Top Ten Most Interesting / Relevant Concepts of Physics

11) Mass

I found the concept of mass to be very interesting when considered in terms of inertia. More specifically, mass is a measurement of an object’s inertia. Inertia is the tendency of an object to resist changes in motion. Thus, the more mass an object has, the greater its inertia. This means that this object will resist outside forces that try to stop its motion or put it into motion from being at rest. I found this concept interesting as the non-physicist world commonly misuses the term “mass” interchangeably with “weight.” These are not the same concept. I like how mass has so much more to it than an object’s matter. It is actually the physical characteristic of a particular object in regards to Newton’s 1^st Law of Motion, which states that any object in motion tends to stay in motion while an object at rest tends to stay at rest, unless acted upon by an outside force.

2) Force ~ acceleration

According to Newton’s 2^nd Law of Motion, acceleration is directly proportional to force and inversely proportional to mass. As a cross-country runner, I find this information very helpful to know. What Newton is saying is that the greater my mass, the lower my acceleration, while the greater my force, the greater my acceleration. Therefore, if I can decrease my mass as much as possible, then I can increase my acceleration. Also, increasing the force with which I accelerate will boost my acceleration as well. Logically, I would want to buy the lightest, or least massive, shoes, wear clothing that would add the least amount of mass to my total mass, and train to decrease my overall mass while increasing my muscle mass in order to run with greater force. All of these things are just small factors that could contribute to my overall success in a race. Although they may seem logically connected, I had not previously seen the connection between the concepts of force and acceleration or mass and acceleration in such a concise, formulaic way. Newton’s 2^nd Law is very relevant to my life as a runner and I also found such a simple physics concept interesting, as it is so basic yet so important.

3) Hitting a home run/ throwing a ball up at a curve

I thought that learning the physics behind hitting a home run was really interesting. I do not play baseball but it was cool to learn the scientific explanation of something that I do not generally associate with physics, or science at all for that matter. We learned how to solve a problem for the picture below and other similar situations. When a ball is thrown straight up with an initial vertical velocity of 20 m/s, gravity causes the velocity of the ball to decrease as it gets higher. This is because it has a constant acceleration (10 m/s^2) so the after the first second that the ball has traveled vertically through the air, its velocity would then be 10 m/s. It would be traveling at 0 m/s after 2 seconds when it has reached the top of its path. The ball then falls back down and gains velocity at the same constant acceleration. This process is important to keep in mind when considering objects being thrown at a curve. But this is not a complete explanation of that object’s path. You also need to consider the horizontal velocity of the ball. The horizontal velocity is constant throughout the entire path through the air. Therefore, while a baseball is following its curved path from home plate to beyond the fence (a homerun), it has a different vertical velocity but the same horizontal velocity at every second. For this specific problem, I know that the horizontal velocity is 30m/s because I used a handy dandy equation. Velocity = distance / time. Since I know the ball has a hang time of 4 seconds and must travel a horizontal distance of 120 meters, 120 ÷ 4 is 30, so the horizontal velocity of the ball is 30 meters per second. Also, it is important to remember that the vertical height of an object controls the time it spends in the air, so using this information we could also find the vertical height of the ball when it reaches the top of its path but this is another problem for another day. Another thing that we found about the baseball was how fast it had to be hit at the angle given. We see in the picture that the ball was hit at a 45º angle so we can form a right triangle from the vectors of the horizontal and vertical velocities at t= 0 seconds. From here we used the Pythagorean Theorem (a² + b² = c²) to find that the ball’s “actual” velocity was 36 m/s (it would need to be hit at this speed to make a homerun. In baseball friendly terms, the ball would be travelling at about 86mph).

4) Newton’s 3^rd Law

Newton’s 3^rd Law of Motion states that for every force, there is an equal and opposite force. Essentially, if I push against a wall, the wall pushes back on me with the same amount of force in the opposite direction of my force. I found this concept interesting, as I had never heard it before. Also, it is relevant as it can be applied to not only every action-reaction pair you encounter in your daily routine but also to some big things that we take for granted. For example, if we are visiting another city and we decide to take a historic carriage ride tour, the horses are able to pull the carriage thanks to and despite Newton’s 3^rd law. The horse and the carriage pull on each other with equal and opposite forces. The carriage and ground do the same with each other, as do the horse and ground, however, the horse and ground put more force on each other (thanks to friction) and thus the entire system of the horse and carriage move in the direction of the horse.

5) Tides

Tides are one of my favorite concepts that we have studied this year. It is easy to take the ocean’s steady, constant rocking for granted- it was so cool to find out that the rhythmic tides of the sea are actually caused by gravitational force. Again it was cool to see physics applied to something that seems a simple part of our lives. To go into a more specific explanation of the topic, I will first say that tides are not caused by the force between the moon and the Earth or the force between the sun and the Earth. When I say they are caused by gravitational force, I mean the difference of force felt on opposite side of the Earth. The side closest to the moon feels more force because force = 1/d^2 and this part of the ocean is much closer to the moon. The side opposite feels a smaller force because it is over more distance. The center of the Earth feels a force that is in between the two. If a hypothetical 25 Newtons of force are felt on the point of the Earth closest to the moon, 15 N felt in the center, and 5 Newtons felt on the far side, we can subtract these forces from each other to find a force of 10 Newtons. (Side A, 25 N, minus the center, 15 N, equals 10 N on side A towards the moon, and side B, 5 N, minus the center, 15 N, equals -10N). Since B is a negative force, it is pulled in the direction opposite from side A (away from the moon). This causes the Earth’s water mass to form a bulge around the planet. It is high tide in the parts of the world that the bulge is most oblong (A and B) at the same time, while low tide is on the other poles of the Earth, as seen in the picture. I thought this was very interesting as I would never have expected tides to be cause by a bulge in the ocean around the Earth.

6) Tangential vs. Rotational velocity

The difference between tangential and rotational velocity is that rotational velocity refers to the total speed of rotation for the object as a whole and is the same at every point on the object, whereas tangential velocity increases the further from the center of the rotating object you get. I found this interesting and relevant because this concept is present on Ferris wheels and other similar things.

7) Rotational inertia – Why do runners bend their legs instead of keeping them straight?

Speaking of rotation, I felt that rotational inertia was particularly relevant to my life. I love running and playing sports so it was interesting to learn physics concepts about my athletic interests. Rotational inertia is an object’s tendency to resist rotation- much like normal inertia and also self-explanatory. Rotational inertia is increased the more distributed an object’s mass is. For example when an ice skater is spinning, she has her arms out, which spreads her mass and gives her greater rotational inertia so she spins slower. When she pulls her arms in, her inertia decreases and she spins faster. Runners bend their legs when they run for a similar reason. Consider your hip to be the axis of rotation in regards to your leg because that is where they rotate. The close to your hip your leg can be, the less rotational inertia you will have and thus the more rotational velocity, so the faster you can rotate your hip joint and you can run faster that way. This was interesting because I never considered there was a scientific reason to this other than that is how your leg works.

 8)  Why do airbags keep us safe?

This question is arguably one of the most relevant issues that we dealt with in our physics course this year. Not only can it be answered in two different ways, using two different concepts, it is also a real-world application of physics, which I find more interesting than memorizing formulas. The first way that we answered this question was in terms of momentum and impulse. During a car crash, your body hits something and goes from moving to not moving. The change in momentum is the same no matter how you are stopped because you are going from moving to not moving (p=mv). Change in momentum is equal to impulse (∆p=J), so the impulse is also the same no matter how you are stopped. Since impulse = force x ∆time, when the time over which the impulse occurs is very long, the force of the impulse is very small. When you collide with a hard surface, it takes a short time to stop you, thus the impulse has a greater force on you. Conversely, an airbag is soft and absorbs the impact so it takes a long time to stop you and there is less force exerted on you. The smaller the force, the smaller the injury. This is why airbags keep us safe in terms of work and momentum. The second way to answer this question is in terms of work and energy. You go from moving to not moving regardless of what you hit, therefore the change in kinetic energy I the same no matter how you stop. KE = ½ mv^2. ∆KE = KE final – KE initial, and ∆KE = work. Since ∆KE is the same, the work done is also the same regardless of what you hit. Work = force x distance. Work must remain the same, but there can be more or less force or stopping distance depending on what you hit in the crash. The dashboard of your car is quite hard and firm, thus it stops you over a short distance when you hit it- this means there is a lot of force and a greater risk of injury. When you hit an airbag, it is soft and absorbs the impact so it takes a greater distance to stop you. This decreases the force on you and thus reduces the risk of injury.

9) Lightning/ Lightning Rods

Lightning is of course something that we see quite often and yet never really understand. We learned that when it is thunder storming, the clouds get very heavy and rub together. The friction causes the clouds to polarize with the negative charges on the bottom and the positive charges on the top. Through induction, the Earth’s surface polarizes with the positive charges along the top of the surface (because opposite charges attract) and negative charges deeper into the previously neutral ground. The air between the ground and clouds wants to equalize and eventually the charges jump across this space to create equilibrium. Energy is released as heat and light- this is lightning. I never knew that the ground was involved so heavily in this process. Lightning rods function as a way to protect our houses from lightning strikes while the charges are equalizing. The rods are metal conductors that stick up from our roofs and provide the lightning the path of least resistance to the ground. The lightning hits the rod and flows through it to the ground, rather than using a house or person to reach the ground. I always thought lightning rods were just metal sticks for lightning to hit instead of hitting people; it was both relevant and interesting to learn that more complex physics concepts are involved in their function.

10) Capacitors (camera flash)

Capacitors are plates that store charge. They transfer electrons between each other- it takes time to build up these charges, however. Once the charge is fully built up, the capacitors release strong currents all at once (including a release of energy). Then they have to build the charge back up all over again. I found this concept especially interesting, as capacitors are what are used to create the flash in a camera. Being very interested and involved in photography, I latched on to this concept. I know understand why you cannot snap multiple consecutive pictures very fast when you are using flash- it needs time between shots for the charges to build back up. Also, the flash itself is the energy released as light when the charges are transferred. It is so cool to view my camera as more than a little toy- it is a complex physics-related object. I would love to explore other physics concepts in photography.

Turbines - A Long WINDed Process

In my physics class, we made wind turbines to wind down the year.  Having just learned about magnetism and the basic concepts of generators and motors, the turbines were not very complicated for us to build.

Background Information:

Wind turbines are used to capture solar energy (wind) to create mechanical energy (the turning of the repellers) and then turn this mechanical energy into electrical energy. The wind turns the repellers which then becomes the input mechanical energy into a simple generator. This is done by rotating magnets that are surrounded by coils of wire. The magnets cause a change in the magnetic field as they move, as in our design we alternated the magnets between north and south poles so that the change was greater. The change in magnetic fields induces a voltage in the coils of wire which causes an electric current. This is how wind turbines generate energy that we are conveniently able to use.

Materials and Methods:

My group used the following materials:
PVC pipe
wooden rods, 2 different sizes
cardboard
wire coil
4 small magnets
two round wooden attachments, two different sizes

We used the PVC pipe as the "tower" of our turbine. Then we used two PVC attachment pieces to create a "neck" at the top of the tower which would house the basic generator we made and connect to our wind catching device. We created blades by hot glueing pieces of cardboard to three wooden rods. We then cut the tips of these rods so that they had a flat surface and we used wood glue to fasten them to a wooden connector piece, the type that is typically used for a knob of some sort. To reinforce the glue, we put a screw through the rods. Tis connector piece has a hole in its center which we widened with a drill. Through this hole we slid another, thinner, wooden rod. We put another smaller wooden connector piece on this rod as well, towards the center of the rod. This piece had four small magnets glued onto it, evenly spaced. The north side of two of the magnets was facing out, while the south side of the other two was facing out. We were sure to alternate the polarity in order to create a consistently changing magnetic field. This piece with the magnets was placed just over the wire coils so that they could affect it with their magnetic field. The wooden rod that held this piece continued through this tunnel of PVC pipe and out the other end. On each side we put a small cardboard disk with a hole drilled in it for the rod to go through so that we could keep the repellers and the magnets suspended. The wire coil was pretty thick so that we could generate voltage. Each end of the wire had to be sticking out so that we could measure the changing voltage and see if our turbine was working. We ran one end of the wire down the PVC tower and taped it to the outside of the pipe. The other end we stuck out the PVC attachments at the "neck" region of the turbine. We scraped the outside of the wire off so that voltage could be read.

Results and Discussion:

The amount of voltage induced by our turbine was effected by the thickness of the coil of wire- or how many turns it had. We wrapped ours for a long time, we did not count the turns but it is suggested that when in doubt, keep wrapping. Also the magnets had to alternate in polarity. This was a problem because the magnets wanted to keep sticking together. We had to cover them with our hands while we were glueing them onto the wooden attachment. Hot glue worked fine but I would advise keeping the magnets away from metallic items once they have been glued on- we had some problems with this and had to re-glue. Keeping our design as simple as possible worked out well for my group. The cardboard worked just dandy as a way of keeping the rod in place that attached the spinning device to the magnet. Our biggest difficulty was inserting the coil into the turbine but we were lucky to have built the tower and neck parts with easily detachable pieces. We forgot to leave both ends of the coil out so that is something to keep in mind when designing your turbine and wrapping your coil.

Here's a video of our turbine in action:

Unit 7 Summary

We began this unit learning about magnetism. The source of all magnetism is moving charges. These charges, electrons, spin in completely random directions when they are not magnetized. Domains are clusters of electrons that are all spinning in the same way. They have no net direction until they are affected by a magnetic field; when magnetized, domains align so that the magnetic field can continue to flow. In each magnetic field, there are two poles- north and south. The magnetic field always comes from the north pole to the south and goes from the south to north.

It is also important to know that the poles are like charges in that like poles repel while opposite poles attract. This is because the magnetic field lines flow towards the opposite poles and repel from the ones that are like the pole they just left.

One of the big questions we asked with this section was : why do paper clips stick to magnets?

A paperclip that is not magnetized will have domains pointing in all different directions. A magnet has a magnetic field. When the magnet is close to the paperclip, the domains of the paperclip align to match the magnetic field of the magnet. The paperclip now has a north ad south pole, just like the magnet. The north pole of the paperclip is attracted to the south pole of the magnet and thus the paperclip sticks to the magnet. Now, the paperclip can act as a magnet and other paperclips stick to it, their domains also aligning to match the magnetic field.

A fun fact about magnetic poles is that the Earth's magnetic south pole is actually at the top of the Earth, which we generally consider the geographic north pole. The Earth's magnetic north pole is on the Earth's bottom, at the southern tip.

All moving charges feel a pull from a magnetic field if they are moving perpendicular to it. The Aurora Borealis, or the Northern Lights, are actually cosmic radiation- moving charges from space that got sucked into Earth's magnetic field and spiral around the field lines and into the Earth's magnetic poles. This is why we can see them in northern parts of the world- they are following the magnetic field lines into the south pole.

Any moving charge that is moving perpendicular to a magnetic field will feel a force from it. this brings us to the "right-hand rule" that explains the relationship between current, force, and magnetic fields. 

In this picture, my pointer finger represents the direction of the current of moving charges. My middle finger represents the direction of the magnetic field and my thumb shows the perpendicular direction that the charges will be caused to move in.

Electromagnetic induction is the process of inducing voltage by changing the magnetic field in loops of wire. Voltage is induced by the motion between wire and magnetic field. The more loops of wire, the greater the voltage that is induced. This is summarized in Faraday's Law, which states that:

The induced voltage in a coil is proportional to the product of its number of loops, the cross-sectional area of each loop, and the rate at which the magnetic field changes within these loops.

There are three ways that voltage can be induced in a wire:

1) moving the loop of wire near a magnet

2) moving a magnet near the loop of wire

3) changing the current in a nearby loop

All of these methods involve changing the magnetic field in the loop of wire.

 This process is used in transformers. Transformers utilize a primary coil, which receive the input of electric current, and a secondary coil, which has an output of electric current. When electric current flows through the primary coil, it causes a change in the coil's magnetic field which spreads and induces voltage in the secondary coil. Voltage causes current, so the electric current is thus transferred between coils. The current must be ac for a transformer to work because this has a constantly changing current, so the magnetic field is constantly changing, which is the only way for the transfer of electricity to be constant. Transformers can either step voltage up or down, depending on the electric power required. The relationship between primary and secondary voltages is described in the equation:

The conservation of energy, which we learned in a pervious unit, comes into play here as well. The initial amount of energy must be equivalent to the ending amount as energy cannot be created. Power is the rate at which energy is transferred. Therefore, the power into the primary coil = the power out of the secondary coil. Electric power = voltage x current. Thus:

primary (Voltage x Current) =  secondary (Voltage x Current).

If the secondary coil has a greater voltage than the primary coil, it will have a weaker current, and vice versa.

Finally, we learned about generators and motors. They work in opposite ways, to put it simply. Generators have an input of mechanical energy, and through electromagnetic induction, have an output of electric energy. They are made of coils of wire and magnets (much like transformers). Motors take electric energy (the input) and through electromagnetic induction produce mechanical energy (the output). They are also made of coils of wire and magnets. The generator effect is: current as a result of motion. The motor effect is: motion as a result of current.

Simple Motors

This week, our class took a leap and designed our own motors. We followed a fairly simple design, which was powered by a 1.5 volt battery and a small magnet. Paper clips were attached to each end of the battery and held a coil of wire; they helped complete the circuit and connect the positive and negative ends of the battery. 

Th current flows from the battery, up one paper clip, through the wire and the motor loop and then back down the other paper clip and back to the battery. Each part of the motor functioned in a different way. The battery supplied voltage to produce a current. The coil of wire, or the motor loop as it is labeled in the diagram above, provided a pathway that allowed current to flow. The paperclips connected the wire to the batter and completed the circuit, also allowing current to flow. The magnet, placed on top of the battery, created a magnetic field that put magnetic force on the motor loop and caused it to turn.

Here is a picture of our actual motor:

We scraped one side of the wire that connected the coil to the paper clip. We did this because we needed current to flow when the loop was in one orientation and not the other. We wanted it to flow while the loop was turning in one direction without causing the loop to turn in the other direction as this would be counterproductive.

The motor turned because of the magnetic pull created by the magnet. The loop had to be vertical. This is because all the charges in the wire were moving the same way, but since the wire was coiled the current would technically be in different directions (this is important when using the right hand rule). The vertical loop felt the force of the magnet in opposite directions on the top and the bottom sides so there was a torque on both ends of the wire, causing it to rotate. If the loop was horizontal, it would not feel this force in a perpendicular way and there would be no rotation. Along the bottom of the vertical loop, the current was going to the right. (This is form the viewer's perspective in the photo above). According to the right hand rule, the magnetic field would be in the upward direction, and the force on the wire would be towards the viewer. On the top of the loop, the current would be going back the other way, the magnetic field is still in the upward direction, and the force on the wire would be in the opposite direction, away from the viewer. Since the sides of the wire are pushed in opposite directions, the wire rotates. When the wire flips and the current is still going in different directions, the magnetic field continues to act on it in this way.

Here is a picture of the hand position for the "Right Hand Rule":

My pointer finger represents the direction of the current, my middle finger represents the direction of the magnetic field, and my thumb represents the direction of the force. In order to find one of these, all you have to do is put your right hand in this position and turn it in the correct way to align with the current, force, or magnetic field given.

This motor is very small, in fact it is too small to effectively power anything. However, it can be used for educational purposes and effectively displays the basic concepts of a motor and how a simple motor is built and works. Plus it is pretty cool to watch- check out this video of ours in action!