the creators made a song about the equation for Newton's 2nd Law of Motion; f=ma. They also had a few clips playing, trying to show how acceleration is directly proportional to force, and is inversely proportional to mass. In the beginning of this video, we are given an example of how this equation is applied when they ask us what force did the hockey player exert on the other to make him fall over. They then show us how to solve the equation on the screen, and move onto the next few clips, showing different things either falling, or bouncing off of another object.

Unit 1 Reflection

In this unit, I learned about acceleration, velocity, netforce and equilibrium, inertia, and using graphs and equations. 

Important measurements to remember are...

Time= Seconds

Force= Newtons

Distance= Meters

Mass= Grams/ Kg

Velocity= Meters/Second

Acceleration= Meters/Second^2

Important equations to remember are:

	Acceleration Equation	Velocity Equation
How Far:	d= (1/2)at^2	d= vt
How Fast:	v=at	v=d/t

Important definitions to remember:

Netforce- the total force acting upon an object

Equilibrium- when netforce is equal to 0N; either when an object is at rest, or at constant velocity

Acceleration- how fast something is speeding up 

mathematical--- acceleration= (change in velocity)/(change in time)

Constant Acceleration- going faster at a constant rate

Velocity- how fast an object is moving over a period of time

Constant Velocity- keeps same speed and direction

Inertia- an object's non-wanting to change states (is measured by an object's mass)

Newton's 1st Law- "An object in motion tends to stay in motion unless acted upon by an outside force. An object at rest tends to stay at rest unless acted upon by an outside force."

Things to remember:

• You can accelerate while keeping the same speed only when going in a circle without changing speed.
• Cannot have changing speed and have constant velocity.

o   A= raising velocity, constant acceleration

o   B=raising velocity, increasing acceleration

o   C=raising velocity, decreasing acceleration

•  Difference between velocity and speed is that v requires a specific direction and speed does not
• Vectors are arrows and they show magnitude and directing
• When something is at equilibrium, net netforce is 0N because all the forces a constant and opposite.
• The force of friction upon an object will always be equal to its opposite force, unless the object is constantly accelerating

What I have found difficult about this section is the problem solving questions. I feel that I didn't show my full potential with them, and get frustrated when I start on the "right track" but then think that I am over thinking it, and do something, ignoring the facts that are right in front of me. How I am overcoming this is really making sure I can "see" the problem in front of me. I feel like the problem is potentially simply solved (at least for me), but they are just a bit tricky. I plan to fully understand why answers can and can't be, and to not "x-out" answers just because I "think", without any concrete proof, they are wrong.

I think that I did well in this unit. I always did my homework before class, I understand the material, and I am consistent with my work. I started off kind of rough, not wanting to be in this class, but once I found my niche, I am much more confident about my skills. I can grasp the concepts easily, which makes me feel good, rather than bored for when I grasp a concept, I put an effort to help others who might have a hard time with it. This helps me understand the concept more, helps others understand the concept, raises my confidence, and helps me with finding helpful ways to explain things (which is both helpful in class and on my blog). I have increasingly and surprisingly enjoyed the blog posts, which I find helpful as well as fulfilling, for I can really tell what my progress is and isn't. It helps me discover what concepts I REALLY know, and what I need to work more on. 

My goal for the next unit is to be more organized. I have barely used my binder and only put things inside my book or in a place in my room. Over the long weekend, I plan on organizing all of my binders which will help me in all of my classes.

I find it easy to name connections between physics and the outside world, or I feel a day doesn't go by when we don't talk about where concepts show themselves and how. An example of inertia is the tablecloth trick. An example of accelerating while keeping the same speed is going in an exact circle at the same speed. It is really easy to connect physics to the outside world because it is basically dealing with the outside world.