Chapter 9 – Section 1 Describing and Measuring Motion

Recognizing Motion An object is in motion when its distance from another object is changing. Whether or not an object is moving depends on your point of view. Reference point – a place or object used for comparison to determine if something is in motion

An object is in motion if it changes position relative to a reference point.

Describing Distance Use units of measurement to describe motion further. International System of Units (SI) is used to communicate clearly. The SI system is based on the number ten. Basic unit is the meter (m) To measure objects smaller than a meter use centimeters (cm)

A centimeter is 100 th of a meter – so there are 100 cm in a meter. For even small measurements use millimeter (mm). There are 1,000 millimeters in 1 meter. To measure very long distances use kilometers. There are 1,000 meters in 1 kilometer.

Common Metric Measurements 1 kilometer = 1,000 meters 1 meter = 100 centimeters 1 meter = 1,000 millimeters 1 centimeter = 10 millimeters

Calculating Speed If you know the distance an object travels in a certain amount of time – you know the speed of the object. Speed = Distance ÷ Time

Label Your Measurements! Speed measurement consists of a unit of distance divided by a unit of time. You must include the label of distance /label of time. Example: km/hour or m/sec.

Constant Speed If the speed of an object does not change, the object is traveling at a constant speed.

Average Speed Most objects do not move at constant speeds for very long. To find the average speed, divide the total distance traveled by the total time.

Average speed = Total distance ÷ Total time Total distance = x km + y km Total time = a hours + b hours

Describing Velocity Knowing the speed of an object does not tell you everything about its motion. It is important to know not only the speed of the object but also its direction. When you know both the speed and direction of an object’s motion, you know the velocity of the object.

Speed in a specific direction is velocity. If you know an object’s velocity – you know it’s speed and it’s direction.

Graphing Motion You can show the motion of an object on a line graph. Time is shown on the horizontal axis. (x) Distance is shown on the vertical axis. (y) A point on the graph (x,y) represents the location of an object at a particular time.

Slope of a Line The steepness or slant of a line on a graph is called its slope. The slope of a line tells you how fast one variable changes in relation to the other variable. In other words it describes the rate of change.

Since speed is that rate of change of distance in relation to time, the slope of a distance-time graph represents speed. The steeper the slope, the faster the motion of the object because it covers a greater distance within a given amount of time.

A distance-time graph with a constant slope represents motion at a constant speed.

Calculating Slope The slope of a line is its rise divided by its run. The rise is the vertical difference between two points. The run is the horizontal difference between two points.

If you have points (3,6) and (8,16) on the same line; to calculate the slope use the following formula: Slope = rise =(y 2 -y 1 ) = (16m – 6m) = run (x 2 -x 1 ) ( 8s - 3s ) 10m = 2 m/s 5s

From the calculation of the slope you know that for every second the object travels, it moves 2 meters. Remember if: y = the distance traveled and x = the time traveled and m = slope; then y = mx

Different slopes Not all objects move at a constant speed. The graph of an object not moving at a constant speed is NOT a straight line.