Motion and Force

Chapter Three: Motion

• 3.1 Position and Velocity

• 3.2 Graphs of Motion

• 3.3 Acceleration

Investigation 3B

• What kind of motion happens when an object rolls down a hill?

Position, Speed and Time Graphs

3.2 The position vs. time graph

• Motion graphs are an important tool used to show the relationships between position, speed, and time.

A runner can learn more about performance by studying data and graphs.

3.2 The position vs. time graph

• Position vs. time data tells you the runner’s position at different points in time.

• The runner is at 50 meters after 10 sec., 100 meters after 20 sec. and 150 meters at 30 sec.

3.2 Graphs show relationships

• A good way to show a relationship between two variables is to use a graph.

• A graph makes it easy to see if changes in one variable cause changes in the other variable (the effect).

3.2 The position vs. time graph

• To graph data, you put position on the vertical (y) axis .

• Time goes on the horizontal (x) axis.

• Data are plotted between x and y axis.

3.2 The position vs. time graph

• An object moving at a constant speed always creates a position vs. time graph that is a straight line.

3.2 The position vs. time graph

• Two variables may have:– a strong relationship, – a weak relationship,– or no relationship at all.

3.2 Graphs show relationships

• This table shows how quickly the car gets from A to B as the angle of the track changes.

3.2 Graphs show relationships

• If we plot the data on a graph, what kind of relationship does the graph show?

3.2 Four steps to make a graph

Step 1: Choose which will be the dependent and independent variables. The dependent variable goes on the y-axis and the independent variable goes on the x-axis.

Step 2: Make a scale for each axis by counting boxes to fit your largest value. Count by multiples of 1, 2, 5, or 10.

Step 3: Plot each point by finding the x-value and drawing a lin upward until you get to the right y-value.

Step 4: Draw a smooth curve that shows the pattern of the points. Do not just connect the dots.

3.2 Reading a graph

• A graph can give you an accurate answer even without doing the experiment.

• Students doing an experiment measured the speed of the car at 20, 40, 60, and 80 cm positions.

• They want to know the speed at 50 cm.

3.2 Slope

• You can use position vs. time graphs to quickly compare the speeds of different objects.

A steeper line on a position vs. time graph means a faster speed.

3.2 Slope

• The “steepness” of a line is called its slope.

• Visualize a triangle with the slope as the hypotenuse.

• The rise is equal to the height of the triangle.

• The run is equal to the length along the base of the triangle.

3.2 Slope

• The slope is the ratio of the “rise” (vertical change) to the “run” (horizontal change).

• The slope is therefore a distance divided by a time, which equals speed.

3.2 Speed vs. time graphs

• The position vs. time graph has position on the y-axis and time on the x-axis.

Which runner has the fastest constant speed?

3.2 Speed vs. time graphs

These graphs each show the same event.What differences do you notice?

Do these graphs display the same data?

3.2 Calculating distance

• A speed vs. time graph can also be used to find the distance the object has traveled.