2 Representing Motion 2 In this chapter you will: Represent motion through the use of words, motion diagrams, and graphs.Use the terms position, distance, displacement, and time interval in a scientific manner to describe motion.
3 Picturing Motion 2.1 Movement Along a Straight Line SectionPicturing Motion2.1Movement Along a Straight LineMotion relates to change in place and time. You must be able to answer the questions of where and when an object is positioned to describe its motion.In the figure below, the car has moved from point A to point B in a specific time period.
4 Click image to view movie. SectionPicturing Motion2.1Motion DiagramsClick image to view movie.
5 DefinitionsMotion Diagram: a series of images showing the positions of a moving object at equal time intervals.Particle Model: a motion diagram in which the images are replaced with a series of single points.
7 Where and When? 2.2 In this section you will: Define coordinate systems for motion problems.Recognize that the chosen coordinate system affects the sign of objects’ positions.Define displacement.Determine a time interval.Use a motion diagram to answer questions about an object’s position or displacement.
8 Where and When? 2.2 Coordinate Systems SectionWhere and When?2.2Coordinate SystemsIn the example of the runner, the origin, represented by the zero end of the measuring tape, could be placed 5 m to the left of the tree.The motion is in a straight line, thus, your measuring tape should lie along that straight line. The straight line is an axis of the coordinate system.
9 Where and When? 2.2 Coordinate Systems SectionWhere and When?2.2Coordinate SystemsYou can indicate how far away an object is from the origin at a particular time on the simplified motion diagram by drawing an arrow from the origin to the point representing the object, as shown in the figure. The arrow is a position VECTOR.
10 Where and When? 2.2 Vectors and Scalars SectionWhere and When?2.2Vectors and ScalarsQuantities that have both size, also called magnitude, and direction, are called vectors, and can be represented by arrows.Quantities that are just numbers without any direction, such as distance, time, or temperature, are called scalars.To add vectors graphically, the length of a vector should be proportional to the magnitude of the quantity being represented. So it is important to decide on the scale of your drawings.
11 Where and When? 2.2 Vectors and Scalars SectionWhere and When?2.2Vectors and ScalarsThe vector that represents the sum of the other two vectors is called the resultant.The resultant always points from the tail of the first vector to the tip of the last vector. “Add vectors head-to-tail.”
12 Vector AdditionClick here for vector practice:Vector Practice
13 Where and When? 2.2 Time Intervals and Displacement SectionWhere and When?2.2Time Intervals and DisplacementThe difference between the initial and the final times is called the time interval.The common symbol for a time interval is ∆t, where the Greek letter delta, ∆, is used to represent a change in a quantity.
14 Where and When? 2.2 Time Intervals and Displacement SectionWhere and When?2.2Time Intervals and DisplacementThe time interval is defined mathematically as follows:Although i and f are used to represent the initial and final times, they can be initial and final times of any time interval you choose.Also of importance is how the position changes. The symbol d may be used to represent position.In physics, a position is a vector with its tail at the origin of a coordinate system and its tip at the place where the object is located at that time.
15 Where and When? 2.2 Time Intervals and Displacement SectionWhere and When?2.2Time Intervals and DisplacementThe figure below shows ∆d, an arrow drawn from the runner’s position at the tree to his position at the lamppost.The change in position during the time interval between ti and tf is called displacement.
16 Where and When? 2.2 Time Intervals and Displacement SectionWhere and When?2.2Time Intervals and DisplacementThe length of the arrow represents the distance the runner moved, while the direction the arrow points indicates the direction of the displacement.Displacement is mathematically defined as follows:Displacement is equal to the final position minus the initial position.
17 Section Check 2.2 Question 1 Differentiate between scalar and vector quantities?
18 SectionSection Check2.2Answer 1Quantities that have both magnitude and direction are called vectors, and can be represented by arrows. Quantities that are just numbers without any direction, such as time, are called scalars.
19 Section Check 2.2 Question 2 What is displacement? The vector drawn from the initial position to the final position of the motion in a coordinate system.The length of the distance between the initial position and the final position of the motion in a coordinate system.The amount by which the object is displaced from the initial position.The amount by which the object moved from the initial position.
20 Section Check 2.2 Answer 2 Answer: A Reason: Options B, C, and D are all defining the distance of the motion and not the displacement. Displacement is a vector drawn from the starting position to the final position.
21 Section Check 2.2 Question 3 Refer the figure below and calculate the time taken by the car to travel from one signal to another signal?20 min45 min25 min5 min
22 Section Check 2.2 Answer 3 Answer: C Reason: Time interval t = tf - tiHere tf = 01:45 and ti = 01:20Therefore, t = 25 min
23 Position-Time Graphs 2.3 In this section you will: Develop position-time graphs for moving objects.Use a position-time graph to interpret an object’s position or displacement.Make motion diagrams, pictorial representations, and position-time graphs that are equivalent representations describing an object’s motion.
24 Click image to view movie. SectionPosition-Time Graphs2.3Position Time GraphsClick image to view movie.
25 Position-Time Graphs 2.3 Using a Graph to Find Out Where and When SectionPosition-Time Graphs2.3Using a Graph to Find Out Where and WhenGraphs of an object’s position and time contain useful information about an object’s position at various times and can be helpful in determining the displacement of an object during various time intervals.The data in the table can be presented by plotting the time data on a horizontal axis and the position data on a vertical axis, which is called a position-time graph.
26 Position-Time Graphs 2.3 Using a Graph to Find Out Where and When SectionPosition-Time Graphs2.3Using a Graph to Find Out Where and WhenTo draw the graph, plot the object’s recorded positions. Then, draw a line that best fits the recorded points. This line represents the most likely positions of the runner at the times between the recorded data points.The symbol d represents the instantaneous position of the object—the position at a particular instant.
27 Position-Time Graphs 2.3 Considering the Motion of Multiple Objects SectionPosition-Time Graphs2.3Considering the Motion of Multiple ObjectsIn the graph, when and where does runner B pass runner A?
28 Step 1: Analyze the Problem SectionPosition-Time Graphs2.3Considering the Motion of Multiple ObjectsStep 1: Analyze the Problem
29 Section Check 2.3 Question 1 A position-time graph of an athlete winning the 100-m run is shown. Estimate the time taken by the athlete to reach 65 m.6.0 s6.5 s5.5 s7.0 s
30 Section Check 2.3 Answer 1 Answer: B Reason: Draw a horizontal line from the position of 65 m to the line of best fit. Draw a vertical line to touch the time axis from the point of intersection of the horizontal line and line of best fit. Note the time where the vertical line crosses the time axis. This is the estimated time taken by the athlete to reach 65 m.
31 Section Check 2.3 Question 2 A position-time graph of an athlete winning the 100-m run is shown. What was the instantaneous position of the athlete at 2.5 s?15 m20 m25 m30 m
32 Section Check 2.3 Answer 2 Answer: C Reason: Draw a vertical line from the position of 2.5 m to the line of best fit. Draw a horizontal line to touch the position axis from the point of intersection of the vertical line and line of best fit. Note the position where the horizontal line crosses the position axis. This is the instantaneous position of the athlete at 2.5 s.
33 Section Check 2.3 Question 3 From the following position-time graph of two brothers running a 100-m run, analyze at what time do both brothers have the same position. The smaller brother started the race from the 20-m mark.
34 SectionSection Check2.3Answer 3The two brothers meet at 6 s. In the figure, we find the intersection of line representing the motion of one brother with the line representing the motion of other brother. These lines intersect at 6 s and at 60 m.
35 How Fast? 2.4 In this section you will: Define velocity. Differentiate between speed and velocity.Create pictorial, physical, and mathematical models of motion problems.
36 SectionHow Fast?2.4VelocityThe displacement, ∆d, is greater for the jogger in red for the same time interval, ∆t.The slope of the position vs. time graph is steeper for the jogger in red.
37 How Fast? 2.4 Average Velocity SectionHow Fast?2.4Average VelocityThe slopes of the two lines are found as follows:We get the formula for average velocity from these slope calculations:
38 How Fast? 2.4 Average Velocity SectionHow Fast?2.4Average VelocityWhat conclusion about velocity can you draw from this graph?
39 How Fast? 2.4 Instantaneous Velocity SectionHow Fast?2.4Instantaneous VelocityInstantaneous velocity: Velocity at a given instant.The instantaneous velocity is the slope of a line tangent to the position vs. time graph at that particular instant.
40 How Fast? 2.4 Using Equations Equation of Motion for Average Velocity SectionHow Fast?2.4Using EquationsEquation of Motion for Average Velocity
41 Section Check 2.4 Question 3 The position-time graph of a car moving on a street is as given here. What is the average velocity of the car?2.5 m/s5 m/s8 m/s10 m/s
42 Section Check 2.4 Answer 3 Answer: A Reason: Average velocity of an object is the slope of the position- time graph.