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PHYSICS 1L03 (115)
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Lecture

Forces (Unit 3).pdf

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Department
Physics
Course
PHYSICS 1L03
Professor
Reza Nejat
Semester
Fall

Description
Forces Forces Concepts: What is force? Force Category of forces Free body diagram Equilibrium Chapter: 3 Pages: 48-69 1 2 Forces Forces •ittbj,t •Werrtlr there are two forces, one on each object wagon a force is being exerted. •idnln trll object, resulting from the interaction between applies a force on the mule, simultaneously; (with the same magnitude but in opposite direction). two objects, such as the person and the cart. 3 – Forces always act in pairs, acting on two 4ifferent objects 1 Force Force Broad Categories of Forces: •Psft:r • Contact forces: require physical contact –Forceisapushorapullthatoneobjectexertson another. between two interacting objects. –Forceeitherisacontact force or is a contact free force. • Contact free forces: require no contact –Forceisavector,havingbothmagnitudeand direction. between two interacting objects and acts over a distance –Forcesarepaired.Twointeractingobjectsexerta force on each other, at the same time. • These forces may be long range forces –Forcesareadditive.Theeffectoftwo simultaneous force on a same object is the same •Contactfreeloclldareas field forces as the net force of the two forces. 5 6 Force Force: What does a force do? •Wealrr 1 on the ball while the two are in contact , changing the velocity (the speed and/or the direction) of the ball. 1 • :-- Force is an interaction between two objects. 2 • :-- Force (external) changes the velocity, causes acceleration , of the object. 7 8 2 Force: Force: What does a force do? Action and effect of a force: •ranote 9 10 Force Brief Review on adding vectors • Unit of Forces: •uortmbrwtd •li(.) vectors in component form, you should watch the tutorial video on •Nd)iI vectors posted on AVENUE 1N ▯ 1 Kgm s2 •DOOTSIHISEORYOU WILL BE LOST FOR THE REST OF THE TERM! 11 12 3 Brief Review on adding vectors Brief Review on adding vectors ▯ ▯ ▯ •W eanlays A ▯ A ▯xA y break any 2D Ax▯ A cos▯ vector up into components Ay▯ A sin▯ ▯ ▯ along two 2 2 2 A ▯ A ▯xA x perpendicular 2 2 axes, such as x A ▯ A x A x and y: A ▯ ▯ ▯ tan▯ ▯ y A ▯ A ,xA y▯ A x ▯ ▯ A ▯ ▯ A, ▯ ▯ A ▯ A ,xA y 13 14 y Review: Adding two vectors: Adding two vectors: Fseot faerhnid 1 and F2, shown below F 2x▯ F 2os ▯ Ry ▯ ▯ ▯ R ▯ F ▯1F 2 F ▯ F sin▯ 2y 2 A simple way to find the resultant of two F1 R F1x ▯ F1cos ▯ F 1y R vectors is finding the components of it. F1 1. Find the x and y components of each F2 F1y ▯ F 1in ▯ vector. 2. x-component of resultant is the Rx▯ F 1x▯ F 2x summation (algebraically) of x- Ry ▯ components of vectors. F1y R y F 1y ▯ F2y F2y ▯ F2 3. y-component of resultant is the F1 R ▯ summation (algebraically) of y- R ▯ R x R y F R x components of vectors. F2y F2 F 1x 2x x tan ▯ ▯ R R Rx▯ ▯ Fix R ▯ F F F 15x y x 16 y ▯ iy 1x 2x 4 Practice: Total Force on an object Practice: Total Force on an object fo eulav tah w rof ,woleb erugi f eht n •I ▯ will •Fitnie ▯ ▯ ▯ C = A + B have: (at home) and direction of D ▯ A ▯ B ▯ C at home a. A minimum value y b. A maximum value o c. Find C for ▯= 90 . B=12 Co6 23 ▯ A=5 23 Force Force Categories of Forces: Fundamental types of forces: Only four types: •FnamFcse • Gravity (long ran)e – contact-free forces – the force between masses – holds planets and stars together, makes things fall, etc – cannot be explained as any component of • Electromagnetic ( long rang) other forces – the force between charges, magnets, – responsible for all familiar “everyday” forces (except • Convenience Forces (non-fundamental) gravity) – Can be explained in terms of fundamental • Strong (short rang) – holds atomic nuclei together forces • Weak ( short rang) 27 – Beta decay (radioactivity) Force Force •Goi()i Gravitational force (Gravity): eno stcar t ta taht ecrof eh •t ▯ F mass to another g ▯ t rexe stcejbo o wt y •A Fg gravitational force on each other. ttrli orbiting the Sun. 29 30 Force Force •ewfli:v Newton's law of universal gravitation: • An attractive gravitational force between two objects is proportional to the product of their masses and inversely proportional to m 1 2 ▯11 2 2 the square of the distance between them. F ▯ G 2 G ▯ 6.67▯10 N m kg r •W:r F ▯ G m 1 2 r2 – m 1nd m a2e the objects masses, – r is the distance between them, – G is the universal gravitational constant. 32 33 Force Weight Example: •W wtomm nalrll •tttj the force of gravity acting on a body 40 cm from each other center. What is the force of gravity of each acting on the other? mM E M E w ▯ F ▯g 2 ▯ m(G 2 ) ▯ mg F ▯ Gm1m2 G ▯ 6.67▯10N m kg2 R E R E r2 •NetEtuf: w ▯ mg • where g is the acceleration of an object due to gravity. On the Earth, it is approximately 34 constant everywhere on the planet. 36 Force Force Example: Example: ,elrala g , at ’2thrip M Mars’s surface, if the mars has a mass of and Mars’s surface. 6.42 x 103 kg and a radius of 3.40 x 10 m. w ▯ mg F ▯ Gm1m 2 G ▯ 6.67▯10▯1N m kg 2 r2 37 39 7 Force Force Example: •iolt m ftll the 20 kg sphere at Mars’s g ▯ 9.8 2 surface, if the mars has a mass of 6.42 x 103kg s and a radius of 3.40 x 10 m. • On the Earth surface, it is approximately constant everywhere. F ▯ Gm1m2 G ▯ 6.67▯10N m kg2 r2 •ttr gravitational field weakens, so g decreases. g ▯ 1 r2 41 42 Force Force Effect of gravitational force: Electromagnetic force .rftgsre tmete cctirs (electric & magnetic force) on each other. Puffy-face and bird-leg look mytptlo body is raised under the influence of gravity. 48 49 8 Force Force •Clb’w Coulomb’s Law • An electric force between two charges is proportional to the product of the magnitude of their chrges and inversely proportional to q q F ▯ k 1 2 k▯ 8.99▯10 N m C 2 2 the square of the distance between them. r 2 •W:r q1q2 – q and q are the magnitudes of the charges (in F ▯ k 2 1 2 r coulomb “C”), – r is the distance between them, – k is the Coulomb constant.
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