NJC Unit 4 Energy & Fields Tutorial

Student’s Copy 2025 Temasek Junior College Tutorial Questions (Solutions to questions 1, 3, 7 and 14 are provided.) Work 1. A horizontal force of 150 N is used to push a 40.0 kg box to a distance of 6.0 m resting on a rough, horizontal surface. If the box moves at a constant acceleration of 3.0 m s-2, calculate (a) the frictional force f, (b) the work done by the 150 N force, (c) the work done by friction, and (d) the final speed of the box. (30 N, 900 J, 180 J, 6.0 m s-1) Ans: (a) Nf .. f mafF 30 03040150    (b) 150 6 0 900 W Fs . J    (c) 30x6 0 cos180 180J W fs .    (d) 2 1 Gain in KE Work done by Work done by friction 1 (40.0) 0 900 1802 6.0 m s F v v       OR ms.v ..v asuv -1 2 22 06 06032 2    6.0 m 150 N f v a = 3.0 ms-2

2 2. A 1.8 kg block is moved horizontally from rest through a displacement of 2.0 m by a constant force F directed at 45o to the horizontal as shown in the figure. Find (a) the magnitude of the force F if there is no resultant vertical acceleration and no contact between block and surface, (b) the work done by the force F, (c) the final velocity of the block and (d) the work done by gravity mg. (25 N, 35 J, 6.3 m s-1, 0 J) 45o F mg

3 3. The graph in the figure shows how the extension of a rubber band changes when it is stretched and released. How much heat is generated each time the band is stretched and released? (0.14 J) Ans: 1 small square represents 1.0 x (0.5 x 10-2) = 0.0050 J Estimated area enclosed = 28 squares Heat generated = net work done by stretching force = 28 x 0.0050 = 0.14 J 4. A catapult fires an 80 g stone horizontally. The graph in the figure shows how the force on the stone varies with distance through which the stone is accelerated horizontally from rest. (a) Use the graph to estimate the work done on the stone by the catapult. (b) Calculate the speed with which the stone leaves the catapult. (24 J, 24.5 m s-1)

4 Energy 5. A 50 g ball is thrown from a window with an initial velocity of 8.0 ms-1 at an angle of 30° above the horizontal. Using energy methods, determine (a) the kinetic energy of the ball at the top of its flight and (b) its speed when it is 3.0 m below the window. Does the answer to (b) depend on the mass of the ball or the initial angle? (1.2 J, 11 m s-1) 6. A 3.2 kg trolley initially moving at 5.0 m s-1 at a height of 4.0 m encounters a hill of height 5.0 m, as shown in the figure. At a later point there is a horizontal spring (k = 120 N m-1) at a height of 2.0 m. (a) Does the trolley reach the spring? Explain your answer. (b) If so, what is the maxim