Unit 5 Reflection!

Tangential and Rotational Velocity:
We started the unit by learning about tangential velocity and rotational velocity. My class found out that rotational velocity is about the number of rotations over a time. When talking about rotational inertia we discovered that if, for example, two kids are playing on a merry-go-round regardless of their position their rotational velocity is the same! So, does that mean their tangential velocity is the same? No! Tangential velocity depends on how far away the body is from the axis of rotation. If the body is far away from the axis of rotation it needs to cover a greater distance per rotation therefore it has a greater tangential velocity, the opposite is true for an object close to the axis of rotation.

tangential speed ~ radial distance x rotational speed 

Rotational inertia:

Rotational inertia is the same as linear inertia but for rotating objects. Just like objects moving in a linear path, rotating objects do not want to stop doing what they are already doing. Rewriting Newton's first law we can say: 

An object rotating about an axis tends to remain rotating around an axis unless interfered with by some external force and an object not rotating wants to remain that way unless an outside force is applied. 

Rotational inertia depends on concentration of mass and how far is it from the axis of rotation. The furthest away the concentration of mass is from the center the greater it's rotational inertia is. I struggled to convince myself that the greater the rotational inertia the hardest it is to start rotating, I always wanted to say that it would start rotating sooner than an object with a small rotational inertia. The secret to understand this concept is always to remind yourself that the inertia is laziness and the lazier the object is the more reluctant it would be to start rotating!

The video above shows that because the hoop has it's concentration of mass away from the axis and the disk is solid (both have the same mass). The hoop has more rotational inertia and therefore is more reluctant to start rotating and therefore loses the race.

Torque:

After talking about rotational inertia we talked about torque which changes the rotation of things. It is important to know that torque is lever arm times the force (force = weight). The concept of torque is used to understand why some things are balanced and how to create a balanced system. The best example for this is a see-saw. If you have too people with different weights it is impossible to maintain the board balanced if they are both sited the same distance from the axis of rotation since for a system to be balanced there needs to be equal torques. To make the board balanced with different weights you need to increase the lever arm (lever arm =  smallest distance from the axis of rotation) of the person with the least weight so that the torques equal.  In practice the heaviest person of a see-saw needs to sit close to the center while the lightest one sits away from it 

Top Ten Real Life Examples of Physics Concepts

After a long year, and a lot of learning this is the ranking of my top ten real life examples for physics concepts.

1.Crumpled vs Flat sheet of paper:

Both sheet's of paper are falling with air resistance, therefore they want to reach terminal velocity (f air = f - weight). There are two ways you can increase your f air: increasing surface area or increasing speed. The sheets of paper have the same weight, so they need the same f air to reach terminal velocity. The flat sheet of paper has more surface area, therefore it does not need to speed up to gain more f air. The crumpled piece of paper does not have the surface area needed to reach the f air needed, therefore it will speed up in order to reach increase the f air.

2. Balloon that sticks to the wall:

When someone rubs the balloon in someone's head the balloon picks up electrons from the hair and becomes negatively charged (by friction). After that, the balloon is placed against the wall. Because the wall is an insulator the electrons of the balloon will not be stolen by the wall, but the charges will rearrange in a way that the positive charges will move close to the negatively charged balloon, and the negative charges of the wall will move as far away from it as possible. This process of rearrangement of charges is called polarization.. You might be wondering why it sticks if there is also a force of repulsion between the like charges of the balloon and wall. Here is when Coulomb's law comes in handy! Coulomb's law says that distance is inversely proportional to force so since the opposite charges are closer together, the force of attraction is stronger than the force of repulsion, therefore the balloon sticks to the wall!

3. Roller Coasters:

Roller coasters work due to conservation of energy. The first hill is always the highest one so that it accumulates enough energy to go through all the elevations. As the car goes down, the PE goes down but KE goes up. If added together at any part of the ride, the KE and PE of the car will equal the PE that the car had on the first hill.

4. Balancing a see-saw:

If you have too people with different weights it is impossible to maintain the board balanced if they are both sited the same distance from the axis of rotation since for a system to be balanced there needs to be equal torques. To make the board balanced with different weights you need to increase the lever arm (lever arm = smallest distance from the axis of rotation, always perpendicular to force) of the person with the least weight so that the torques equal. In practice the heaviest person of a see-saw needs to sit close to the center while the lightest one sits away from it.

5. Fouette Turns

A ballerina turning is an example of conservation of angular momentum. The girl extends her leg and arms and that increases her rotational inertia (because her mass is farther away from the axis of rotation) and therefore decreases her rotational velocity since angular momentum is conserved.

6. Bouncing Ball vs. solid ball - Which can knock a block over?

The answer to that question is the bouncing ball! The reason why the bouncing ball knocks the block over is because there is not only one force acting on that block. Because of Newton's third law, we know that for every action there is an equal and opposite direction therefore ball pushes block, block pushes ball in both cases. The bouncing ball not only causes that force, the ball will bounce on the block adding another force that causes the block to fall over.

7. How to find out the height of a building
Go to the top of the building drop a ball from the top of it and record how long it took to reach the ground. Do this a few times plug in the average time for t in the how far equation (d = 1/2 gt2) and 9.8 for g! Do the math... and now you know how tall the building is!

8. Forgetting something on top of the car and driving away

Have you ever stopped to observed that the object is at the same place it was before but on the ground? This concept is explained by Newton's first law! The object was at rest and therefore wanted to stay at rest, there was no outside force to stop it from doing so therefore the object continued in the same place but on the floor.

9. Best way to break a brick

In the equation J = f Δt , in order to prove that time determines the force necessary to achieve a certain
impulse, the impulse needs to be proven constant. To prove impulse is constant you need to state that
regardless of the time, the hand of the fighter is moving and comes to a stop therefore since the mass is
obviously the same and P = mv, the change in impulse is the same (ΔP = Pfinal - Pinitial). We stated before
that J = ΔP therefore J is constant! To continue to answer the question, you should now state that J is also

J = fΔt. Because of the last equation is possible to afirm that the smallest the time of contact the greater the force will be in order to achieve the constant J. Since to break a brick you need the greatest force possible, the time of contact should be as small as possible.

10. How a credit card works:

A credit card works because it contains magnets placed in a specific pattern. The credit card machine has coil of wires that cause a change in magnetic field when the card is swiped. This process is called electromagnetic induction. The change in magnetic field induces a voltage that creates a current that is used to signal your account information to the machine.

This is a contrived picture of a ballet step called an attitude turn. For this move the dancer needs to go from a position where she has her feet on the floor and knees bent to pushing up and lifting her leg in a 90 angle. In order to come up the dance pushes down into the ground, because of Newton's Third Law we know that for every action there is an equal and opposite reaction, that means that the floor also pushes the dancer up. Notice the arms of the dancer, one of her arms go up distributing her weight closer to the axis of rotation. That redistribution of weight reduces the rotational inertia and therefore increases rotational velocity, since the two are inversely proportional. The dancer's other arm is held beside her body and that helps her maintain her balance. The last thing the dancer wants is to have a torque in any direction, and since the leg needs to be held away from her body a torque is bound to occur, but the arms stand in a way to create a torque in the opposite way and therefore stops the dance from unbalancing as fast as she would have if her arms were simply by her side.

Unit 8 Reflection

This Unit was all about magnetism and the benefits of this great discovery! On the chapter about electricity we learned that charged particles follow the Coulomb's Law - that means that the magnitude and the distance between two charges is the determinant of their attraction. When it comes to Magnetic Force the motion of the charged particle are also very important.

Every magnet has a south and a north pole, just like electrical charges, opposites  poles attract and like poles repel. It's important to remember that if there is a south pole, there is a north pole and the other way around - one cannot exist without the other. For because north and south pole can't ever exist without each other if a magnet was broken, regardless of the number of times, all pieces would have a north and south pole and therefore they would remain being magnetic. The space around a magnet is called a magnetic field.



http://www.geo.arizona.edu/xtal/nats101/s04-10.html

The earth itself has a magnetic field! This magnetic field explain why we only see the northern lights on the poles of the earth and not in the equator. Cosmic rays move towards the earth, and in the equator that movement is perpendicular to the earth's magnetic field, while in the pole it is parallel. When the rays move perpendicular there is a force of repulsion that causes the rays to be repelled by the magnetic field. When the rays move parallel there is no repulsion force, therefore the rays come in and are the lights we see!

http://www.unc.edu/depts/oceanweb/turtles/geomag.html

The earth's magnetic field is also the reason why a compass works. The key to a compass is a magnetized needle that is free to move. The needle will line up with the magnetic field of the earth and point north and south. If you want to build a compass, this is how you can do it!

The picture above shows the direction of the field outside the magnet - it goes from north to south pole. Do not assume that the direction of the field is the same inside the magnet! Inside the magnet it moves from south to north pole.  In order to understand why some things are magnetized and others are not, it is helpful to understand what domains are. Domains are a cluster of  atoms spinning in the same direction. A bar of iron, for example, is composed of many domains. The difference between a magnetized and a nonmagnetic iron bar is that the first one has its domains aligned with each other. Have you ever used one of those magnetic paper clip holders, like the one bellow?

http://dolphindentist.blogspot.com/2011/04/fun-theory-worlds-deepest-bin.html

If yes, have you ever noticed that the paper clips do not only stick to the holder but to each other?  That is because the paper clips have become magnetized, meaning that their domains are in line with the domains of the magnet in the paper clip holder. The paper clips now have a north and south pole as well and therefore are magnetized!

We also learned about electromagnets in this unit. Electromagnets are simply a current carrying coil of wire. They are really powerful and therefore are used, for example to lift up cars in a junk yard or to levitate trains!

After learning about electromagnets we learned about an important concept that is the reason why a motor works. That concept is: A current carrying wire feels a force in a magnetic field. The direction of the force can be easily determined by the right hand rule (picture in previous post). A motor works by this exact idea. There is a current carrying wire (current provided by the battery), a magnetic force due to the magnet and therefore there is a force that causes the copper loop to turn. For a more detailed explanation of motors, check out the previous post about building a simple motor.

The rest of the unit was based on electromagnetic induction! Electromagnetic induction happens when a magnet moves close to a wire (or the other way around) and therefore changes its magnetic field. The change in magnetic field results in the induction of voltage and therefore the creation of current. This concept is what explains the coils of wire on the floor in front of the traffic lights and the function of transformers. When cars drive by the wires on the floor, it acts as a magnet and therefore changes the magnetic field and induces a voltage. That induced voltage creates a current that signals the light to change.

A generator has a magnet and a coil of wire that receive the input of some type of mechanical force, the magnet creates a magnetic field that is changed when the coil of wire moves towards it. The change in magnetic field induces a voltage that creates a current. 

A transformer changes the voltage from a primary source to a secondary. The voltage is changed by using the idea of power (P = IV) and the number of turns on each side of the transformer. The number of turns (loops) in the transformer is the determinant of the output voltage. If the secondary has more turns than the primary the transformer steps up the voltage, if it has less it will step down the voltage. A transformer works on the principle that a current carrying wire feels a force in a magnetic field. With AC current there is a change in the magnetic field that induces voltage and creates current. Because of the difference in number of loops the ratio of current vs. voltage can be changed to fit the needs of an appliance. A laptop, for example, needs less current than the one coming from our outlets therefore it has a transformer that changes that voltage before it harms the computer. We learned about two equations related to transformers:

number of turns/voltage = number of turns / voltage

IV primary = IV secondary 

The first equation can let you find out how many loops of wire a transformer needs to have in order to step up/ step down a voltage. The second equation serves to find the amount of current or voltage on the transformer.



Reflection:

One of the hardest things of this unit was to remember the difference between a generator and a transformer. My teacher made the table bellow that was really helpful in understanding the difference between the two.

Besides that, the unit was simple. The idea of a compass confused me at first but after our review I understood that the needle of the compass lines up with the earth's magnetic field, or a stronger field - like on ships or big metal things that have not been moved.