Friction is a force that resists the motion between two objects in contact with one another.

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AP Physics rictio rictio! You ve heard that word, hec, maybe you ve eve used it. So what i the world is frictio? We've all heard about it ad blame it for all sorts of thigs.

1 AP Physics rictio rictio! You ve heard that word, hec, maybe you ve eve used it. So what i the world is frictio? We've all heard about it ad blame it for all sorts of thigs. It is the cause of may problems, but we could't get alog without it. Press your had dow o your destop ad the pull it sideways. The force that tugs o your had tryig to stop it is frictio. rictio is experieced by cars rollig dow a road, a horse walig i a field, a sailboat glidig through the water, or a airplae cruisig at 45,000 feet. rictio is a force that resists the motio betwee two objects i cotact with oe aother. or a facier defiitio, try this: rictio force caused by iteractio of a body with its surroudigs. Cause of rictio: rictio is brought about by may thigs. Oe major cause of frictio has to do with the electros of the two surfaces i cotact with oe aother. Whe the objects are close, electros from oe object will form wea bods with the atoms of the other (ad vice versa). These bods, idividually wea, are preset i great umbers ad add up to a sigificat force. Aother cause of frictio is the surfaces themselves. Eve the smoothest surface is, whe examied uder high magificatio, rough as a cob. Whe a object lies o a surface, its little hills ad dales presses dow ad gets stuc i the rocy moutais of the other surface. To mae them move you have to dislodge all the little catches ad hoos that have developed. Oce the thig is movig, the surface irregularities still try to catch ad lodge with each other. Aother cause of frictio is deformatio of the surface. A object sittig o a surface causes the surface to sag a little. So the object is sort of sittig i a little depressio. To mae it move, you have to dislodge it from the depressio it s i Value of rictio: Is frictio bad? Yes! rictio is a force that slows thigs dow, maes airplaes go slow, uses up eergy, maes you tired, costs a lot of moey, wears thig out, &tc. If frictio good? Yes! You could't wal without frictio. Sas frictio, cars would't go ad they could't be stopped. Your pats would fall off. &tc. orces Ivolved i rictio: rictio opposes motio. If you pull o a object to mae it slide across the floor, the frictioal force opposes that pull ad will be i the opposite directio. The frictioal force is always parallel to the surface ad i a directio that opposes the motio of the object. There are several forces actig o a object that is experiecig frictio. The weight of the object which pushes it dow oto the surface it sits upo. This is g (or mg), the weight vector. The force exerted o the object by the surface, pushig it perpedicular to the surface. This is, of course, our old fried, the ormal force,. The force that is pullig the object (or pushig it). This is called the applied force,. The frictioal force, parallel to the surface ad opposig the applied force, f.

2 If the surface is horizotal ad there are o other vertical forces = g. The ormal force,, is always perpedicular to the surface. If is greater tha f, the object must accelerate with a et force of f. If f is greater tha, the object wo't move at all. If f = the the object is movig at a costat speed (or is at rest) as the et force o the object is zero. Types of rictio: There are several types of frictio. The oe we'll be dealig with is called slidig frictio. There is also rollig frictio (which is much smaller). This is the frictio of wheels, rollers, ball bearigs, &tc. There is also frictio caused by passig through fluids - water resistace ad air resistace for example. f g igurig rictio: rictio exists betwee two thigs. No oe, sigle thig has frictio. rictio is the result of the iteractio of two surfaces. A rocet travelig through the atmosphere experieces frictio with the air but a rocet travelig i the vacuum of space experieces o frictio at all. (Well, maybe a teey amout because the vacuum of space does have a slight smatterig of loose atoms ad thigs, but they are so far apart that we ca virtually igore them.) The magitude of the frictioal force depeds o the ormal force ad the material of the two objects i cotact. Wood o wood would have a differet frictioal force tha steel o wood, ad so o. Heavy objects have more frictio tha very light oes, &tc. Whe a object is at rest, it taes a larger force to mae it move tha it taes to eep it movig oce it s got goig. Restig objects have to have their static frictio overcome before they ca move. s static force of frictio If a object has a force applied to it but remais at rest, the f s =. The static frictioal force ca have a max value of. Movig objects require a applied force to eep them movig that overcomes the ietic force of frictio. Kietic is based o a Gree word ad meas movig. f ietic force of frictio. If the object moves, the the applied force has to be greater tha the static frictioal force (to get it started) ad must be as big or bigger tha the ietic frictioal force.

3 Kietic frictio is always smaller tha static frictio. This is because a object at rest o a surface has its microscopic surfaces embedded i the surface. Also, the electros form their little bods (as metioed before), ad you have the depressio deal. Whe a object is i motio, you just have the rough surfaces boucig off each other. The electros do t have time to set up bods, so that force is ear zero, ad the depressio thig gets elimiated or redered pretty isigificat. Therefore the ietic frictioal force is always less tha the static frictioal force. idig the frictioal force is pretty simple. The frictioal force is proportioal to the ormal force. Sice there are two types of frictio, static ad ietic, we have two equatios: fs s f = s coefficiet of static frictio coefficiet of ietic frictio The value of the coefficiets depeds o the two surfaces i cotact with oe aother. These values are foud by experimet. Useful tables ca sometimes be foud that have the differet coefficiet values for commo materials wored out ad ready for use by the eterprisig physicist. Let s do a simple frictio problem. A 25.0 N wood bloc is pulled across a woode table at costat speed. What is the force eeded to do this? or the coefficiet of ietic frictio use The et force must be zero i the x ad y directio. This is because the bloc is ot movig up or dow, so the sum of the vertical forces must be zero. It is movig at a costat speed horizotally, so, agai, the sum of the forces i the x directio must be zero as well. x = f = 0 so = f Behold! The frictioal force is equal to the applied force! y = g = 0 therefore = g Sice everythig is flat ad there are o extra vertical forces, the ormal force equals the weight of the bloc. Kowig the ormal force, we ca calculate the value of the frictioal force, which is, of course, equal to the applied force. f f = = ( ) f N = = 8.8 N g Here is a table of values for some example coefficiets of frictio.

4 Coefficiets of rictio Materials Static rictio Kietic rictio Steel o steel Alumium o steel Wood o bric Copper o steel Rubber o cocrete Wood o wood Glass o glass Waxed wood o wet sow Waxed wood o dry sow Metal o metal (lubricated) Ice o ice Teflo o teflo Syovial Joits i humas Let s loo at a slightly more complicated problem where the applied force is at a agle. The force that frictio will oppose will be the horizotal compoet of the applied force. You pull a 65.0 g crate of mass m across the floor with a force at a agle of The coefficiet of frictio betwee the surfaces is If the crate moves at a costat speed, what is the applied force? The complicatio i this problem is that has a horizotal ad vertical compoet. We draw the BD. The compoets are si ad cos. m O = cos f = 0 x = + si mg = 0 y Normally we assume that the ormal force which is resposible for the frictioal force is equal to the weight of the object, but sice there is a upward compoet from the applied force, this is ot true ay loger. So we write out a equatio for the sum of the forces i the vertical directio ad use this equatio to fid the ormal force. si mg cos + si mg = 0 = mg si We also ow that: f = We ow substitute the value for foud i the first equatio ito the frictio equatio:

5 ( si) f = mg f = mg si We substitute i the value for the frictioal force ito the equatio for the sum of the horizotal forces ad solve the thig for the applied force: ( ) = cos f = 0 so cos mg si = 0 x cos mg + si = 0 cos si mg + = ( cos si ) + = mg = mg ( cos + si ) We ow what all this stuff is, so we ca ow plug ad chug. m 0.235( 65.0 g ) N = s = = ( cos si 34.0 ) 156 N

PROBLEM Copyright McGraw-Hill Education. Permission required for reproduction or display. SOLUTION. v 1 = 4 km/hr = 1.

PROBLEM Copyright McGraw-Hill Education. Permission required for reproduction or display. SOLUTION. v 1 = 4 km/hr = 1. PROLEM 13.119 35,000 Mg ocea lier has a iitial velocity of 4 km/h. Neglectig the frictioal resistace of the water, determie the time required to brig the lier to rest by usig a sigle tugboat which exerts