UNIT 3 REVIEW

In this Unit, we reviewed 6 major topics

1. Newton’s 3rd Law and  Action/Reaction Pairs
2. Horse and Buggy Problems
3. Gravity and Tides
4. Momentum and Impulse Momentum
5. Conservation of Momentum

Newton's 3rd Law
Newton's 3rd Law States that every action has an equal and opposite reaction
When talking about this law, we use the equation F=ma

When there is greater mass, the acceleration will be less:

F= ma

F= ma

When mass increases, the acceleration will decrease to equal the force and when mass decreases, the acceleration will increase (as shown above)

Action Reaction Pairs
When explaining Newton's 3rd Law, we use action-reaction pairs. There are 3 rules to remember when writing these action reaction pairs:

1. Verb stays the same
2. Direction stays the same
3. Objects stay the same, but switch

Here is an example:

To explain the blue arrow, you would say:

Apple pushes Table down

To explain the yellow arrow, you would say:

Table pushes Apple up

Here is a video to explain:

Perpendicular Forces

To explain perpendicular forces, we draw vectors.  For example, how does a block slide down a ramp?

Draw lines perpendicular to the force of gravity and the support force.  The point where these lines meet becomes the direction of the F Net.

Horse and Buggy (or Tug of War)

One difficult problem is the "horse-and-buggy" problem.  The table below shows each vector you need to know to explain how a horse and buggy move forward:

Here is how you would write each reaction pair for each pair of arrows:

A: Buggy pulls Horse back, Horse pulls Buggy forward

B: Earth pushes buggy back, Buggy pushes Earth forward

C: Ground pushes Horse forward, Horse pushes Ground backward

  However how does the system, as a whole, move forward? As illustrated, the forces between the horse and ground are greater than those between the buggy and ground.  This is because the horse has more force of friction between it and the ground, therefore it overpowers the buggy's backward force, and the system moves forward.

Gravity and Tides

The Earth's natural tides can be explained through the pulls of gravity and Newton's 3rd Law.

Each day there are 4 tides in total:

• 2 high tides
• 2 low tides

You can explain these tides through Newton's 3rd Law.

Notice how, in the image below, the distance A is lesser than the distance B.

Because the distances are different, point A and point B will experience different forces from the moon's pull.

We know that Force and Distance are inversely proportional.

Therefore, 

• Since B has a greater distance, the force will be lesser,
• And since A has a lesser distance, the force will be greater.

This DIFFERENCE IN FORCE causes the water surrounding Earth to create an oval:

However, the sun also affects the tides, but less drastically, This is why we have Spring Tides and Neap Tides:

During Spring Tides, since the sun is in line with the moon, you end up getting:

• Higher high tides
• Lower low tides

Momentum and Impulse Momentum

Momentum= movement of mass

The character used for momentum is P
The equation that expresses momentum is:

P=mv

Change in momentum is the same, regardless of speed.

Impulse=force upon something and HOW LONG that force is applied
The character used for Impulse is J
The equation that expresses impulse is:

J = F * ∆ t

Momentum and impulse are connected through this equation:

J= ∆ P

Bullet-proof Vests

This means that time is inversely proportional to force when entered in an impulse equation.  Therefore, in order to decrease the force of impulse, you must increase the time of the impulse.  For example, a bullet proof vest.  To decrease the impact (F) of the bullet, the material used in a bullet-proof vest increases the change in time (∆t) of the impulse of the bullet.  Therefore, the the impact of the bullet is "absorbed" and the person wearing the vest experiences less force.

Conservation of Momentum

Conservation of momentum states that momentum can neither be destroyed nor created.

The two most important equations to remember when explaining conservation of momentum are for objects hitting and bouncing and objects and objects hitting and sticking.

When an object hits and bounces you use the following equation:

MaVa+MbVb=MaVa+MbVb

When objects stick you use the following equation:

 MaVa+MbVb=Ma+b(Vab)

I struggled most with change in tides. I overcame this confusion when I realized tides are not caused by distance, but by CHANGE in distance

Tides Resource

UNIT 2 REFLECTION

In UNIT 2, we learned about 5 major topics:

1. Newton's 2nd Law of Motion
2. Air Resistance
3. Free Fall (straight down)
4. Free Fall (thrown straight up)
5. Free Fall (projectile/ at an angle)
6. Free Fall (thrown up at an angle)
7. Air Resistance

NEWTON'S 2ND LAW

Newton's 2nd Law states that acceleration is proportional to force, and inversely proportional to mass.  This can be illustrated symbolically like so:

OR they can be combined, creating the equation:

One important concept involved with Newton's 2nd law is the relationship between weight and mass - they are NOT the same thing.  If in a problem, you are given the mass and must figure out what the weight is, you can use this conversion equation:

The number (10) is the constant physicists use in place of gravity (g).  In labs, to be more precise, use 9.8 as the (g).

FREE FALL - STRAIGHT DOWN

Free fall is when an object falls due to the effect of gravity only.  This means no force of air resistance.  In real life, an object can only be in free fall when in a sealed vacuum.  The equation used for an object free falling straight down is

To find how fast the ball was moving when it hit the ground, you can use the equation:

AIR RESISTANCE

There are two factors that change air resistance: speed and surface area.  As speed or surface area increases, so does air resistance, making them directly proportional.

If a person were to jump off a cliff, his force of weight (fweight) is much greater than his force of air resistance (fair).  Speed is directly proportional to air resistance.  Since all falling objects want to reach equilibrium, the only way for the person to increase his air resistance is to also increase his speed, to reach terminal velocity.  

FREE FALL - STRAIGHT UP

How is throwing something up different from dropping? 

• When throwing an object up, it has an initial velocity
• This changes our equations (d = 1/2(10)t²)and(v = gt) because they assume an object starts at rest

However, we can still use these equations to find the total distance traveled by the object.  To do this, you can still use the (d = 1/2(10)t²)This only gives us half the parabola, so you must then multiply that amount by 2 to get the total time or distance.

The best way to answer problems involving straight up free fall is by drawing a picture like this:

Note the number of seconds and the velocity of the ball at each second.

FREE FALL - PROJECTILE/ AT AN ANGLE

Projectile throwing is much like free fall straight down, however, one must take the horizontal velocity into account as well as the vertical.  Finding horizontal velocity is easier because it stays constant (due to Newton's first law).

If given the distance (d) in the graph shown above, you can use the  (d = 1/2(10)t²) equation to solve for the time (t).  Once you know the time, you can use the (v=d/t) equation to find the horizontal velocity.  

FREE FALL - THROWN UP AT AN ANGLE

Throwing an object up and at an angle is simply a compilation of the skills we learned from straight up free fall and free fall at an angle

All the problems we are given this year will have an angle of 45 degrees.  You can determine the height, distance, horizontal velocity and vertical velocity of the object using skills from the lessons above. 

 However, to find the diagonal velocity of the ball, you must remember your special right triangles:

(Note: The square root of 2 equals around 1.41, so if you see a measurement of 14.1, 141, 1,410, assume the triangle is this special right triangle)

For example, if you know the horizontal velocity is 30, and the vertical velocity is 40, you can assume the diagonal distance id 50:

Free Fall Video Resource

Free Fall: An object falling without the force of air resistance, only being affected by the force of gravity on the object (about 10N)
This video shows how a feather and penny fall in a tube in contrast to in a vacuumed tube, without any air resistance.  In the vacuum, because both objects have only one, equal force of gravity, they will reach the bottom of the tube at the same time by then end of the video.

Newton's 2nd Law Video

This video covers the basics of Newton's first law very well.  It gives the formulas as well as explaining them in terms of real life situations, demonstrated through fun, cartoon animations.

UNIT 1 REFLECTION

In Unit 1, we learned about 5 major concepts:

1. Inertia (Newton's First Law)
2. Equilibrium and Net Force
3. Speed and Velocity
4. Acceleration and Constant Acceleration
5. Graphing Data and Using Linear Equations 

INERTIA

Inertia can also be referred to as "Newton's First Law".  This law states that 

• An object in motion will stay at motion and an object at rest will stay at rest unless acted upon by an outside force

To remember this, Mrs. Lawrence told us that all objects are "lazy".  Objects don't want to change what they're doing unless a force acts upon them to do it for them.   A force is a push or pull on an object, measured in Newtons (N).   

The diagram above is an example of an object in motion staying in motion.  The ball, along with the cart, are both moving at the same speed.  Once the ball is ejected from the cart, it continues in the same path as the cart because there are no forces causing it to slow down.  therefore, it lands in the same spot along with the cart in motion. 

Another example is dishes placed on a tablecloth on a table's surface.  If someone were to pull the cloth from underneath the dishes, they would stay in the same place on the table rather than crashing to the ground.  This is because they were at rest on the table, and although the table cloth has a force causing it to move, the force is not applied to the dishes, so they remain on the table.

One important thing to remember about inertia is that it IS NOT A FORCE.  Inertia is simply a property of motion.  If a question asked:

1. You throw a ball upward, what causes the ball to continue to move upward?

The answer IS NOT inertia.  The answer is, there is NO force causing it to move forward.  All objects simply have the property to stay in the motion they are given.
Also, inertia is related to the mass of the object.  Simply put, inertia increases as mass increases.  A tennis ball traveling at 20 mph will be easier to move/stop than a 300 lb. wrecking ball.

Here is a video created by me and two of my classmates explaining the property of inertia:

EQUILIBRIUM AND NET FORCE

A force is the push or pull exerted on an object, measured in Newtons (N).  

The net force of an object is the result of 2 or more forces acting upon an object (also measured in N).  

This example shows 2 separate forces of 5 N being exerted on one side of a box.  To calculate the net force, you add the forces because they are in the same direction  5 + 5 = 10.  Therefore, the net force on the box equals 10.

However, if 2 forces are directed on an object from opposite directions, it's possible that they can equal out to give an object a net force of 0 N.  This is know as the state of equilibrium.

The example above shows equilibrium.  Because there is a force of 5 N being exerted on the object from opposite sides,  5 - 5 = 0 , the object's net force has been equaled out and is now in equilibrium, at rest.

An object can be in equilibrium in 2 ways

• at rest (as showed above)
• in motion, at a constant velocity, which is explained in the next section 

SPEED AND VELOCITY

Speed, as most commonly know, is how fast an object is moving.  Velocity is similar, however there is one important difference.  Velocity is "direction aware".  Velocity is the speed of something in a given direction.  If an object's direction is changing, so is the velocity.  In order for an object to maintain a constant velocity, it must also maintain the same direction. However, an object can maintain a constant speed while changing direction.  Therefore, an object changing direction is always changing velocity, even if it's keeping a constant speed.

In order to find an object's velocity, one must know it's distance and time.  They can then calculate the velocity with the following equation:

You can also calculate the distance an object moved if given the velocity and time, or how kong it took given the velocity and distance.

Velocity and speed are both measured by the unit meters/second.

ACCELERATION

Acceleration is the rate at which an object is speeding up.  this can be calculated by the following equation:

This equation reads, "acceleration equals the change in velocity over a time interval".

The acceleration of an object can be constant, increasing, or decreasing depending on the slope of the plane it's traveling on.  These slopes each give the ball a different acceleration:

Notice how the acceleration of the last graph is decreasing.  Remember, although the acceleration is decreasing, the object's velocity is still increasing.  Even though the acceleration is going down, it is still accelerating at some rate, so the object is still going faster per second.

The example above shows a ball rolling down a slanted plane.  As you can see, at each second interval, the ball increases it's velocity by 7m/s.  Since this is happening per second, you must write acceleration as m/s².  This is the unit you will always use to describe acceleration.  One constant you should remember is that an object at free fall will always have an acceleration of 10 m/s².  Also, an object moving at a constant velocity across a plane will have no acceleration 0 m/s².  This brings us back to the state of equilibrium!

There is a different equation for finding the distance traveled by an object when given constant acceleration:

GRAPHING DATA AND USING LINEAR EQUATIONS

You can graph the data from an acceleration experiment by using time as the x coordinates and distance as the y coordinates, like we did in the "constant Velocity versus Constant Acceleration Lab".  Once you graph your data, you can use Excel to determine the equation of the line your data formed.  The equation of a line is:

The equation used for distance and acceleration is :

In an experiment, the "b" value of the equation is usually so close to 0 that it can be omitted.  Therefore, one can deduct that:

The y coordinate is the distance, the (1/2a) is the slope, and the x coordinate is the time.  Knowing this is important because you can take the slope from your equation (calculated from Excel) and set it equal to (1/2 a), allowing you to solve for a, giving you your acceleration!

REFLECTION

What I have found difficult...

• In this unit, I found it hard to understand how to translate an Excel graph's line into the equation of constant acceleration.  I wasn't sure how Excel graphs translated into the data we were retrieving from our experiments

I overcame these difficulties when, on the board, Mrs. Lawrence wrote one equation under the other and showed how the variables line up and how you can then solve for acceleration knowing the slope of the line.  That's when it all clicked why we were plotting our data onto excel and creating this line.

At first, I feel like I didn't have a clear understanding of the formulas for acceleration, velociy, etc.  I confused them and had trouble solving problems using equations.  However, I made a list of these formulas and clarified them and now solving with them is simple.  At the beginning of the unit, I was skeptical of blog posts and why we do them.  However, I understand now that it's like making our own study guides as we go along.  Reading through my blog posts from the unit, I was able to remember each class, what we learned, and how well I learned it.  I was able to decide what I need to study more and what I have concretely understood.  I think I've already improved since the first week of class.  As far as my group's Inertia video, I think I gave as much effort as I possibly could, and even got excited about the subject.  And with this unit reflection, I feel confident in my explanations, and ready to explain concepts to anyone else who needs an explanation.  Looking back at my class notes helped me a great amount while writing my reflection, so I think my in-class listening skills have also benefitted my understanding of the unit.

My goal for he next unit is to never miss watching an assigned video.  The videos assigned help me understand the textbook's context on a deeper level, especially because I take notes and then answer questions with these notes.  This system if fool-proof and if I don't miss any video assignments next unit, I should have a thorough understanding of all the concepts.

CONNECTIONS

When I was driving, I began traveling down a slight slope.  I noticed that the car was still accelerating, however, I was not pressing the gas pedal.  Therefore, my acceleration was not increasing, however my  speed was still increasing.  I realized that although my acceleration may have been decreasing (because I took my foot off the gas), the car was still in a state of acceleration, therefore the speed was still increasing.

ACCELERATION VIDEO

A helpful video describing acceleration as well as it's equations and units

CONSTANT VELOCITY AND CONSTANT ACCELERATION LAB

What was the purpose of this lab?
I believe the purpose of this lab was to solidify our understanding of constant velocity and constant acceleration by creating a real life situation to study.  By physically watching the ball roll and sometimes accelerate, our group was able to take the diagrams and vocab we've been studying and see how it plays out in life (the purpose of our physics class).  The lab also introduced us to recording information as well as graphing it and seeing how results look on a linear graph and how one can use this graph, algebraically, for later predictions.

Distinguish between constant velocity and constant acceleration
Constant velocity is when an object is moving at the same speed and same direction without change, where as constant acceleration is when an object keeps accelerating the same distance each second.  Therefore it isn't keeping a constant velocity, but it's speed is changing constantly.

Describe in your own words how you conducted this lab:
In this lab, we began with a flat table (without any elevation of any kind) and a ball.  Using a metronome, we rolled the ball and used chalk to mark the distance the ball traveled per second (or tick from the metronome).  Then, we elevated one side of the table using two books.  Using the same method, we made similar marks as the ball rolled more quickly down the table.

What did you find out on how constant velocity and constant acceleration compare with each other?
I found out that although both are constant, an object with constant acceleration is changing speed unlike one with a constant velocity.


What formulas did you use?
For constant velocity:  V = DT
For constant acceleration:  A=1/2 DT^2

How do the lines in a graph for constant acceleration and constant velocity compare?
Both graphs have similarly graphed points, starting at the same point with a positive incline.  However, the graph for constant acceleration is a curved line unlike the constant velocity graph which is straight.

How did you use the graph you created and the equation of the line to support your data?
We were able to find the slope of the lines of our graphs in order to predict distance and time for different equations

What were 3 important things you learned from this lab that you will use in future labs and problems?
- Before you begin a lab, be sure to remember the correct definitions of your vocab words, especially if they are similar
-If an answer seems wrong, check over your work
-Maintain good communication with all members of your group, and make sure everyone understands all the steps your taking

INERTIA SOURCE

A good source explaining inertia

HOVERCRAFT LAB

Thursday, September 12, 2013

Today in Physics, each member of our class rode in a home-made hovercraft.  Riding in a hovercraft feels unlike riding in any car, bike, or other form of transportation.  Since there's virtually no friction between the vehicle and the ground, you glide along the floor at a constant speed without slowing down.  The only way to stop the hovercraft is to approach it with an opposing force.  This is because of inertia, which is an object's tendency to remain at a state (in this case, in motion) unless acted upon by an opposing force.  This opposing force is also known as net force. In this case, the net forces acting upon the hovercraft were the person pushing it forward and the person stopping it.  Soon after the hovercraft is pushed, it reaches a state of equilibrium, which means all forces acting upon it are equal and it begins to glide at a constant velocity.  As a class, we also noticed heavier people were harder to stop.  We discovered that objects with more mass have greater inertia, therefore build up a faster speed and are harder to stop.

INTRODUCTION POST

What do you expect to learn in physics this year?

• How physics affects our everyday lives, such as how physics is used in vehicle safety as well as the running of machines we use everyday.
• How physics connects to my previous science courses, chemistry and biology and how/why it pertains to my high school education if I'm not considering a physics major.
• How I will use physics in my future

Why do you think studying physics is important?

• I think there are many concepts in physics that can easily be applied to everyday life
• There are many concepts that seem like basic knowledge that physics justifies
• I think it will serve as a good basis for later science and math courses

What questions do you have about physics?

• What kind of jobs require a thorough understanding of physics, and how one would pursue a job like this through their majors in college.
• How long has physics been explored, and how has it developed over time with developing technology and ideas

What goals do you have for yourself in physics this year?

• Complete my homework as early as possible
• Make personal study guides before difficult tests.
• Understand physics in a more personal way. Less of memorizing terms and more deep, perceptive learning.