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Physics 8A Summer 2020 MIDTERM #1 I certify that I did not seek help from any other person in solving the problems on this exam. Name: ____________________________ Signature:__________________________ 1. A football is thrown at an unknown speed and angle (although we know the vertical component of the initial velocity is upward). It then lands at a location 15.0 m below the level at which it was thrown. Someone observes the football to be traveling at a speed of 25.0 m/s and in a direction 65.0 below the horizontal just before striking the ground. (a) What was the initial speed of the football? At what angle was it thrown above the horizontal? (b) How much time did the ball spend in the air? (c) How far horizontally did the football travel? 15 m at Vfx n UfcosOf fy UfSia Ef Vix a Vij Ufy Vic t a Viy n l 10.6mls a 22.7 m s 25.0 4s sin C 65,07 1cos C G5 O 25.0 lo G Ms 2g Oy VI t 2gOY VI T 2gOY 1 22.7 14.9 Vi an 219 fm sYL 15.0 ms T Vii Vix't VI Gi Vix Gi t 54.60 18.34 n 14.9 4 10.6ms m lol Vfy 22.7 g0 ms 14.9 Ot as T Vi y 3.84 s 9.8 4 Ot Physics 8A Summer 2020 MIDTERM 1 2. Block A has a mass of 20.0 kg. It is attached to a rope, which is slung around a massless, frictionless pulley and attached at the other end to block B, which has a mass of 25.0 kg. The incline makes an angle of 15.0 with the horizontal. There is no friction between block B and the incline. The blocks are initially at rest. (a) What minimum static friction coefficient must there be between block A and the surface to prevent the blocks from slipping? (b) Suppose the blocks do slip, and accelerate. If the kinetic friction coefficient between block A and the surface is 0.10, what is the tension in the rope? y F ha f twi Ta Y TB T f A is Block B If system is at rest Block B sin 0 Mpg Ti mpgsin 0 T T Misgsin 0 fo f O Block A Fnet a T f f static friction is being pushed to its limits Block At Fnet Fnet T fr MA a 0 25.0 Ks 9 8m64 sin 15.00 this is also Fornax But also fsmay lls h Us MAg G3 f ma Lzo.oksk9 fm s2 µs Mag Los I T 63.4 N Since BY ti W X Block A Fnet a T Mu ma g 0.324 tlwmag mn.AT T Block B Fnet Fnet T Mrsgoin G Mrs a MBgsinGT T M.rs easiest to find the Algebraically it might be tension from that acceleration then get Add equations T Uk Ma g t mpg sin 0 T Ma at Mpa Ma MBg sin G MvcMag MB Jin G llama a Mat MB 25 0kg sin 15 20 0 kg t T 0,974 µ ma g t MA U g lo lol 20.0kg 9.8 25.0 Kg 2 0.1016200kg1 9 fm 39 IN t Mp3 a s4t 20.0kg 0.974 2 4 Physics 8A Summer 2020 MIDTERM 1 3. A block of mass 5.00 kg is pushed against a horizontal elastic spring with a spring constant k = 75.0 N/m. The block is then released from rest, and moves up a 25.0 incline that is 15.0 m long. It then travels along another horizontal surface before entering a loop of radius 10.0 m. All surfaces are frictionless. Find the minimum spring compression needed in order for the block not l e c n ac i h he l hen i a he f the loop. is.com flueefT c 25no work is done by nonconservative forces mechanical energy is conserved But what speed does the block need to have in order not to lose contact at the top Since no I j 14th 4 Finely h t ng ht mg mu at mr mg 0 needstobe 3 Mgr and the kinetic energy at the top n So M Fnety n m a minimum K Ef KEI t Ui O t Iz k X r tn rYCld.om f mu't Iz MS lzc5ookssc9 245J K Est Ut r k Ef t ng y G t Zr ya Isin a I 5 Om Sin250t 2 loom 26.3 in 75.0mm X 245J t 1534 J X 64 m Fooks 9 sn s4C26.3m