The concept of force. By the end of this topic you should be able to : state the difference between mass and weight ; draw vectors representing forces acting on a given body ; identify situations in which frictional forces develop and draw those frictional forces ;
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Bytheend of thistopicyoushouldbeableto:
statethedifferencebetweenmass and weight;
drawvectorsrepresentingforcesactingon a givenbody;
identifysituations in whichfrictionalforcesdevelop and drawthosefrictionalforces;
Why do objectsmovelikethey do?
Thisforceistheresult of thegravitationalattractionbetweenthemass in question and themass of theearth. Theweight of a bodyisthegravitationalforceexperiencedbythatbody, whichonearthisgivenbythe formula
W = mg
A stringthatistautissaidto be undertension. Theforcethatarises in anybodywhenitisstretchediscalledtension. A tensionforce in a stringiscreatedwhentwoforces are applied in oppositedirections at theends of thestring.
Tosaythatthereistension in a stringmeansthatanarbitrarypointonthestringisacteduponbytwoforces (thetension T).
If a bodytouchesanotherbody, thereis a force of reactionorcontactforcebetweenthetwobodies. Thisforceis perpendicular tothebodyexertingtheforce.
Dragforces are forcesthatopposethemotion of a bodythrough a fluid (a gas or a liquid).
Anyobject placed in a fluid experiencesanupwardforcecalledupthrust. Iftheupthrustforceequalstheweight of thebody, thebodywillfloat in the fluid. Iftheupthrustislessthantheweight, thebodywillsink.
Frictionalforcesopposethemotion of a body. Frictionariseswheneveronebodyslidesoveranother. In this case wespeak of kineticfriction. Frictionalsoariseswheneverthereisjust a tendencyformotion, notnecessarilymotionitself. In this case, wespeak of staticfriction.
A free-bodydiagramis a diagramshowingthemagnitude and direction of alltheforcesactingon a chosenbody. Thebodyisshownonitsown, free of itssurroundings and of anyotherbodiesitmay be in contactwith.
Ifwe try toextend a spring, a forcepullsthespring back toits original length; ifwe try tocompress a spring, again a force tries topullthespring back toits original length. Theforce in thespring has a simple relationshiptotheamountbywhichthespringis extended orcompressed.
T = k
Theextensionorcompression of thespringmustnot be toolarge, otherwiseHooke’slawisn’tapplicable. Thatrangeiscalled as theelasticlimit.