Describing Motion 2 1 Motion occurs when an

Describing Motion 2. 1

Motion occurs when an object changes its position.

Motion and Position Mark Ransom Although you did not see the truck move, you know it moved because its position relative to the mailbox changed.

Movement can be measured only with reference to something that is assumed to be fixed into place – frame of reference Motion is relative to the observer

Distance How far an object has moved SI unit is meter, km, or cm Displacement Distance and direction of an objects change in position

Rules for Adding Displacements 1. Add displacements in the same direction. 2. Subtract displacements in opposite directions. 3. Displacements that are not in the same or in opposite directions cannot be directly added.

Speed is the distance an object travels per unit of time.

Graphing Motion The motion of an object over a period of time can be shown on a distance-time graph. For example, the graph below shows the distance traveled by three swimmers during a 30 -minute workout. Time is plotted along the horizontal axis of the graph, and the distance traveled is plotted along the vertical axis of the graph.

Change in Position Describing how far and in what direction you moved was an important part of describing your motion. An object’s displacement is not the same as the total distance that the object traveled. The runner’s displacement is 20 m east of the starting line. However, the total distance the runner traveled is 80 m.

30 km 15 km ! 20 km 15 km His Displacement is only 15 km.

Essential Questions How are distance and displacement different? How is an object’s speed calculated? What information does a distance-time graph provide?

Review Vocabulary meter: the SI unit of length, abbreviated m

New Vocabulary motion displacement speed

Assessment 1. Which of the following do you calculate when you divide the total distance traveled by the total travel time? A average speed B constant speed C variable speed D instantaneous speed CORRECT

Assessment 2. How is speed defined? A acceleration/time B change in velocity/time C distance/time D displacement/time CORRECT