What I learned this unit: Work is a force applied over a distance. Work= Force x Distance( force and distance must be parallel to have work) Unit for work is in Joules (J) Power = Work/Time Unit for power is in Watts (J/s) or Horsepower (746 watts) Kinetic Energy (KE) is the energy of movement and can be found with the equation KE= 1/2mv^2 Change in KE is equal to work. Potential Energy is the energy an object has at some height and can be found with the equation PE=mgh KE and PE are inversely related. When an object is at rest it has no KE, but if at a height it has PE. Energy is never created or destroyed, only transferred into something else (heat, sound etc...) A simple machine is something that makes work easier, you are still doing the same work but it feels easier due to the fact that you use less force because of increased distance. Examples are Ramps, Levers, and Pulleys Work in = Work out Machine Efficiency= (Work out/Work in) x 100 In this unit, I had trouble with the concept of potential energy, but after a short time I mastered the idea. My problem solving skills, effort, and learning: My problems solving skills remained about the same as this unit went on, I simply started to see problems in a different way. My effort has been a bit elevated in this unit as I was working with concepts I had never seen before. My goal for next unit is to keep up strong effort and contribute more to the class. My previous goal of trying to work harder on having better effort has worked and I felt that I was trying harder in this unit than before. This unit helps me understand another reason for why Newton's Cradle works.
This video was very helpful review as it applies exactly to what we have learned in class. This video was made by the creator of the textbooks that we use and therefore uses basically the exact terminology that we use in class. I would recommend this video for anyone who is just learning about simple machines or to anyone who wants to review.
The above video provides a perfect example of work and power and the differences between them. This helps me understand the concepts because lifting weights is something I do every day. The only concept that they left out from the video was that the force and the distance must be parallel to be classified as work. But it does provide the basics of work= force x distance, and the fact that power is work over time.
Tangential speed- the distance an object covers in a given time. Depends on radial distance.
Rotational speed- the amount of rotations or portion of a rotation an object performs in a given time
Gears: Same tangential
speed, the smaller gear will have to have a faster rotational velocity
to account for the equivalent tangential speed.
Example- 8 teeth vs 4 teeth.
Smaller gear will have twice the rotational velocity as the bigger one
because it has to rotate 2 times to go the same distance it takes the
bigger gear to rotate once.
DIFFERENT RADIUS FROM AXIS OF ROTATION= DIFFERENT TANGENTIAL VELOCITY
Rotational Inertia + Conservation of Angular Momentum
Rotational inertia- property
of an object to resist changes in the spin. Distribution of mass affects
RI. Easier to spin= less rotational inertia, harder to spin= more
rotational inertia.
The greater the distance a mass is from its axis of rotation, the more rotational inertia it will have.
Angular momentum before= Angular momentum after
RI x RV before= RI x RV after
A hoop will have more
rotational inertia than a steel ball because its mass is further from
its axis of rotation, it will therefore lose a downhill race. (same goes
for frozen water bottle vs water bottle)
Torque and Center of Mass
Torque- something that causes a rotation, = Force x Lever Arm
Lever arm- the distance from an object's axis of rotation.
When an object is balanced its counterclockwise torque is equal to its clockwise torque
Center of mass- average position of mass
An object will fall when its center of gravity is outside of its base of support
Examples of torque + center of mass problems:
Why are football players told to keep their legs bent and shoulder width?
To widen their base of
support and get their center of gravity closer to the base of support,
they will then have to be pushed harder to get their center of gravity
outside of their base of support and fall.
Large wrench= more torque
10 N ball rests on one end of
a meter stick with a 1m lever arm. How much must a ball on the other
side weigh if it is 5m away from its axis of rotation?
10 x 1= 10
10/5= 2N
Centripetal Force
Centripetal force is a center seeking force
Centrifugal force does not exist
Why are racetracks banked?
So that they can have more centripetal force from the x component of the support force coming from the track.
In this unit I had difficulty with the concept of the racetrack being banked, but after being shown the force vectors I felt completely fine with the subject. I overcame this difficulty by learning to think of force vectors whenever provided such a picture.
My Problem Solving Skills, Effort, and Learning
Throughout the unit my problem solving skills improved by learning to think of things in different ways such as in force vectors or as in centripetal forces interacting with objects in the world. My effort in the class has been fairly consistent with a few dips in effort due to the fact that I grasped the material sooner than classmates and waited for them to see the reasoning behind a problem.
My goal for next unit is to have my effort back to where it was before.
My previous goal of not procrastinating has improved greatly and I often feel better prepared for class.
This unit helps me understand why in football I have to keep my legs spread out and my knees bent as to not be hit over so easily.
The above image includes the pictures for step 1 parts A, B, and C. A. The equations that we used for our initial plan are as follows:
Torque= Force x Lever Arm
Torque Clockwise = Torque Counter-Clockwise (when balanced)
W=mg
Our initial plan was to multiply the mass of the added weight by 9.8 to give us the total weight added to the ruler. We would then find the torque on the side of the weight (we'll use counter-clockwise torque to describe this) by multiplying the added weight by the distance from the pivot point (Lever arm, 30cm).
Since the center of mass of the meter stick itself wasn't changed by adding weight, its center of mass remained at 50 cm. We then found the distance to the pivot point from 50cm and found that it was 20cm for the clockwise lever arm. We did not know the weight of the meter stick, so we plugged in x for that.
The Clockwise torque = the Counter-Clockwise torque when balanced, so we set the two equations of (0.98N)(30cm)= (X)(20cm).
We would then solve for the weight and convert the weight to mass by dividing it by 9.8.
The above image includes all the work that I did in step 3 describing our procedure. B. Our initial plan worked.
Balancing point was at 70cm, the lever arm of the weighted side was 30cm, the added weight was 0.1 kg, which weighs .98 N. The center of mass of the meter stick was at 50cm. the lever arm from the center of mass of the stick to the pivot point was 20cm. Our final mass of the meter stick was 150g. C.
For the measurements we took see part B.
Our initial plan worked to solve the problem. When things are balanced the Counter-Clockwise torque will equal the Clockwise torque. The center of mass of the meter stick was unchanged throughout the whole lab. We remembered that when we found the weight of the meter stick we needed to convert it to mass by dividing it by 9.8. The actual mass of the meter stick was only 0.8 off of what we had calculated. D.
The above video is really helpful to understand the concept of Center of Gravity. It provides physical examples of how a person's center of gravity can affect what they do every day. This video gets to the point quickly and is very helpful for people who are struggling with the subject, or people who need to review it. Thought the quick speed of the video may cause them to leave out a few details about center of mass and such.
The above video concerns Angular momentum and how it relates to rotational motion. This video touches on everything that we have talked about (it was made by the same guy who wrote our physics books). This is a really good video to watch if you are having trouble with the concept of angular momentum or if you are just learning. (I used this video because all of the other videos were for AP physics that I found and had a lot of different formulas).
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