The concept of force

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The concept of force - PowerPoint PPT Presentation

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

Bytheend of thistopicyoushouldbeableto:

statethedifferencebetweenmass and weight;

drawvectorsrepresentingforcesactingon a givenbody;

identifysituations in whichfrictionalforcesdevelop and drawthosefrictionalforces;

use Hooke’slawcorrectly.

Dynamics

Why do objectsmovelikethey do?

Weight

Thisforceistheresult of thegravitationalattractionbetweenthemass in question and themass of theearth. Theweight of a bodyisthegravitationalforceexperiencedbythatbody, whichonearthisgivenbythe formula

W = mg

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).

Normal reactionforces

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

Dragforces

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

Upthrust

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

Friction

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

Free-bodydiagrams

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

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.