Snow Physics, cont.

After a wonderful spell of blessed warmth, we are again in the grips of the demon snow. Here is another reason why I hate snow. Walking dogs in snow is a task best left to the experts. Even on a flat, horizontal surface, the danger of slipping is large, but when you have to dodge cars that have blocked the sidewalk, this puts an extra measure of danger into the situation. You must now walk on the sloping driveways and this is a three-way problem, from a physics standpoint.

1. Snow and ice have a lower coefficient of friction with shoes than does cement, brick or asphalt. The coefficient of friction can be thought of as a way of telling how large a frictional force will be encountered when one object is in contact with another. A lower coefficient of friction, means less frictional resistance. When you walk, the static friction is greater than your force applied against the sidewalk, so you don’t slide. With snow and ice, however, you meet far less resistance and it is more likely that friction won’t be able to keep you in place.




2. When you walk across a flat, horizontal surface none of your weight is directed in the direction of your motion. Your weight points towards the center of the earth and you are moving forward. On an in incline, however, that changes. Even though the direction of your weight does not change, the angle of the surface has and it sets up a situation where a component of your weight is directed downwards along the slope. Without trying, you have now set up a force that is pulling you down the slope. The steeper the slope, the greater the proportion of your weight that is acting to pull you down the incline.




3. Your center of gravity is positioned roughly in the middle of your gut. As long as that point stays above your base of support (the area bounded by your feet), you stay upright. If that center of gravity finds itself away from your base of support, you fall. Walking on a flat surface, your center of gravity stays happily above your feet, on an incline, the chance is greater that your support base will get away from your center of gravity (your body leans out away from your feet). This makes your position very unstable.

Taken in sum, all of this means that I had to endure the indignity of landing at the bottom of a neighbor’s driveway, backside down, with two dogs licking my face to make sure that I was still alive to fill their food bowl. Happy New Year….

Have a Great Vacation

However you choose to spend the holiday season - enjoy yourself!

Here are some songs to add to your holiday repertoire - Physics Carols!

The End is in Sight

Vacation is almost at hand and we can at least count on the fact that there will be plenty of snow. Like everyone else, I would have liked a delayed start this morning, but we made do nicely. Physics - the exam is over and we will dip our toes into Chapter 8 tomorrow. Honors Physics - equilibrium problems can be a bear. Make sure to carefully write out the relationships for the 1st and 2nd conditions of equilibrium, take close note of the direction of each force and torque and choose an axis of rotation for the net torque evaluation that eliminates one of the unknown forces from the equation. We'll go over the problems in class tomorrow and pick up with more rotational dynamics when we return. Physical Science - tomorrow's lab will center around pulleys. This will give us one lab for the lever family and one lab for the inclined plane family for this unit. After vacation, we will add energy to our study of work, power and machines.

Snow

All of you who say you love the snow are crazy. Plain and simply crazy. However, here is a good site about snowflakes and snow crystals that even includes a good bit of physics...

SnowCrystals.com

Here is a site with a collections of cartoons about snow...

Off the Mark

And this is just a waste of good coffee...

Lots of Stuff Whirling

Centripetal force and torque were the two lab topics today. Physics - complete your centripetal force lab tonight - all graphs and calculations - so that we can discuss it tomorrow. Also, make sure that you have finished your review worksheet so that we can use that to prepare for Friday's exam. Honors Physics - make sure all calculations for the torque and equilibrium lab are completed. We will discuss torque in detail tomorrow and consider examples where applied force and the lever arm are not perpendicular. Then, we begin to examine the concept of rotational dynamics and evaluate, mathematically, the two conditions of equilibrium. Physical Science started their discussion of simple machines with the lever family and tomorrow's lab will center around levers. Our next long block, we will do a lab examining pulleys. This website has a nice flash lesson on simple machines and mechanical advantage.

Get the Office Together

Let me inform you of something - putting together a tv stand, office chair and desk with the enthusiastic help of two dogs is somewhat of a trial. Fortunately, they chose not to assist with the unpacking and setting up of the flatscreen. Home office with HDTV and Xbox 360 - I'm not sure if productivity will he high, but at least work will be fun.

Physical Science - inclined planes are an example of a simple machine, something that makes work easier. Notice that the amount of force you used decreased, but work increased by using the inclined plane. Mechanical advantage varied with slope angle, but efficiency was pitiful each time. Exactly as I would expect. Tomorrow, we will go into depth about machines and their connection to work and will stay with this topic a few days, with an additional simple machine lab to reinforce the ideas.

Physics - gravity and its effects on motion was the topic of the day. Tomorrow, some math practice with gravity and centripetal force problems and then a centripetal force lab on Wednesday. Thursday, review - Friday, exam. Week over.

Honors Physics - tonight's reading on torque will give you some background for tomorrow's lab. The lab will center on torque, balance and the two conditions of equilibrium. Pay attention to the sign conventions for torque, as you did for rotation, when working torque problems and analyzing the data in lab.

Flying Pigs!

Honors Physics investigated circular motion using a flying pig today and a fun time was had by all (except the pig who had to fly in a circle all day). Centripetal force is not always the tension in a cord, as for the last lab, it might be a component of that tension and you have to use vector analysis to puzzle that out. Tomorrow - review. Monday - exam.

Physics started defining the descriptors of angular motion and will take it further tomorrow. If you have used radians before, then this is nothing new for you. If you haven't covered that unit in math yet, we are not going to go in any deeper than we did today in class.

Physical Science - as you work on the Work worksheet (!), remember to keep track of the direction of force. Not always is weight the force you want for calculating work. Read the problems closely and consider direction of motion and direction of forces. Note: the answer for the last problem on the worksheet should really be a negative value, although the answer key lists it as positive. From our discussion of positive and negative work, you should be able to tell me why work, for this situation, is negative.

Busy, Busy, Busy

And I mean me, not you, so I've gotten behind posting. Oh well, life goes on...

Physical Science - tomorrow starts a new unit: Work and Energy. Both will still use the ideas that we have been building concerning motion and forces, we'll just add some new concepts to the pot and work towards tying things together into a tighter package. Physics - you guys start rotational motion tomorrow. We will not explore the mathematical aspects as much as the conceptual aspects, so do not worked up seeing all of the formulas in the chapter. Honors Physics - finished up talking about basic circular motion today and with an additional lab tomorrow and review on Friday, we are now in test preparation mode. The next chapter will examine interesting applications of circular motion that many of you engineers will appreciate.

Whirling Stoppers

Honors Physics investigated centripetal motion and how centripetal force and radius affect tangential velocity. Most groups seemed to show the appropriate patterns - with greater force (radius constant), tangential veloticy increases and greater radius (force constant)tangential velocity again increases. Think about those relationships when reflecting on the types of forces that can act as centripetal forces - gravity, friction, tension, etc.

Physics reviewed perfectly inelastic collsions and began work on elastic collisions. We will review this tomorrow and any other material that people request in preparation for Wednesday's exam.

Physical Science reviewed forces and fluids. Do not let go of your basic math skills. There is nothing in these problems that goes beyond the basic middle school math frameworks. Read the problem, identify what you are given for information, choose a formula or series of formulas to use that information to determine the final desired quantity. Although I give you the formulas on exams, as does the MCAS, you need to know how, when and why to use them. Tomorrow, we discuss Bernoulli's principle and the exam is on Wednesday.

Friday!

A few classes were low on students due to the band/chorus hoohah, but we made do nonetheless. Physics discussed yesterday's lab and used it to introduce the concept of collisions. Remember that both perfectly inelastic and elastic collisions demonstrate conservation of momentum and the same problem-solving skills apply. Only elastic collisions demonstrate kinetic energy conservation; however, inelastic collisions demonstrate a loss of kinetic energy. Physical Science students spent the period working on problems dealing with buoyancy and pressure. I posted the solutions on the course webpage for your two worksheet sets. Honors Physics discussed tangential velocity/acceleration and their connections to their angular counterparts. Centripetal acceleration was introduced and Monday's lab will allow you to investigate centripetal force and the resulting effects on motion. Have a good weekend!

Covering New Ground

Honors Physics had a test today and the homework assignment that drops you into the wonderful world of radians. For some of you, the the initial discussion of the unit circle and the relationship between radians and degrees will be a review. For others, this may be new - regardless, you will be using radian values for angle measurements and for then reporting values describing angular motion. Note: when you look through this and the following chapter, you may feel overwhelmed by what seem to be a whole suite of new formulas. Relax. You've seen them before, only wearing their translational cape. Pick each formula in Chapters and 8 apart and you find that it is the same as one we've used before, only with the angular equivalents of the variables. Here's a little video about the Unit Circle, for those who might need a review:



Physical Science - remember that forces are still the only things that can cause changes in motion, so when you discuss floating and sinking, discuss it in terms of the object's weight and the buoyant force of the fluid. Today's density discussion described that property of matter and your homework problems will give you some practice calculating density. However, density does not make something float or sink - density determines the objects weight for its size and the buoyant force of the fluid for the displaced volume. Regardless, it all comes back to forces and we'll tie these ideas together tightly tomorrow. For you, here's a video about density and buoyancy:



Physics - tomorrow's lab will have you review the concept fo 2-dimensional motion and vectors and use them to analyze momentum. It is easy to forget that momentum is a vector quantity when all the collision we study occur in one dimension. Refresh your mind about horizontal projectile motion, as that is what will occur after the collision in lab takes place and how to add vectors to find the resultant.

Still on Forces

Regardless of the specific concept we've attacked so far, forces have been lurking in the background. Welcome to the wonderful world of matter. Physics, today, discussed momentum and impulse, which branch from and look back towards our discussions of Newton's laws of motion. Tomorrow, we begin looking at conservation of momentum, which is a consequence of, specifically, Newton's third law of motion. Physical Science examined buoyant forces in lab. A couple of groups had slightly quriky results, but other groups work nicely demonstrated that the apparent lost of weight of an object (the buoyant force) was equal to the weight of the displaced water. The homework questions should be approached in terms of the forces involved. We'll add the ideas of floating, sinking and density to the mix tomorrow. Honors Physics prepared for tomorrow's exam. Even though I make light of it, do make sure that as we move into the next chapters that you remember to do your work with your calculators in radian mode. Every year, people get very frustrated that they aren't getting their homework problems right and it turns out they are working things in degrees.

Talent Showcase

Nate B. is a great woodworker and made these bottle/decanter stoppers:



The work is just lovely and he has other styles and types of wood. These would make very good holiday gifts; however, I am keeping mine for myself...

Different Places

Every course is working on something different right now, so things are pretty busy. Physics - the lab today on momentum and impulse, helped demonstrate the concpets that A block discussed on Wednesday and that E block will discuss tomorrow. Due to the less-than-frictionless lab situation, don't expect your impulse and momentum change values to be dead-on exact. They should be reasonably close, though, and we'll discuss the ranges of people's data tomorrow.

Honors Physics - tidied up the remnants of the momentum chapter. Exam on Wednesday. Bring questions for tomorrow's review and today's chapter review problems to discuss.

Physical Science - An overview and a couple of demonstrations of buoyancy. Tomorrow's lab will highlight those ideas and add in the concepts of calculating buoyant forces (rather than measuring them) and predicting floating and sinking for an object placed in a fluid. Even though water will act as our fluid in tomorrow's lab, the concept of buoyancy, buoyant forces and Archimedes Principle apply to all liquids and gases.

General Note: Some groups are getting a little careless in lab, in terms of equipment. First off, keep an eye on all the pieces for your sensors. Already, a couple of force sensors have lost their thumbscrews. Secondly, only put the sensors, LabPros and cables in the containers. I should not find heavy masses, string, scissors, etc. in there. Lastly - CLEAN UP WHEN YOU'RE FINISHED!!! Some groups do a great job with clean up and others do a terrible job. I am goign to have to start holding groups after class to clean up their stations if you can't take the initiative on your own. Repack materials, throw away trash (in the trash can, not the sink or lab drawers), return materials you borrowed from other groups to them, etc. Saves time for me and groups using the equipment after you.