Category: physics

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Friction for Children: 4 Tricks to Help Children Understand Friction

By Lorie Moffat

If you’re looking for ways to aid in teaching friction for children then keep reading. So your child comes home from school with questions about friction. How can you help your child understand this concept? Without friction life as we know it would not exist. Every surface would be more slippery than ice. You could not walk, run, write, or even feed yourself without friction. Friction for children is as easy as using common examples to guide your explanation.

Friction for children starts with the basics. Friction is a push, pull, or a force which works against the motion of objects that are in contact as they move past each other. When objects are touching their surfaces tend to stick together, like the tiny loops and hooks of Velcro. Heat and sound are also produced by friction. If you rub the palms of your hands together quickly your hands get warm and you can hear the sound that friction creates.

There are three types of friction; sliding friction, rolling friction, and fluid friction. Sliding friction is caused by two objects touching each other that slide past one another. For example, when you push a large wooden crate across a floor you push against sliding friction. The entire surface of the crate that is in contact with the floor slides against the floor, slowing down the motion of the crate. Rolling friction uses wheels. If you move the identical large wooden crate with a wagon then you exert a force against rolling friction. Only the bottom of each wheel is in contact with the floor. Rolling friction is less than sliding friction; it takes less effort to push the crate on the wagon than to push the crate that is directly resting on the floor. When an object is in contact with a fluid, a liquid or a gas this is considered fluid friction. Airplanes and race cars are streamlined to reduce fluid friction. They have smooth, curved surfaces to reduce the friction, called drag, with the air.

When teaching friction for children it’s important to stress how friction can be advantageous. You light a match using friction. As you strike a match, friction creates enough heat to ignite a chemical compound in the match head that then burns the rest of the match head. Automobile brakes work because of friction. As the brake pads rub against the car’s wheels, the car slows down. Shoes designed for some sports have special soles to use friction to your advantage. Baseball shoes and football shoes have cleats to increase friction by sticking to cracks in the ground. A violinist puts rosin on his bow to increase friction between the bow and the violin strings, therefore producing sound.

However, friction can also be disadvantageous. If a door hinge squeaks, the noise is caused by friction. The space shuttle’s nose and wings heat up dramatically as it returns to Earth from orbit. The ceramic tiles on the shuttle’s nose and wings are designed to dissipate this heat caused by friction. The moving parts of a car’s engine rub against each other and can stick together, causing the engine to seize and to stop working. Using oil in a car’s engine protects the parts from friction. Cooked foods tend to stick to pans. Teflon on non-stick cookware reduces friction between the food and the pan, causing the food to slide. Competitive swimmers wear specially designed racing suits to reduce the friction between themselves and the water so that they can swim faster. A bowler wears extremely flat-soled shoes to slide on the lane right before he releases the bowling ball. Silicone aerosols, oils, grease, graphite (the very soft form of carbon in “lead” pencils), and ball bearings are all used to reduce friction.

By using every day examples, you can teach friction for children and help them better understand this concept. The three types of friction, sliding, rolling and fluid, can either be beneficial or detrimental to the motion of objects. Friction between your pen or pencil tip and the paper you write on allows you to write on the paper. Friction between the ground or the floor and your feet allows you to run or walk along these surfaces. Friction between your food and a spoon or fork allows you to eat with these utensils.

Lorie Moffat has 20 years of teaching experience in both public school classroom and science museum settings. Contact her about special summer online tutoring packages.

Source: http://www.homeschool-articles.com/friction-for-children-4-tricks-to-help-children-understand-friction/

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Everyday Science: Contrails

It’s fall, and that means leaves falling, pumpkin pie baking, calendars rapidly filling, and ….. contrails forming.  So what exactly is a contrail?

Contrails (short for condensation trail) are man-made “clouds” created by aircraft engines when the temperature and humidity conditions are just right.  Usually occurring at temperatures below -40°C, and at altitudes above 26,000 ft, contrails are formed when water vapor condenses and then freezes on the particulate matter in aircraft exhaust.  To form a contrail, the temperature must be low enough or the humidity must be high enough for water to condense on the exhaust particles.  Since the process of condensation is well understood, it is possible to predict when contrails will form based on prevalent temperature and humidity conditions.

Typically, with fall weather comes cool, moist air aloft and conditions at are perfect for contrail production.  Think about it: do you see very many contrails in the summer months when it is hot and dry?  Sometimes contrails dissipate rapidly with high winds or high turbulence aloft, and sometimes contrails linger for quite some time.  Contrails dissipate rapidly with low humidity, since the newly formed ice crystals evaporate quickly.  With high humidity, persistent contrails are visible, as ice crystals grow in size, absorbing water from the surrounding (humid) atmosphere.  You might see a contrail which has been stationary for some time right next to one which is dissipating rapidly.  These contrails are at different altitudes, with different conditions at each level.

Since different gas composition, exhaust gas temperatures, and water vapor content exist in different aircraft engines, not all aircraft engines will produce a contrail at the same altitude (with identical weather conditions) at the same time.  Contrail formation has nothing to do with the type of aircraft or its speed.

Another form of contrail is produced when a portion of an aircraft such as a wingtip or winglet causes air cavitation in humid conditions.  Have you ever seen these vapor trails, perhaps at an airshow, when aircraft flying low over the airfield pitch up rapidly in high humidity conditions?  And since we’re mentioning winglets (shown below), you might be interested to know why they’re on the wing.  The winglet serves to reduce aerodynamic drag so the aircraft burns less fuel in flight.  This fuel efficiency is created by breaking up the turbulent wingtip vortices (pronounced VOR-ti-sees) produced by the pressure differential between the bottom and top of the wing.  No vortices = less drag.

Southwest Winglets

Back to the contrails, though… have you ever seen patterns in the sky produced by contrails?  Perhaps all paralleling one another or converging on a point?

This is no coincidence.  The world is criss-crossed with jet routes flown by all aircraft above a certain altitude.  At the junctions of jet routes are navigational aids, which you may have seen while driving or flying yourself.  Here is one navigation aid (navaid) in Oregon, shown courtesy of Wikipedia:

The aircraft whose contrails you see are each navigating with navaids similar to the one shown above.  Therefore the aircraft fly over the navaids as they transit the jet routes.  Parallel tracks of contrails are aircraft flying along the same route.  If you are ever curious about a contrail track you observe, you can check http://flightaware.com.  A screen shot near Dallas, Texas shows multiple aircraft tracks transiting along an east-west jet route using navaids.

Contrail predictions have been used since WWII, when the ability to spot enemy aircraft “conning” could make the difference in advance notice of an airstrike or positioning for air combat.  Here’s a graphic from p. 55 of the March 1943 edition of Popular Science identifying two types of vapor trail formation:

How about an activity to predict contrail formation?  NASA has an activity page where you can access  an Appleman chart.  Using the chart, you can construct a temperature profile to forecast contrail activity in your area.  The activity (recommended for 5th grade and up) begins this way:

…”Military planners have been interested in condensation trail (contrail) forecasts since World War II. Contrails can make any aircraft easy to locate by enemy forces, and no amount of modern stealth technology can hide an aircraft if it leaves a persistent contrail in its wake. In 1953, a scientist named H. Appleman published a chart that can be used to determine when a jet airplane would or would not produce a contrail. For many years, the US Air Force Global Weather Center used a similar chart to make contrail forecasts.

The first published reports of contrail formation appeared shortly after World War I. At first, scientists were not sure how contrails formed. We now know that they are a type of mixing cloud, similar to the cloud that sometimes forms from your breath during a cold winter day. Appleman showed that when the air outside of the airplane is cold enough and moist enough, the mixture of the jet exhaust and the air would form a cloud.

An example of a contrail-forecasting chart is shown below. We will use the chart to make our own forecasts, and make observations to determine whether they are true or false.

Appleman Chart

… Using either the temperature information provided by the teacher or the temperatures obtained from an Internet location (see note below), complete the table on the “Student Data Sheet”, then plot the temperatures corresponding to each pressure level on the “Appleman Chart: Student Graph Worksheet”. Connect the points to create a temperature profile of the atmosphere.

Note: Temperature and humidity information can be obtained from weather balloon soundings launched twice a day from several locations around the country. Several locations on the Internet including http://weather.uwyo.edu/upperair/sounding.html provide detailed sounding information. Choose the location nearest your school.”

This activity would make a perfect accompaniment to a physical science class, a weather unit study, or just a fun diversion.  If you have a weather station in your home, it would pair up nicely as an associated activity.  For younger elementary students, the NASA site also has a word search and a downloadable “Clouds and Contrails” craft.  Have fun!

Resources:

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Activity: Experiencing Centrifugal Force

To perform this activity, you will need a merry-go-round and a small, light-weight ball.

1.  Get onto the merry-go-round and hold on tight.  Have someone spin you very fast. 
    a)  How do you feel yourself being pulled? 
    b)  If you were to let go– which you certainly won’t do– which way do you think you would go? 
    c)  Try moving to the center of the merry-go-round, to a point mid-way between the center and the edge, and then the edge.  At which location do you experience the most “pull”?

2.  Now have the person spin you at a moderate rate while you hold the small ball. 

3.  Look for a landmark object, such as a tree.  The next time you come to the tree, let go of the ball.  Make sure you just let go, don’t throw it.     
    a)  Where does the ball end up? 
    b)  Did it go directly towards the tree?

Have you ever been on one of those amusement park rides that you stand up in while it spins around very fast?  If you have, it feels as if you are being thrown out against the side doesn’t it? 

It’s called centrifugal force, but it is not a force at all.  It is an effect you feel that results from the curved shape of the ride and the forward motion you have.  An object moving in a circle, acts as if it were experiencing an external force when in fact it is not.  Centrifugal force is dependent on the the object’s mass, rotation speed, and the distance from the center of rotation. 

If the walls of the amusement park ride were to suddenly disappear, which way would you go– straight back, or to the side?  Remember which way the ball went when you let go of it while spinning around.

Think of what happens to you when you are in a car that turns suddenly.  You are thrown up against the side of the car.  The reason is explained by understanding an insight from Galileo.  He said things like to keep going the same direction and the same speed.  It takes a force to change the direction or speed of something.  So when the car turns, your body wants to go straight, at least until it is pulled back by the seat belt or hits the side of the car!  Thus, it feels as if you are being thrown up against the side of the car, when in reality you are just going straight as before, while the car is turning.  To better understand this, imagine that the amusement park ride is a square box instead of a circle.   What would happen?  You can try it with a marble and a cardboard box.  Spin the box with the marble in it and watch what happens!