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Transcript
____ Name: Friction and Newton’s Second Law *Use the following three relationships to solve the problems below: a=1! Fgrav = m*g Use Acceleration due to gravity, g = Ffrict IJ = * Fnorm 10 m/sN 1) An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.) + çoi norm Ffrictl 0 app Fqray iOv’,4t = 50 N 80 N m=___ / 5a4,/’L 6 - F- a4i\i fry’ F net =40t% 2) An applied force of 20 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the coefficient of friction (p) between the object and the surface, the mass, and the acceleration of the object. (Neglect air resistance.) ( Fnorm= FfrictlO N !app =20 N M N j1= m= net . io ‘I’ - cc€\.y) ___ ________ 3) A 5-kg object is sliding to the right and encountering a friction force that slows it down. The coefficient of friction (p) between the object and the surface is 0.1. Determine the force of gravity, the normal force, the force of friction, the net force, and the acceleration. (Neglect air resistance.) ( : A . J Ffrict..j 2*’ - 5W 1 = I Fgrav 0i’ m = oN 1 no y Fnorm=W cotJ 5 kg kL*” 1 F net =5tJ a = rç ) — —I I I2 6 ‘i 4. Edwardo applies a 4.25-N rightward force to a 0.765-kg book to accelerate it across a tabletop. The coefficient of friction between the book,,9cj the tabletop is 0.4 10. Determine the acceleration of the book. iv4 c1; 2 c n k: 4 F %) 1itJ CA:*E: I . ( 5. In a physics lab, Kate and Rob use a hanging mass and pulley system to exert a 2.45 N rightward force on a 0.500-kg cart to accelerate it across a low-friction track. If the total resistance force to the motion of the cart is 0.72 N, then what is the cart’s acceleration? 4Sr1 1 ( fr’ 2 /s N A _________. Name: Forces in Two Dimensions <6y Inclined Plane Analysis Read from Lesson 3 of the Vectors and Motion in Two-Dimensions chapter at The Physics Classroom: http://www.physicsc1assroom.com/C1ass/vectors/u313e.htiu1 MOP Connection: Forces in Two Dimensions: sublevels 5 and 6 Review: A normal force is a force that is always directe4 1. c. perpendicular to the surface the object is touching b. sideways a. upwards 2. An object is upon a surface. The normal force is equal to the force of gravity b. only when the object is at rest a. in all situations d. only when there is no vertical acceleration accelerating c only when the..objçct thereis no vertical accelerabon AND 1 iorm and Fgrav are the only vertical forces .. Getting the Forces Right: 3. The object at the right has been placed on a tilted surface or inclined plane. If there is enough tilt, it will accelerate from rest and begin its motion down the incline. Draw a free-body diagram for the object ? sliding down the rough incline. Label the three forces according to type (Fgrav Ffricb Fair, Ftens, Fapp etc.). Physics Tip: When you encounter a situation involving a force directed at angles to all other forces, immediately convert the uncooperative force(s) into two perpendicular components. Use SOH CAH TOA to resolve any uncooperative force into components directed at right angles to each other. One component should be in the direction of the acceleration; the other should be perpendicular to it. In the case of inclined planes, resolve the uncooperative force into components parallel and perpendicular to the inclined plane. The force of gravity (or weight vector) is the uncooperativeforce. It is 4. typically resolved into two components one parallel to the plane and the other perpendicular to the plane. Given the diagram at the right with the two components of gravity represented as i and Fj., use trigonometric functions to write equations relating these components to the force of gravity. - FU=()) 5. fr’() II.= ( () 1F1 For the three situations described below, use <, >, or = symbols to complete the statements. Object accelerates down Object moves at constant speed. Object at rest. incline. Fim Ffrict Ffnct Fti : , 1 F F = 1A ru Ffrjd Fnorm = -__ “ © The Physics Classroom, 2009 Ffrjd 11 F 1 F > = Ffrjd Fnorm )> Page 1 ___ ________N _____N _____N Forces in Two Dimensions Use equations for calculating the components of gravity (#4) and Newton’s laws to fill in the blanks. 6. A 4.50-kg object cceler rig down an inclined plane indined at 36.0° (with the horizontal) and having a coefficient of friction of 0.548. NVI,FIXiCt S NF !9,%EN () (9’15,v cç (ZJP : 4.5Zk /€/ • ;1) (3k ) 1F1( k4#1,) :S() i4 a 7. A 65.0-kg crate remains at rest on an inclined plane that is indined at 23.0° (with the horizontal). 9 Pi. “mFils ,1iSEN L,, W 0 — = 2jj 1 r N - O.4Z. = 5 Fj= N (0 1 S 4 0 = ic 8. tl i(z3)2s4p4 = 2S4(LN 1 r, =____ N Acc a =_____ rv’ A 41.3-kg box slides down an inclined plane (inclined at 29.1 d’egrees) at a constant speed of 2.1 m/s. C6 (29, i) :(4i3 .ic/,) 3( c 0 J (6$($I) IL FJ3CcJ N ZOIN =____ 4 3Co0 = N frict T pMsCji(2’.1) j=Q m ’ 4 3icg (41.3w N ./i)e (i.) Zoi,i net= W) 0 N Ac 1 h ’. a QmFils The Tilted Head Trick Inclined plane problems can be easy. Resolve gravity into its components. Then, ignore the force of gravity. Finally, tilt the paper or your head and the problem becomes a simple Fnet= m•a problem. Fd FMa \ Pr \ / 1 F / Fftjdr / I r it 1 \? (s\’Je() © The Physics Classroom, 2009 jifrô cpfleVS N Page 2