Friction (1950)

when Jane and Bobby go for a ride early little Bobby who does the writing well big sister Jane does the pulling the chain doesn't mind runners are smooth the sled glides easily over the heart well pack snow well it did glide easily until it reached this bare spot where there isn't any snow have you ever wondered why it is so much easier to pull a sled on snow or ice than it is to drag it over a bear sidewalk or dry ground a simple experiment will help to show us the reason rub a piece of cotton over a mirror it slides easily on the smooth so that it doesn't it now stop it over a rough piece of wood the splinters and bumps on the would catch on the cotton and hold in fact it doesn't slide so easily on a rough surface does it if you look closely you'll see that the ground is pretty up to and full of little bumps which hold back the sled and make it hard to probe we say there is a great deal of friction between the ground and the runners of the sled whenever two objects rub together there is always a certain amount of friction which tends to keep the objects from moving the smooth of the objects the less friction there is do you think there is much friction between the snow and the sled runners friction because it keeps things from sliding easily makes them wear out for instance the rough surface of the sidewalk rubs against the bottom of your shoes and tears off tiny pieces of leather that is why the soles and heels of your shoes wear out because of friction automobile tires wear off because of friction between the road and the rubber tires knives and scissors become dull pencils need sharpening machines wear out all because of friction we cannot do away with friction entirely but can you think of ways to reduce it one way of course is to make the parts that rub against each other as smooth as possible the moving parts of machinery army very very smooth in order to reduce friction and have you ever heard people talking about seventeen jeweled watches the jewels help to reduce friction you cannot see them by looking at the outside of the watch they are inside at the points where parts rub together Jules such as rubies and sapphires or harder than steel and can be polished to a very fine smoothness watches having a large number of jewels in them usually lasts longer than watch is made entirely of metal yes smoothness helps a great deal but in most regimes ordinary smoothness does not reduce friction enough here is a piece of metal that looks and feels very smooth but if we could examine it under a microscope we would see that the smooth surface is actually covered with thousands of tiny bumps and holes it to pieces of metal like this thrive rapidly against each other those tiny bumps still cause some friction how can we reduce the friction even more by putting grease or oil between the two moving parts we can fill up all the little holes now the moving parts are actually sliding on a thin smooth layer of oil instead of rubbing against each other without oil no machine could last very long because friction would wear it out automobiles must be greased and oiled regularly sewing machines must be oiled and your bicycles and roller skates will run more smoothly and will last longer if you all them regularly have you ever tried to move a barrel if you have you know that it is much easier to roll it than it is to slide it and so another way of reducing friction is to change sliding friction to rolling friction of course that's easy if the object is around like a barrel but suppose the object is square like a box how can we use rolling friction here that's right by putting rollers under it and we can make our work still easier if we change the rollers two wheels the wheel has been called man's most important invention without wheels to reduce friction modern land transportation would be impossible to make wheels roll more easily we use ball bearings to reduce the friction between the wheel of the axle you have seen lit by reducing friction we can make things last longer and we can make our work easier but you should also know that friction is very useful to have you ever tried to turn a doorknob with wet soapy hands the soap and water reduce the friction between your handle allowed now you need more friction to clip the knob hard enough to turn it and so you'll dry your hands in order to increase the friction we increase friction in many other ways to bottle cats are usually made with ridges on them to keep your fingers from slipping the ridges increase the friction between your fingers on the cat automobile tires have rough threads on them to increase the friction between the road and the tires so they are not scared we use friction as a brake to stop moving objects Albright's work by increasing friction and we use friction and walking without friction you couldn't walk because your shoes would not grip the ground now see if you can answer these questions how does friction help you to write on the blackboard what does this sign have to do with friction and how is this boy scout making use of friction