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Force and Motion Notes

8-5.1

Use measurement and time-distance graphs to represent the motion of an object in terms of position, direction, or speed.

 

Motion occurs when there is a change in position of an object with respect to a reference starting point.  The final position of an object is determined by measuring the change in position and direction of the segments along a trip.  The following terms are used to describe and determine motion:

 

Position          

  • Position is the location of an object. 
  • An object changes position if it moves relative to a reference point
  • The change in position is determined by the distance and direction of an object’s change in position from the starting point (displacement).

 

Direction        

  • Direction is the line, or path along which something is moving, pointing, or aiming. 
  • Direction is measured using a reference point with terms such as up, down, left, right, forward, backward, toward, away from, north, south, east, or west.

 

For example, given the following data table, determine the change in the object’s position based on its final position, distance, and direction, from a starting point.

 

Segment

Distance (m)

Direction

X

10

East

Y

7

North

Z

10

West

 

 

  • Draw a line to scale representing 10 meters in an easterly direction.
  • At the end of that line, draw a line representing 7 meters in a northerly direction.
  • From the end of the second line, draw a line representing 10 meters in a westerly direction.
  • Connect the end of the third line to the starting point.
  • Measure the distance and direction from the starting point to the end of the third line.
  • The position at the end of the trip is 7 meters north of the starting point.

 

Motion can also be described by the relationship between distance an object travels and the period of time it travels.  This measurement of motion is a rate.

 

Speed              

  • Speed is a measure of how fast something moves a particular distance (for example, meters) over a given amount of time (for example, seconds). 
  • Therefore, speed is the rate of change of the position of an object, or how far something will move in a given period of time. 

Speed does not necessarily mean that something is moving fast

 

Distance-Time Graph  extraordinary A graph that can be used to represent how both speed and distance change with time. 

  • For this type of graph, time (the independent variable) is plotted on the x-axis and the distance (the dependent variable) is plotted on the y-axis. 

 

Speed              

  • The slope of the line can tell the relative speed of the object. 
  • When the slope of the line is steep, the speed is faster than if the slope were flatter. 
  • When the slope of the line is flatter, the speed is slower. 
  • When the slope of the line is horizontal to the x-axis, the speed is zero (the object is not moving).  For example: 

    

 

Data can be represented in a table.  For example:

 

Time (s)

Distance (m)

0

0

1

5

2

10

3

15

4

15

5

15

6

30

7

45

 

 

This data can then be represented on a distance-time graph.

 

 

C

B

A

 

 

This distance-time graph can then be used to describe the speed of the object.  For example, the speed of segment A is slower than segment C.  The speed of segment B is zero, the object is not moving.

 

 

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