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ism_chapter_02

# ism_chapter_02 - Chapter 2 Motion in One Dimension...

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33 Chapter 2 Motion in One Dimension Conceptual Problems 1 Determine the Concept The "average velocity" is being requested as opposed to "average speed". The average velocity is defined as the change in position or displacement divided by the change in time. t y v = av The change in position for any "round trip" is zero by definition. So the average velocity for any round trip must also be zero. 0 0 av = = = t t y v *2 Determine the Concept The important concept here is that "average speed" is being requested as opposed to "average velocity". Under all circumstances, including constant acceleration , the definition of the average speed is the ratio of the total distance traveled ( H + H ) to the total time elapsed, in this case 2 H / T . correct. is ) ( d Remarks : Because this motion involves a round trip, if the question asked for "average velocity," the answer would be zero. 3 Determine the Concept Flying with the wind, the speed of the plane relative to the ground ( v PG ) is the sum of the speed of the wind relative to the ground ( v WG ) and the speed of the plane relative to the air ( v PG = v WG + v PA ). Flying into or against the wind the speed relative to the ground is the difference between the wind speed and the true air speed of the plane ( v g = v w v t ). Because the ground speed landing against the wind is smaller than the ground speed landing with the wind, it is safer to land against the wind. 4 Determine the Concept The important concept here is that a = dv/dt, where a is the acceleration and v is the velocity. Thus, the acceleration is positive if dv is positive; the acceleration is negative if dv is negative. ( a ) Let’s take the direction a car is moving to be the positive direction: Because the car is moving in the direction we’ve chosen to be positive, its velocity is positive ( dx > 0). If the car is braking, then its velocity is decreasing ( dv < 0) and its acceleration ( dv / dt ) is negative. ( b ) Consider a car that is moving to Because the car is moving in the direction

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