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.