#### Transcript Dynamics Example (Pushing Blocks)

DYNAMICS Worked Example 4 – Pushing Blocks. 56 N acts on 3 blocks as shown. 56 N A B C 7 kg 14 kg 7 kg Each block experiences 6 N of friction. a) Calculate the acceleration of the blocks. 1st Free body Diagram + sign convention 56 N is -ve Total mass = 28 kg is +ve 18 N F u = 38 N a = ? Fu= 38 N m = 28 kg a= ? a = Fu m = 38 28 = 1.4 ms-2 b) What force does block A exert on block B ? Blocks B and C are affected. is -ve Mass of B& C= F from A Fu= ? a = 1.4 m/s2 m = 21 kg 21 kg is +ve F f = -12 N Fu= m B+C x 1.4 = 21 x 1.4 = 29.4 N Fu = FA + F f FA = Fu - F f = 29.4 – (-12) = 41.4 N Over to you – Problems up to 75. Internal forces tutorial. Revision questions for Higher Physics, page 17, Q. 1 – 7. Reminder - Components of forces. Read through your Components of a Vector written notes. Force is a vector therefore the components are found using this method. Components of forces tutorial. Revision questions for Higher Physics, page 18, Q. 1 – 4. Reminder – Forces on a slope. Read through your forces on a slope notes. Remember – use sin for component of weight parallel to the slope. Forces on a slope tutorial. Revision questions for Higher Physics, page 18, Q. 1 – 5. Lifts tutorial Revision questions for Higher Physics, page 20, Q. 1 – 6. Problems 52 – 75 52. 4900 N 53. a) (i) 0.015 ms-2 (ii) 3 x 106 N b) -0.0027 ms-2 54. OA … decreasing acceleration as air resistance increases AB…constant velocity as air resistance balances the weight BC … parachute opens so dramatic deceleration to lower speed CD … constant velocity as new air resistance balances weight DE …dramatic deceleration as parachutist lands. 55. 0.02 ms-2 56. 150 N 57. a) 120 N b) -20 N 58. a) 1200 N b) 108 m c) 2592 N 59. a) (ii) 7.7 ms-2 b) mass decreases as fuel is burned, air resistance decreases as rocket leaves Earth’s atmosphere, weight decreases as rocket leaves Earth’s gravitational field. c) 15.9 ms-2 d) Once out of Earth’s gravitational pull weight drops to zero so engines can be switched off and rocket will continue at a constant velocity. 60. a) 1778 kg b) 62 424 N 62. a) 1960 N b) 2260 N 63. a) (i) 2450 N (ii) 2450 N 61. 28 600 N c) 1960 N (iii) 2950 N d) 1660 N (iv) 1950 N b) 4.2 ms-2 Problems 52 – 75 (cont.) 63. c) An empty lift so resultant force would increase giving a higher acceleration. 64. 51.2 N 65. a) 0.5 a (m/s2) b) 37.2 N, 39.2 N, 43.2 N. t (s) 0 -1.0 66. a) 8 m/s2 b) 16N 67. a) 5100N b) 2500N c) 700N, 500N 69. a) 2 m/s2 b) 40N c) 20N d) 10,300N 68. 24m d) 12N e) No – A onto B affects 2 blocks so must balance 8N of friction. B onto C only affects 1 block so just has to balance 4N of friction. 70. a) 3.27 m/s2 73. 9.8 N b) 6.54 N 74. 0.735 N 71. 0.541 m/s2 72. a) 225.5N b) 0.376 m/s2 75. a) 2283 N b) 2.2 m/s2 c) 14.8 m/s d) 8520N