Unit 3 Blog Reflection -- Dec.

Part A: What We Learned 

1.) What is Newton's 3rd Law? Action/Reaction Pairs

2.) Tug of War; Horse and Buggy Problems

3.) Adding Forces/Vectors with Angles

4.) Gravity and Tides

5.) What is Momentum? Relationship with Impulse

6.) Conservation of Momentum

A1.) N's 3rd Law; Action/Reaction Pairs                                                                                                                    

Newton's 3rd Law states that "every action and has an equal and opposite reaction." 

Here are some examples:

• Road pushes tire forward, tire pushes road backward.
• Rocket pushes gas backward, gas pushes rocket forward
• Girl pushes water backward, water pushes girl forward.

Now-- to make a correct pair, we must keep the action the same, and also the subjects; the direction is the opposite!

Action/Reaction with Different MASSES:

• Cannonball shot out of cannon
• Cannonball: F = m*a
• Cannon:       F = m*a

A2.) Horse and Buggy & Tug of War!                                                             

So if the forces are equal and opposite, why don't the forces cancel to zero? Well....you have to look at the entire system involved in the situation. The horse and buggy provide a great example for understanding the concept of a system of forces:

http://www.batesville.k12.in.us/physics/phynet/mechanics/newton3/Images/HorseCart0.GIF

The image shows a horse pulling a buggy. Both are on top of the road. If Newton's 3rd Law is true, why do they move forward?

• Label the action/reaction pairs first.
• Horse pulls buggy forward, buggy pulls horse backward.
• Horse pushes ground backward, ground pushes horse forward.
• Buggy pushes ground backward, ground pushes buggy backward.
• The arrows representing the forces of the horse/ground should be longer than those of the buggy/ground. This is because the horse exerts more force.
• Summing it up,  the horse pushes harder on the ground, moving itself forward, than the buggy does. Therefore, the system moves.

Tug of War is the same concept of Newton's 3rd Law. Whichever person pushes harder (exerts more force) against the ground wins in this case:

http://www.bbc.co.uk/bitesize/standard/physics/images/tug_of_war.gif

A3.) Adding Forces/Vectors with Angles                                                         

Before going over this sub-topic, let's watch the podcast to get a view of things:




I used Powerpoint (screenshots of slides) and iMovie to create this review.

The main uses/functions of adding vectors are:

• Box and ramp (or snowboarder and slopes)
• Boats and current (or airplane and wind)
• Rope tension and ball
• Pool balls

A4.) Gravity and Tides!                                                                         

[Everything with mass attracts all other things with mass]

What does the force of gravity depend on?
-- mass of objects (F_g ~ m) 
-- distance between objects (F_g ~  d²)

The "Universal Gravitational Formula" is what we use to find the Force of Gravity:

F = G*m1m2
           d²

Tides are those wave-things you see on the ocean's shore when you go on vacation. 
 *Correction: they are caused because of the difference in force felt by opposite sides of the Earth.

There are 2 high tides and 2 low tides during an entire day or 24 hour period. (6 hours apart).

http://4.bp.blogspot.com/-0OCrG-5Cv-0/UGtb75p3BNI/AAAAAAAAABs/N85TYJ4l-yo/s1600/SpringNeap-Tides-color.gif

Spring Tides: highest of high tides/lowest of low tides; just after new or full moon

Neap Tides: moderate; least difference between high and low tides; just after 1st/3rd quarters

~ Practice problems:

• When the distance between 2 objects is doubled, what is the resulting force?
• When the distance between 2 objects is halved, what is the resulting force?
• When the distance between 2 objects is quadrupled, what is the resulting force?
• When the distance between 2 objects is cut in 1/3, what is the resulting force?

*Answers:

• F = cut in 1/4
• F = quadrupled
• F = cut in 1/16
• F = tripled

A5.) What is Momentum? Relationship w/ Impulse                                         

Momentum is "inertia in motion" or the (mass*velocity) of an object.

P = mv [units: kg*m/s]

EX: A 10kg ball is moving at a speed of 5m/s. What is its momentum?

       P = mv

       P = (10)(5)

       P = 50 kg*m/s

Impulse is "force on a time interval" or the (force*change in time) of an object.

J = FΔt [units: N*s]

J = ΔP, therefore

ΔP = FΔt 

• Increasing momentum -- greatest force for as long as possible
• Decreasing momentum -- stopping a moving object by way of haystack or wall
• Decreasing momentum over a short period of time -- punching someone; the amount of time determines the force.
• J = F*t
• J = F*t

How does an airbag keep you safe?

• No matter how the person is stopped, he/she is going from moving to not-moving.
• P=mv
• The change in momentum is the same regardless of how the person is stopped.
• ΔP = Pfinal - Pinitial
• Because the ΔP is zero, (Pfinal is always zero), Impulse must also be zero.
• J = ΔP
• The airbag stops the person over a longer period of time. This causes the force on the person to be smaller. 
• J = F*t
• J = f*T
• The smaller force results in the person not being injured. 

A6.) Conservation of Momentum                                                            

mV = Mv (inversely proportional)

"In the absence of an external force, the momentum of a system remains unchanged."

Elastic Collision: objects bounce off each other

Inelastic Collision: objects stick to each other

~ There are 2 formulas we learned in class: one for each type of collision...

• m_av_a + m_bv_b = m_av_a + m_bv_b 
• m_av_a + m_bv_b = m_av_a (V_ab)

~ We plug in the velocities before and after to find the final velocity!

OPPOSITE MEANS NEGATIVE DIRECTION;

ONE VELOCITY WILL ALWAYS BE NEGATIVE IN THESE PROBLEMS!

Which exerts more force: an ball that bounces off a person's head, or a ball that sticks to a person's head? Which will hurt more? Why?

-- When the ball bounces, it changes momentum twice. Once to stop, and once to start moving again.

-- Therefore, its impulse is double, and it exerts twice the force. 

-- The bouncing ball will injure the person more. Sorry!

Difficulties and How I Overcame Them                                                     

I thought the tides were hardest to understand because it is confusing to know why and how the water level of the ocean changes and when. The gravitation force was alright, but I wish we'd had some tides review going into making this reflection.

Also, labeling the Fnet, Fsupport, and Fweight on each vector diagram lost me points on quizzes. I hope to practice this before the test.

I overcame these difficulties by doing textbook practice problems for help and homework. I got more and more familiar with the topics and feel good about the problem-solving! That's what made the lightbulb click!

Problem Solving Skills, Effort, and Learning                                              

My ability to solve and work on mathematical problems and critical thinking in this class has improved as the semester has progressed. I usually enjoy contributing to class discussions and helping my table-classmates with the warm-ups. 

My effort has been guided by my desire to do well and master the material. 

My goals for the next unit are to be more of a presence in class even if I am not fully engaged. I will do this by entering the room prepared and ready to learn. Also, I want to have a great time with my 5th form friends.

Part B: Connections                                                              

On my Thanksgiving Break, I noticed the tides each day. On the airplane, I thought of the gas pushing plane forward and plane pushing gas backward. I thought of the gravitational pull on me at sea-level. 

The current and boats problem, tug-of-war (I played on the beach and won!), and airbag problem are influential to my life and anyone's life.

I look forward to learning more physics that is applicable to my life.

CHEERS.