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Problems 49 1. The speed of sound in air is 331 m/s. During the next thunderstorm, try to estimate your distance from a light- 2. The average velocity of a particle moving in one dimen- sion has a positive value. Is it possible for the instanta- If the average velocity of an object is zero in some time in- 4. Can the instantaneous velocity of an object at an instant of time ever be greater in magnitude than the average veloc- 5. If an object’s average velocity is nonzero over some time interval, does this mean that its instantaneous velocity is 6. If an object’s average velocity is zero over some time inter- val, show that its instantaneous velocity must be zero at x (t ) is a continuous function. 7. If the velocity of a particle is nonzero, can its acceleration be zero? Explain. 8. If the velocity of a particle is zero, can its acceleration be nonzero? Explain. Two cars are moving in the same direction in parallel lanes 10. Is it possible for the velocity and the acceleration of an ob- ject to have opposite signs? If not, state a proof. If so, give Consider the following combinations of signs and values 11. 9. 3. Velocity Acceleration a. Positive Positive b. Positive Negative c. Positive Zero d. Negative Positive e. Negative Negative f. Negative Zero g. Zero Positive h. Zero Negative Describe what a particle is doing in each case, and give a 12. Can the equations of kinematics (Eqs. 2.9–2.13) be used in a situation where the acceleration varies in time? Can they 13. A stone is thrown vertically upward from the roof of a building. Does the position of the stone depend on the lo- 14. A child throws a marble into the air with an initial speed v i . Another child drops a ball at the same instant. Compare A student at the top of a building of height h throws one ball i and then throws a second ball downward with the same initial speed, v i . How do the final velocities of the balls compare when they reach the ground? 16. An object falls freely from height h. It is released at time zero and strikes the ground at time t. (a) When the object 17. You drop a ball from a window on an upper floor of a build- ing. It strikes the ground with speed v. You now repeat the 15. Q U E S T I O N S Section 2.1 Position, Velocity, and Speed 1. The position of a pinewood derby car was observed at vari- second, (b) the last 3 s, and (c) the entire period of obser- 0 1.0 2.0 3.0 4.0 5.0 x(m) 0 2.3 9.2 20.7 36.8 57.5 1, 2 , 3 = straightforward, intermediate, challenging = full solution available in the Student Solutions Manual and Study Guide = coached solution with hints available at http://www.pse6.com = computer useful in solving problem = paired numerical and symbolic problems P R O B L E M S |