## Monday, January 13, 2020

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Therefore, speed = length of the tape chosen Time interval for that length You are provided with a runway and a data-logging system with a motion sensor. Describe how you should use the apparatus to conduct an experiment to demonstrate the relationship between the net force and the acceleration of a trolley. You may use additional apparatus if necessary (5M) Adjust the runway for friction compensation (1 M) Pull a trolley down the runway using an elastic thread. Keep the extension of the elastic thread constant all the way.M) Record the motion ot the trolley wit data-logging system and find the acceleration of the trolley. (1 M) Repeat by pulling he trolley with 2, 3 and 4 identical elastic threads, side by side and extended by the same length as before. (1 M) From the experiment, it is found that the acceleration of the trolley is directly proportional to and in the same direction of the net force. (1 M) It is not preferable to carry an excessive amount of goods on the bicycle when he is riding in the street, why? (3M) The larger the mass, the larger the inertia the bicycle has. 1 M) By newton's second law of motion, the deceleration of the bicycle would become smaller when John applies the brake. (1 M) As a result, the stopping distance ncreases (1 M) So the chance of having an accident is larger. In supermarkets, bottled drinks are usually stored in refrigerators with shelves inclined at an angle B with the horizontal.Explain why there is such an arrangement. Also state the minimum angle required to achieve the purpose above. Given mass of a bottle Ã¢â‚¬ËœM', friction between a I-kg bottled drink and the inclined shelf is f'. 3M) If the bottled drinks are put on inclined shelves, they would slide down automatically when the first bottle is taken. It is more convenient for customers. (1 M) MgsinB > f (f/M9 (1M) Therefore, the least inclination angle is sinA-1 ( f/Mg). In some parts of the world, it is common for people to shoot firearms straight up into the a ir during celebrations. Falling bullets pose a significant danger to bystanders. As a bullet falls, its velocity increases until it reaches a constant terminal velocity. Explain, in terms of forces acting on the bullet, why this happens.Also, assume the air resistance is bv , derive the terminal kinetic energy of the bullet. (5M) As the bullet falls, its downward weight and the upward air resistance (or friction) are acting on it (1 M) Firstly the bullet accelerates. The air resistance opposing the motion of the bullet increases with its velocity. (1 M) Finally, its weight is balanced by the air resistance at a certain velocity. (1 M) As the net force acting on the bullet is zero, the bullet will continue to fall at this constant velocity which is called terminal velocity. mg = bv) (1M) KE=O. 5mv?2 = 0. 5mA3 g,N2 /bA2 (1M) A basketball player Jumps up such that his height reaches a height of 3m. Using Newton's laws of motion, explain why the player can gain an initial speed to leave the ground vertically. (3M) The player exerts a force onto the ground when he is about to ump. (1 M)This results an equal normal reaction force from the ground according to Newton's 3rd law of motion. (1 M) As the normal reaction force is greater than the weight , the player will be forced to accelerate up according to Newton's 2nd law of motion. 1 M) Chapter 8 What is the advantage ot using the spannerÃ¢â‚¬Å¾3z to turn a screw* effort(force) required can be reduced. (2) The efficiency can be increased. X (3) The energy required can be reduced. X For (2) and (3), they are wrong as moment is not related to energy. (Force is perpendicular to the displacement, the work done should be zero. ) Chapter 9 There are two methods of raising the block from the ground to a certain height. (2M) Method 1 : Pulling the block up a inclined plane Method 2 : Lifting the block vertically upwards.Method 1 is better than method 2 in terms of forces as smaller force is needed to raise the block in metho d 1. (1 M) However, method 1 has a lower efficiency because some energy is lost in overcoming the friction between the block and the plane. (1 M) What are the functions of the counterweight in a lift system? (3M) To reduce the power required to raise the car (1 M) To reduce the load acting on the motor (1 M) To improve the efficiency of the lift (As less energy is lost to overcome the friction between the cable line and the drum) (1 M) Verify the conservation of mechanical energy.Given a simple pendulum, a protractor, a light gate, a ruler. (7M) Set up a simple pendulum, a protractor and a light gate. (1 M) Displace the bob and read the angle B between the string and the vertical from the protractor. (1 M) Start data-logging and release the bob. (1 M) Record the time t taken for the bob to pass through the light gate. Measure the diameter of the bob(this is the distance travelled by the bob while passing the light ate, which is very small so the instantaneous speed of the bob in the lowest position calculated is relatively accurate), and hence estimate the speed v of the bob at the lowest position. 1 M) Use a ruler to measure the length I of the pendulum. Apply the conservation of mechanical energy and calculate the speed v of the bob at the lowest position. (1 M) V = {square root (2gh)} , where h = I(1-cosB) (1 M) Conclusion, the two values of speed calculated is quite close so the law of conservation of energy is proved.