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The concept of force

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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The concept of force

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  1. The concept of force Bytheend of thistopicyoushouldbeableto: statethedifferencebetweenmass and weight; drawvectorsrepresentingforcesactingon a givenbody; identifysituations in whichfrictionalforcesdevelop and drawthosefrictionalforces; use Hooke’slawcorrectly.

  2. Dynamics Why do objectsmovelikethey do?

  3. Weight Thisforceistheresult of thegravitationalattractionbetweenthemass in question and themass of theearth. Theweight of a bodyisthegravitationalforceexperiencedbythatbody, whichonearthisgivenbythe formula W = mg

  4. Tension 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).

  5. Normal reactionforces If a bodytouchesanotherbody, thereis a force of reactionorcontactforcebetweenthetwobodies. Thisforceis perpendicular tothebodyexertingtheforce.

  6. Dragforces Dragforces are forcesthatopposethemotion of a bodythrough a fluid (a gas or a liquid).

  7. Upthrust Anyobject placed in a fluid experiencesanupwardforcecalledupthrust. Iftheupthrustforceequalstheweight of thebody, thebodywillfloat in the fluid. Iftheupthrustislessthantheweight, thebodywillsink.

  8. Friction Frictionalforcesopposethemotion of a body. Frictionariseswheneveronebodyslidesoveranother. In this case wespeak of kineticfriction. Frictionalsoariseswheneverthereisjust a tendencyformotion, notnecessarilymotionitself. In this case, wespeak of staticfriction.

  9. Examplequestion

  10. Free-bodydiagrams A free-bodydiagramis a diagramshowingthemagnitude and direction of alltheforcesactingon a chosenbody. Thebodyisshownonitsown, free of itssurroundings and of anyotherbodiesitmay be in contactwith.

  11. Hooke´slaw 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.

  12. Questions

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