H1.02. 2022 Kinematics Answers to Examples, Quizzes _ Mini-tests

Answers to Examples, Quizzes & Mini-test [Kinematics] Quiz A cyclist riding at 7 m s-1 sees a stop sign and comes to a complete stop in 4 s. He then, in 6 s, returns to a speed of 5 m s-1. Which of the following shows his motion diagram? Solution: Answer is either B or C based on the relative directions of the acceleration and velocity vectors. Since the cyclist is slowing down in the first part of his motion, the acceleration and velocity vectors should be in opposite directions. Since he speeds up after the stop, the acceleration and velocity vectors should be in same direction. Answer is B not C because the final velocity is smaller than the initial velocity, so the last arrow should be shorter than the first arrow. Quiz The graph of velocity against time for a moving object is shown. Which of the following is the corresponding graph of displacement against time? (N95/I/5) A B C D

Solution: Answer: C • Velocity is always +ve, hence gradient of s-t graph should be always +ve (i.e. should neither be -ve nor zero). Eliminate A & B. • Velocity doesn’t change abruptly, so eliminate D where ‘sharp’ points indicate abrupt change in gradient and thus abrupt changes in velocity. MINI-TEST 1 A car is travelling along a straight road at a constant velocity initially for 1 minute. After that, the driver steps on the accelerator, causing the car’s speed to increase uniformly for 30 s. It then travels at constant velocity again for another minute before the driver slows down uniformly for 30 s to a momentary stop and then makes a U -turn back with the same acceleration for 30 s. Sketch the displacement -time, velocity -time and acceleration -time graphs to illustrate the motion of the car. My solution: t/s velocity t/s 60 90 150 180 210

Quiz 1) A ball is thrown vertically upwards. While the ball is in free fall, its acceleration A) increase, B) decrease C) remain constant 2) After a ball is thrown vertically upward and is in air, its speed A) increases, B) decreases, C) decreases and then increases, D) remains the same. Solution: Answer: Both (C) Quiz A stone is thrown upwards from the top of a cliff. After reaching its maximum height, it falls past the cliff- top and into the sea. The graph below shows how the vertical velocity v of the stone varies with time t after being thrown upwards. R and S are the magnitudes of the areas of the two triangles. Solution: What is the height of the cliff-top above the sea? (N98/I/3) A) R B) S C) R+ S D) R - S

Quiz Would a more massive object (e .g. elephant) fall faster than a less massive object (e .g. mouse) in a vacuum? Solution: All objects under free fall accelerates at 9.81 m s-2. Hence both objects speed up from rest at the same rate (speed increases by 9.81 m s-1 per second). MINI-TEST 2 A tenni