14E. Sketch a graph that is a possible description of position as a function of time for a particle that moves along the x axis and. at   = 1 s, has (a) zero velocity and positive acceleration; (b) zero velocity and negative acceleration; (c) negative velocity and positive acceleration: (d) negative velocity .and negative acceleration. (e) For which of these situations is the speed of the particle increasing at I = 1 s?

15E. What do the quantities (a), (dx/dt)~ and (b) d~x/dl~ represent? (c) What are their SI units?
16E. A frightened ostrich moves in a straight line with velocity
described by the velocity-time Z graph of Fig. 2-19. Sketch accel- E.
eration versus time. -10
17E. A particle had a speed of 18 m/s at a certain time. and 2.4 s later its speed was 30 m/sin the opposite direction. What were the magnitude and direction of the average acceleration of the particle during this 2.4 s interval? ssm

18P. From I = 0 to I = 5.00 min, a man stands still. and from I = 5.00 min to I = 10.0 min. he walks briskly in a straight line at a constant speed of 2.20 m/s. What are (a) his average velocity I·,.g and (b) his average acceleration a.y~ in the time interval 2.00 min to 8.00 min? What are (c) l and (d) a v in the time interval 3.00 min to 9.00 min? (e) Sketch versus I and I’ versus I. and indicate how the answers to (a) through (d) can be obtained from the graphs.

19P. A proton moves along the x axis according to the equation x = 501 + 1012.where x is in meters and I is in seconds. Calculate (a) the average velocity of the proton during-the first 3.0 s of its motion. (b) the instantaneous velocity of the proton at I = 3.0 s. and (c) the instantaneous acceleration of the proton at I = 3.0 s. (d) Graph x versus I and indicate how the answer to (a) can be obtained from the plot. (e) Indicate the answer to (b) on the graph. (f) Plot I’ versus I and indicate on it the-answer to (c).

20P. A electron moving along the x axis has a position given by
x = 161t’-‘ m. where I is in seconds. How far is the electron from
the origin when it momentarily stops?

 21P. The position of a particle moving along the x axis depends on


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