SHATIN TSUNG TSIN SECONDARY SCHOOL FIRST EXAMINATION PHYSICS S4 Date : /12/2008

Transcription

1 P1 SHATIN TSUNG TSIN SECONDARY SCHOOL FIRST EXAMINATION PHYSICS S4 Date : /12/2008 Full marks : 67 marks Instructions 1. There are TWO sections in this paper. 2. Answer ALL questions. 3. Only calculators approved by HKEAA can be used. 4. Take g = 10 m s -2 when necessary. 5. The diagrams in this paper are NOT necessarily drawn to scale. Time allowed : 90 mins. Section A : Multiple Choice (25 marks) There are 25 multiple choice questions in this section. Answer ALL questions. Each question carries equal marks. Choose the best answer and marked on the Multiple Choice Answer Sheet. Two or more answers will score No Mark. 1 The following is a displacement-time graph of a ping-pong ball that is released from a height of 1 m above a table with zero initial speed. The ping-pong ball bounces for several times. Neglecting the air resistance, which point in the graph represents the second bounce? A. E B. F C. G D. H 2 If there is no air friction, a raindrop falls freely from rest under the action of gravity from the cloud m above Hong Kong can have incredible speed. Find the speed before it hits the ground and try to think whether it is dangerous or not. A m s 1. It is dangerous. B. 200 m s 1. It is dangerous. C m s 1. It is not dangerous. D m s 1. It is not dangerous. 3 Which of the following is a vector quantity? A. The magnitude of acceleration due to gravity B. Distance C. Speed D. Instantaneous acceleration

2 P2 4 If there is no reaction force, which of the following cases will occur? (1) We cannot swim. (2) We cannot jump. (3) We cannot walk. A. (1) only B. (2) only C. (3) only D. (1), (2) and (3) 5 A lift and three men inside have a total mass of 600 kg. If the lift is accelerating upwards with acceleration of 1.1 m s 2, find the tension in the cable of the lift. A N B N C N D N 6 A parachutist opens a parachute and an upward force is acting on it. The parachutist and parachute is still accelerating downwards with 1 m s 2. Find the force of air friction given that the mass of the parachutist and parachute is 50 kg and 5 kg respectively. A. 495 N B. 480 N C. 470 N D. 505 N 7 Which of the following statements is / are CORRECT? (1) Weight depends on the position of objects from the earth. (2) Weight depends on the mass of the objects. (3) Weight is the same as mass on the earth surface but different on different planets. (4) Weight of an object is the same on different planets. A. (2) only B. (1) and (2) only C. (3) and (4) only D. (1), (2) and (4) only. 8 In which of the following cases does the bead have the greatest speed just before hitting the ground? A. B. C. D..

3 P3 9 Which of the following statements is / are CORRECT? (1) Inter-conversion of kinetic energy and potential energy is only valid in the system of roller coaster and simple pendulum. (2) A hydroelectric power plants use the inter-conversion of kinetic energy and potential energy to generate electricity. (3) We can make use of conservation of energy to create energy. A. (2) only B. (1) and (2) only C. (3) only D. (1) and (3) only 10 As shown in the figure, Tarzan travels from one place to another by swinging in the forest. Assume that the rope is just long enough to swing from point A to point B, what is his initial speed in order for him to reach point B? A m s 1 B m s 1 C. 8.5 m s 1 D m s 1 11 In a circus show, a demonstrator uses a rigid rod to swing from point A in the horizontal direction to point B in the vertical direction as shown in the following figure. What is his initial speed at point A in order to achieve this motion? Given that the rigid rod is 2 m long. A m s 1 B m s 1 C m s 1 D m s 1

4 P4 12 A 5 kg object is accelerating uniformly with 0.5 m s 2 from rest for 10 s. Find the change of momentum of the object. A. 25 kg m s 1 B. 32 kg m s 1 C. 30 kg m s 1 D. 18 kg m s 1 13 Which of the following quantity represent(s) the unit of force? (1) J s 1 (2) kg m s 2 (3) W m 1 s A. (1) only B. (3) only C. (2) and (3) only D. (1), (2) and (3) only 14 Which of the following designs is/are based on the principle of increasing the impact time during a collision? (1) Gloves of the baseball players. (2) Booster of a rocket. (3) The bumper of a car. (4) Cushions used by the firemen when an accident breaks out. (5) The swinging of a pendulum. A. (3) and (4) only B. (1), (2) and (5) only C. (1), (2), (3) and (4) only D. (1), (2),(3), (4) and (5) 15 Which of the following statements is/are CORRECT? (1) Conservation of momentum is a result of conservation of energy. (2) The Newton s first law of motion can proof the conservation of momentum. (3) When there is an external force, momentum will change accordingly. A. (1) only B. (2) only C. (3) only D. (2) and (3) only 16 As shown in the figure below, bob A is hung on the ceiling with a 50 cm string. It is pulled aside to a vertical displacement of 10 cm. It is released from rest and it hits another identical bob B at the lowest position, which is initially at rest. Bob A comes to rest, while bob B will swing up. Find the maximum angle θ reached by bob B, assuming that the collision is an elastic one.

5 P5 A B C D The figure below shows a Newton s cradle. How many balls will swing up on the right hand side after the balls on the left hand side are released? A. 1 ball B. 2 balls C. 3 balls D. 4 balls 18 A 2 kg stationary block explodes and breaks into two parts as shown in the figure. Find the final speed of the heavier mass. A m s 1 B m s -1 C m s -1 D m s Which of the following quantities is/are constant if a ball falls freely from a certain height to the time before it hits the ground. Assuming there is no air friction. (1) Potential energy of the ball (2) Kinetic energy of the ball (3) Total mechanical energy of the ball

6 P6 A. (1) only B. (2) only C. (3) only D. (1), (2) and (3) only 20 Block A of 3 kg is moving on a frictionless surface at 5.5 m s 1. Suddenly it hits another identical block B, which is moving at 0.5 m s 1 in the same direction. Find the velocity of A after the collision assuming that the collision is elastic. A. 0.5 m s 1 B. 5.5 m s 1 C. 3.5 m s 1 D. 1.5 m s 1 21 Which of the following statements is/are CORRECT? (1) The explosion of a bomb is an example of conservation of momentum. (2) The bouncing motion of basketball with the ground is a counter example of conservation of momentum. (3) Bowling is a game that based on the conservation of momentum. A. (1) only B. (3) only C. (1) and (2) only D. (1) and (3) only 22 Which of the following is/are CORRECT? (1) The momentum of a bowling must be larger than that of a ping-pong ball. (2) Dropping an egg to the ground is an example of inelastic collision. (3) Conservation of momentum can be used to investigate the speed of cars in car accidents. A. (1) only B. (2) and (3) only C. (1) and (2) only D. (1), (2) and (3) 23 In a car accident, car A of kg is hit by car B of kg. Car A is initially at rest. From the marking of the tyre on the road, police found that the speed of car A and B after the collision is 10 m s 1 and 8 m s 1 respectively. Find the speed of car B before the collision. (Assume it is valid to use conservation of momentum here.) A m s 1 B m s 1 C m s 1 D m s 1

7 P7 24 Which of the following quantities does not represent energy? (1) Mass acceleration displacement (2) Force velocity (3) Momentum time displacement A. (1) only B. (2) only C. (1) and (2) only D. (2) and (3) only 25 Car A of mass 500 kg is moving with uniform velocity of 10 m s 1 and having a head on collision with a car B of 500 kg moving with velocity 5 m s 1. Given that the collision is elastic, find the velocity of car B after the collision. A. 10 m s 1 B. 0 m s 1 C. 5 m s 1 D. 7 m s 1 - End of Section A -

8 P8 Shatin Tsung Tsin Secondary School First Examination Physics S4 Student Name : ( ) Date : Section B : Conventional Question(42 marks) Answer all questions in the spaces provided. 1 As shown in the figure, a force F pulls a cart along the horizontal direction. The pendulum bob of mass 0.7 kg is tilted to an angle of 20. (a) Draw a free body diagram to show the forces acting on the pendulum bob. (3 marks) (b) Find the Tension in the string connected to the bob using the free body diagram in (a). (c) Hence, or otherwise find the acceleration of the pendulum bob along the horizontal direction.

9 P9 2 A 0.15 kg ball is dropped from rest at a height of 2 m. It bounces to a maximum height of 1.9 m. (a) Find the energy loss in this bounce. (b) After reaching 1.9m, the ball falls again and has the second bounce. Assuming the energy loss in each bounce is the same, find the maximum height reached by the ball after the second bounce. (c) What can we do, if we want the ball to bounce back to the original height?

10 P10 3 The figure shows a toy model. A block is pull aside to a vertical displacement of 1 m, and then it is released from rest. It hits a stationary bead of equal mass at the lowest position and the bead rises to certain height along the curved surface. (a) Find the maximum height reached by the bead if the block becomes stationary after the collision. Assume the collision is elastic in this part. (b) What is the velocity of the block before it hits the bead at the lowest position? (c) Then, find the maximum height reached by the bead if the block moves with velocity 1 m s 1 at the lowest position after the collision (4 marks)

11 P11 (d) There is a game in carnival that uses the same principle as this toy model. Suggest a method to increase the height reached by the bead. Explain briefly please. (3 marks) 4 During an explosion, a stationary 5 kg mass explodes into two parts A and B. Part A is 2 kg and part B is 3 kg. Part A is moving at 6 m s 1 after the explosion. (a) Find the velocity of B after the explosion. (b) Find the total KE before and after the collision. (3 marks) (c) Is this an elastic process? Where does the energy come from? Explain briefly. (4 marks)

12 P12 5. The following v-t graph shows how the velocity of a skydiver varies with time after jumping off his carrier. Read the graph carefully and answer the subsequent questions. v/m s t/s (a) What is acceleration of the skydiver right after he jump off the aircraft. (1 mark) (b) What is the acceleration of the skydiver at the 30 th second? (1 mark) (c) What is the terminal velocity of the skydiver before he opens his parachute? (1 mark) (d) Describe and explain, in terms of net force, weight, air resistance, the velocity-time graph. (6 marks) End

13 P13 Answer Part 1: Multiple Choice(Answer) 13 C 1 C 2 A 3 D 4 D 5 D 6 A 7 B 8 B 9 A 10 D 11 B 12 A 14 C 15 C 16 A 17 C 18 B 19 C 20 A 21 D 22 B 23 C 24 D 25 A Part 2: Conventional Question(Answer) 1 (a) (b) T cos 20 = mg T = 7 cos 20 = 7.45 N (C) 0.7 a = 7.45 sin 20 a = 3.64 m s 2 2 (a) Energy loss = mg h = = 0.15 J (b) For each bounce, energy loss is the same, so decrease in height ( h) is the same. Energy loss = mg h 0.15 h = = 0.1 m (0.15)(10) Therefore, the maximum height reached by the ball after the second bounce = =1.8 m (c) We can input energy to the ball when it bounces to the maximum height. 3 (a) For an elastic collision, the total KE of the bodies is conserved. Therefore KE of the bead just after the collision equals to PE of the block initially. We have: KE of the bead initially = PE of the bead finally = PE of the block initially Thus the bead can reach the initial height of the block, that is vertical displacement of 1 m.

14 P14 (b) Velocity of the block at the lowest point = 2 gh = 20 m s 1 (c) Let the velocity of the bead after the collision is v, By conservation of momentum, m 1 u 1 + m 2 m 2 = m 1 v 1 +m 2 v 2 since, m1=m2 therefore, u 1 +u 2 =v 1 +v 2 20 = (1) + v v = 3.47 m s 1 Therefore the height reached by the bead is: 2 v h = = 0.6 m 2g (d) Giving an initial speed to the block, thus the energy transfer to the bead at the lowest position will be larger, which in turns increases the maximum height reached by the bead. 4 (a) By the law of conservation of momentum, m A u A + m B m B = m A v A +m B v B m A v A = m B v B, v B = (2)(6) = 4 m s 1 3 The negative sign means that part A and B move at opposite direction after the explosion. (b) Total KE before the collision = 0 J Total KE after the collision = 1 (2) (6) (3) ( 4) 2 = 60 J 2 (c) This is not an elastic process, because the initial KE is not equal to the final KE. The energy comes from internal energy, for example, chemical energy from chemical process. (5) (a) 9.81 m s -2 or 10 m s -2 (b) 0 m s -2 (d) 60 m s -1 (d) When the spydiver jumps off his carrier, he gains speed at a rate of 9.81 m s-2 (he accelerate at 9.81 m s-2) initially due to his own weight. As he gains speed, the air resistance increases. Thus the net force and the acceleration decreases. Eventually, the air resistance increases so much that it balances the weight of the skydiver. The net force actign on the skydiver and thus the acceleration become zero. Then the skydiver moves with a constant speed downwards (terminal velocity).