DYNAMICS Worked Example 4 – Pushing Blocks. 56 N acts on 3 blocks as shown. A 7 kg B 14 kg C 7 kg 56 N Each block experiences 6 N of friction. a) Calculate the acceleration of the blocks.

1 st Free body Diagram + sign convention Total mass = 28 kg 56 N18 N is +veis -ve F u = 38 N a = ? F u = 38 N m = 28 kg a = ? a = F u m = = 1.4 ms -2

b) What force does block A exert on block B ? Blocks B and C are affected. Mass of B& C = 21 kg F from A F f = -12 N F u = ? a = 1.4 m/s 2 m = 21 kg F u = m B+C x 1.4 = 21 x 1.4 = 29.4 N is +veis -ve F u = F A + F f F A = F u - 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. Components of forces tutorial. Revision questions for Higher Physics, page 18, Q. 1 – 4. 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. 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. Reminder – Forces on a slope. Read through your forces on a slope notes. Remember – use s in for component of weight parallel to the s lope.

Problems 52 – N 53. a) (i) ms -2 (ii) 3 x 10 6 N b) ms 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 ms 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) N N 62.a) 1960 N b) 2260 N c) 1960 N d) 1660 N 63.a) (i) 2450 N (ii) 2450 N (iii) 2950 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 N 65.a)b) 37.2 N, 39.2 N, 43.2 N. 66.a) 8 m/s 2 b) 16N 67.a) 5100N b) 2500N c) 700N, 500N d) 10,300N m 69.a) 2 m/s 2 b) 40N c) 20N 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/s 2 b) 6.54 N m/s a) 225.5N b) m/s N N 75. a) 2283 N b) 2.2 m/s 2 c) 14.8 m/s d) 8520N a (m/s 2 ) t (s)