Unit 5 Blog Summary

In this unit, we have studied different forms of energy and the measures of work and power. We did a lab to find out how work and power are related, and that was really helpful when energy came into play because all three of these ideas are intertwined. We learned that kinetic energy, the energy of movement, is different than potential energy, but both are equal to work and are all measured in Joules. Towards the end of the unit, we touched on the conservation of energy which was a nice refresher about how the conservation of momentum works. Seeing similarities between these two topics made it easier to understand the purpose of the conservation of energy.

The first topic we discussed was work. Work is equal to (force)(time), and is measured in Joules. Next, we learned that power is the time in which work is done, and is measured in watts. In the lab we did where we walked, ran, and walked with weights up three flights of stairs, we were able to calculate our weight in joules, the work we did, and the amount of power it took. Through the course of this year, I think that this has been the best hands on experience of physics. Although I have been able to recognize it in my daily life, the lab helped me better understand the topic of work and power.
To help myself review, I made a Quizlet with the formulas I needed.

work= (f)(d) :joules

power= work/time :watts

1 horsepower= 746 watts




After we were comfortable with the concept of work, we brought in kinetic energy, which is the energy of movement. Obviously a resting object cannot have kinetic energy, but all objects do have potential energy. In the example we often used in class, a ball is resting on the top of the cliff, where it only has potential energy. However, just before it hits the ground, it has the same amount of energy, but it is now kinetic since it is moving. An imperative concept to grasp is that an object's potential and kinetic energy values are equal no matter which type it is. So if the ball has 2,000J of potential energy at the top, it still has 2,000J of energy during it's fall, but it is now kinetic.

Kinetic energy= 1/2mv^2 :joules

potential energy= mgh :joules

Change in pe=work=change in ke



As I said earlier, a good refresher from a past unit was the reintroduction of the conservation of momentum and how it is similar to what we recently learned, the conservation of energy. In a machine, which I will get to later, energy is mostly conserved while only a small amount of energy is lost through heat or sound. An important formula to know when talking about the conservation of energy is that the work in=work out of a machine. We know that work=(f)(d), and because of the law of conservation of energy, the work in must equal the work out. A common misconception is that all machines are 100% efficient, but as I said earlier, some energy is lost due to heat and sound. In the example of a ramp that is 2m in vertical height and 8m in length, we want to push a 600N box up the ramp. Using the formula work in = work out, we can plug in these numbers to find out how much force was needed to push the box up the ramp.
Work in = work out
(f)(d)     = (f)(d)
600(2)   = (f)(8)
1200=f8
150N=f
Key formulas to know for this topic are:

Work in = work out

Change in PE= change in KE


Lastly, after we understood the conservation of energy, we applied our knowledge to simple machines such as ramps, pulleys, and jacks. The formula used in the previous topic, work in = work out, is vital to understanding how simple machines help us. The main goal of a machine is to decrease either the force or the distance, but not the work as a whole. In the ramp example, the factors for work in are the weight of the box and the vertical height, and the factors for work out are the length of the ramp and the unknown force.


By writing this blog, I have made connections that I hadn't before between topics and also it helped me to solidify my prior knowledge of some information. It was helpful to list the different formulas and also bold the key words so that I will remember what the unit was really about, and not get caught up in the specifics as I have in the past.

Unit 4 Summary

Conservation of Angular Momentum

When an ice skater goes into a spin, she speeds up rapidly. This happens because of the Conservation of Angular Momentum. The former momentum formula still applies here: p before=p after, except the object is rotating. When her arms are extended, it slows her down because her mass is not all close to her axis of rotation. When she changes her spin and pulls her arms in, that moves more of her mass to the center and increases her rotational velocity.

Rotational and Tangential Velocity

When you're riding a train and feel a slight shift horizontally, that is because of rotational and tangential velocities. Train wheels are designed especially to keep themselves on the track, and they actually self correct because of physics. The wheels have a tapered design where they are smaller on the exterior and larger on the interior. The two sides of the wheel make the same number of revolutions in a certain amount of time (rotational velocity), but the interior side of the wheel has a higher tangential velocity because it has less time to make a larger rotation because it has a larger circumference.

 

Rotational Inertia

As you can see in this picture, a hollow disk and a solid disk are going to race down a ramp. Initially, I thought that the hollow disk would win because it has less mass but that is not the case. Rotational inertia is the willingness an object has to rotate. An object with higher rotational inertia is harder to rotate. The closer the mass is to the axis of rotation, the lower rotational inertia it has. Likewise, the hollow disk will have a higher rotational inertia because its mass is distributed to the outside of the object. Due to these properties, the solid disk will win the race because its mass is distributed evenly and around the axis of rotation.

Torque

As we know, torque = force x lever arm. The force can either be gravity, or an added weight onto the system. The lever arm is the distance from the axis of rotation to where the weight falls. When one lever arm is created, another one is too, so we use the formula (f)(lever arm)=(f)(lever arm) to ensure that the two sides are balanced. Because f x lever arm = torque, we can say that the clockwise torque is equal to the counterclockwise torque. In the example below, adding a rope to the end of a wrench does not increase the lever arm. It may increase the force because you get a better stance, but it does not increase the distance from the axis of rotation to the point where force is being applied.

Center of Mass/Gravity

As a kid when my family and I traveled to Italy, we went to visit the leaning tower of Pisa and I was appalled by it’s slant and how it doesn’t fall over. This is because its center of gravity has not yet fallen over the alignment of its base of support. Two problems that contribute to the falling of an object are a high center of gravity and a narrow base of support. In wrestling, the players do just the opposite to stay standing. They widen their base of support and lower their center of gravity so that there is more distance in their base of support so they are less likely to fall over when wrestled with.

Centripetal/Centrifugal Force

The word “centripetal” seems daunting because it is used in really advanced math, but it is pretty simple. Centripetal means center-seeking. So a centripetal force is a force that acts on an object, forcing it inwards towards the axis of rotation (the center of an object). If when a car makes a sharp turn, the centripetal force pushes you towards the center, but why do you fly the other way? This is because of centrifugal force. It isn’t actually a force, but rather the reaction to centripetal force.

Mass of Meter Stick Lab Reflection

In the demo's, Ella and I drew various examples of the meter stick, its lever arms, its attached weights (forces), and in result, its clockwise and counterclockwise torques. During the planning phase, we were sure that we'd use the equation w=mg because we want to find the mass of the meter stick, not the weight. The obvious first step was to find its center of gravity, and where it would balance equally on the table and in the air.
When we began our lab, we immediately wanted to find the center of gravity with the 100g weight attached to one end. The numerical value for the center of gravity was 21.6 centimeters. Next, we used our knowledge that (f)(lever arm)=(f)(lever arm). Before this step, I understood what that meant, but I had an "aha" moment that the force and lever arm from each side of the center of gravity multiply to equal the same equation from the other side. This was huge for me! Our goal for this step was to find the weight of the whole stick, and that equaled 6.3N. Using the formula, w=mg, we found that the predicted mass of the meter stick was 130g.
Next we massed the meter stick on the scale and found that its actual mass was 119g. Here is a photo explaining our work and how we used formulas to solve for the mass.

Unit 4 Resource Post (Rotation)

I found this image really helpful because it explains what happens when an ice skater's arms and legs are extended and when they are tucked. In the image on the left, the skater's limbs are extended therefore creating a large rotational inertia because it is harder to spin. It is harder to spin because her mass is farther from the axis of rotation.
In the image on the right, the skater's arms and legs are tucked creating a lower rotational inertia due to her mass being centrally located near her axis of rotation. Therefore, her rotational velocity is large and she spins much faster.



In this yoga pose, the woman has a low center of gravity and a large base so that she won't fall over. Because of her wide base of support, she is able to maintain her center of gravity and stay balanced.

Unit 3 Blogpost

Throughout this unit, most things we studied revolved around Newton's Third Law which states that, "Every action has an equal and opposite reaction." At first, this was hard to comprehend because in a car crash, one car is impacted more than the other, but how is that possible if they have equal and opposite reactions to each other? After learning more about what factors in to these problems, I learned that the acceleration of the respective cars has a huge impact on the result of the crash. If a truck and a sports car collided, the truck would have a bigger force not because it was going "faster" but because its acceleration was higher because of its greater mass.

A tug of war game is surprisingly similar to the car crash in that the weight of each team is irrelevant because the forces are equal and opposite. However, the amount of friction that the teams put in the floor determines the winner.
An easy example to help understand how forces work in perpendicular directions is which way a boat will go in a river. If the current is going downstream, and the boat is traveling to the right, the boat will go southeast in order to get to shore. A connection I made was using the special right triangles from Geometry last year in order to find the third velocity value. If two were labeled 3 and 4, I could assume that the hypotenuse would be 5, and that was comforting to have that knowledge up my sleeve.
My favorite topic we studied during Unit 3 was what forces happen in order for ocean tides on earth to do what they do. There are two types of tides that occur on earth, called Spring and Neap tides. The difference between the two is the amount of force the moon is pulling on earth with, and therefore how far the moon is from the earth. I know this because F~m and F~1/d. These formulas mean that Force and mass are directly proportional while Force and distance are inversely proportional. Spring tides occur when the sun, moon, and earth are perfectly lined up, while neap tides occur when the sun and earth are lined up, but the moon is off to the right or left of the earth. The results of these two tides are very different. Because F~1/d, the closer the moon is to earth, the greater force there will be. Spring tides create extreme high and low tides and neap tides are less extreme and normal. This is because when Spring tides occur, there are two strong forces on the earth: the sun and the moon. Although the sun does have a larger force on the earth because of its mass, some of that is diluted because of its far proximity to earth. That is why the moon has a greater affect on earth's tides. Tidal bulges occur when the moon is pulling on that part of the earth thus creating high tides. High tides occur simultaneously on opposite sides of the earth because the tide changes every six hours (from low to high). It takes twelve hours for a beach to have a high tide and then another one.

One of the biggest topics we learned about was momentum. Momentum is the measure of inertia in motion, and the symbol for momentum is p. The formula to find the momentum of an object is p=mv. If the momentum changes, then the mass and velocity increase as well. One phrase that was stressed throughout the section was that the momentum before a collision is ALWAYS equal to the momentum after the collision (p total before=p total after). After understanding momentum, we were ready to introduce Impulse (J) which is Force(time interval)=J. Another way to say this is that in relation to momentum, J=△p.
To find the velocity of two objects that recently collided, we would use the formula maVa+mbVb=ma+mb(Vab). By using the masses and velocities of two objects that become one, we can find the velocity of the new object.
After learning about this, we delved into the Law of Conservation of Momentum which states that the p total before = p total after. This law is vital for our understanding of momentum and how we solve for it.

Tide Resource

Each day, there will be four changes of tide. Every six hours, it will change from low to high or high to low. Therefore, every twelve hours, each type of tide will repeat itself. They are distanced in this increment because of the time it takes the moon to orbit the earth.

The difference in force is caused by the position of the moon in accordance with the sun. If the moon is alligned with the sun and earth, then there will be high tides where the moon pulls on the earth as well as the part of the world opposite that. In result, the other two corners of the world that are opposites will experience low tides.

When the moon pulls on a certain part of the world, it creates a tidal bulge which is an excess amount of water which then creates a high tide.

A spring tide occurs when the sun, moon, and earth are all lined up with each other and will result in extremely high tides. This only happens during a full or new moon. In contrast, a neap tide is when the moon is positioned on the right or left of the earth and there is less of a direct pull from the moon. This results in extremely low tides.

Right now (9:34 pm), the tide in Bogue Inlet is rising. The moon at this beach is not full or new, which means that it is currently experiencing a neap tide.

Newton's Third Law Resource

This image supports Newton's Third Law which states that every action has an equal and opposite reaction because both people are pushing equally on each other. This creates the action/reaction pair that explains how the interaction is possible. When the woman pushes on the man, the man is automatically pushing on the woman because of Newton's Third Law.