# CH 12.pdf - 12.5 PROBLEM vr5re raoxlelle |]CI In the...

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€vr5re. ra ox lelle |]CI PROBLEM 12.5 In the brakingtest of a sports car its velocity is reduced fiom I l0 km,/h to zero in a distance of 5l m with slipping impending. Knowing that the coefficient of kinetic ftiction is 80 percent of the coefficient of static friction, determine (a) the coefficient of static friction, (r) the stopping distance for the same initial velocity if the car skids. Ignoreair resistance and rolline resistance. SOLUTION (a) Coefficient of static fiiction. l'? /a\ 'oJio-zr > 4 - 0 , N - m g = Q N = m g vo = I l0 km/h = 30.556 m/s + +=o,(s-s6) u2 - rl, o - (30.556)' ) a, = -u = -9. I )J m/S- ' 2(s - s6) (2X51) For brakingwithout skidding 7r = 1t,, so that p,N = mla'l -1-:4 = ma," -P"N = ma' ffia, a, 9.153 llr = - - mg g 9.81 (b) Stopping distance with skidding. tJse p= pr = (0.80X0.933) = 0.746 4" = 0.913 { :-2F = ma,: ltoN = -ma, u'N = -7 '322'7 n/s2 m Since acceleration is constant, u'-ri o-(30.556)' (s -sor = -i =AX-'.zzztl r - r o = 6 3 . 8 m <

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@i ,1"" ,,,r-- a- ri r.':.:j -a\ PROBLEM 12.19 Each ofthe systems shown is initially at rest. Neglecting axle friction and the masses of the pulleys, determine for eachsystem (a) the acceleration of block l, (b) the velocity of block ,4 after it has moved through 5 ft, (c) the time required for block,,1 to reach a velocity of l0 ft/s. SOLUTION Lety bepositive downward for both blocks. Constraint of cable: !,t r !s = constant a 4 + a B = 0 @ t l l r n | -1; aA\ + \a^ tt I / B \ T Wsr P /G1 Ito" For blocks I and B, + i 2F = ma'. Block,4: BlockB: r,rru T =Ytou c or T=w^ Y-n oo I P+ll8 T-!!qs=-3oo W, W. P + WB - W t + -t-L au - - :-L o u . W,_W" P S o l v l n g t o r a t . a 4 - 4 9 ' WA +WB (r) 'i -0")l =zouly n-(r,)ol *itr'(u,L = o (2) a / ,o- lzoolyn-(yn)of -(v1)o= ant with (vr)o = 0 (3)
(a) PROBLEM 12.19 CONTINUED Acceleration of block l.

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• Winter '17
• Friction, faceofthe cavity, insidea small cavity, car skids.Ignoreair resistance

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