pes of orces pes of forces. orce of Grvit: his is often referred to s the weiht of n object. It is the ttrctive force of the erth. And is lws directed towrd the center of the erth. It hs nitude equl to the ss of the object ties the ccelertion due to rvit, or.. Norl orce: When n object rests on nother surfce (which could be nother object in proble), the surfce exerts force on the object which is perpendiculr (or norl) to the surfce of contct. Exple: Book on tble N N Exple: Book on sooth incline Lecture 8: orces
pes of orces pes of forces 3. riction orce: When n object slides over surfce, there is usull soe resistnce to this slidin. his is due to friction force, nd is lws directed opposite the intended direction of otion lon the surfce. he size of this force is dependent on the prticulr object nd surfce - for exple, it is hrder to push hocke puck lon n sphlt drivew thn it is lon n ice rink. Often, we will refer to soe surfces s sooth, nd in this cse we will ssue tht the friction force (which is rell lws present) is sll enouh tht we cn inore it ltoether. 4. ension force: his is the force exerted b rope, cble, or strin, when it is ttched to n object nd pulled tut. It is directed w fro the object nd lon the rope t the point of ttchent. We will ssue (unless otherwise stted) tht the ropes re ssless nd unstretchble. Ropes nd cbles re soeties used in conjunction with pulles, which we will ssue (unless otherwise stted) re ssless nd frictionless. In this cse the pulle does not chne the nitude of the tension in the rope, it just chnes its direction. Lecture 8: orces
Exple: (Ch 5, Prob 43) An elevtor of ss.60 x 0 3 N ovin downwrd t.0/s is brouht to rest with constnt ccelertion in distnce of 4.0. ind the tension in the supportin cble durin this tie. I) Drw picture of the object, showin the forces which re ctin on it: Exple (Hnin block) Cble exerts tension on the elevtor. x Grvit exerts force equl to on the elevtor. II) Now ppl Newton s nd Lw to the elevtor: ( ) But wht is? Lecture 8: orces 3
Exple: (Ch 5, Prob 43) Continued. Exple (Hnin block) o find the ccelertion of the cble, we cn tret the otion of the elevtor s otion under constnt ccelertion. We know the initil velocit of the elevtor, its finl velocit (zero), nd its initil nd finl position. v v t 0 0 0 4.0.0/s 0?? v 0 v0 44 44 8 ( ( 4.7/s 0) 0) Wh is the ccelertion +? Now substitute this vlue of bck into the erlier eqution to find : 600(9.80 846N 3 8.4 0 N.7) Lecture 8: orces 4
Exple: wo blocks lie in contct on frictionless tble s shown below. A force =3.N is pplied to one of the blocks. Wht is the norl force between the two blocks? Exple (wo blocks bein pushed) =.3k =.k I) Drw the forces which ct on ech block. Block #: Block #: is the norl force (or contct force) of block on block. is the norl force (or contct force) of block on block. It is the Newton s 3rd Lw equl nd opposite force of the norl force of block on block. Lecture 8: orces 5
Exple: wo blocks continued Exple (wo blocks bein pushed) II) Now ppl Newton s nd Lw to ech block: Block #: Block #: block block substitute ( ) 3.N 0.9/s 3.5k nd then NOE: he ccelertion of block is equl to the ccelertion of block..n Lecture 8: orces 6
Exple (Continued): Exple (Continued) Wht is the interprettion of the force? his is the force needed to ccelerte block t 0.9/s. Wht hppens to the force if the force is pplied insted to block? Will the ccelertion be the se? Wht bout the interprettion of the force? Lecture 8: orces 7
Exple ( blocks connected b strin) Exple: wo blocks re connected b strin. Block is on frictionless surfce, while block hns freel. he strin psses over ssless frictionless pulle. ind the ccelertion of block, the ccelertion of block, nd the tension in the strin. 3.3k.k ) Drw the forces on the two blocks Block : N Block : x Lecture 8: orces 8
Exple (Continued): Exple (Continued) ) Appl Newton s nd Lw, seprtel, to both blocks: block block x x x N N 0 0 Lecture 8: orces 9
Exple (Continued): Exple (Continued) 3) IMPORAN: Since the strin does not stretch, the nitude of the ccelertions of the two blocks re the se! We defined to the riht s positive x nd upwrd s positive. If block ccelertes to the riht, then block ust ccelerte downwrd, so the ccelertions ust hve opposite sin (even thouh the hve the se nitude). Cll this coon ccelertion : x Put this into the equtions for the two blocks: x x x Lecture 8: orces 0
Exple (Continued): Exple (Continued) 4) Now substitute: into: (.)(9.80) 3.3. 3.8/s (3.3)(3.8) 3N 5) Checks of nswer: If then 9.80/s If then 0 Lecture 8: orces
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