Chapter Introduction Lesson 1 Describing Motion Lesson 2 Position and Motion Lesson 3 Acceleration Chapter Wrap-Up What are some ways to describe motion? Position and Motion • How does the description of an object’s position depend on a reference point? • How can you describe the position of an object in two dimensions? • What is the difference between distance and displacement? Position and Motion • reference point • position • motion • displacement Describing Position A description of a location usually states the location relative to a certain point. relative Science Use compared (to) Common Use a member of your family Describing Position (cont.) • A reference point * • A position *. Describing Position (cont.) A complete description of your position includes a distance, a direction, and a reference point. Describing Position (cont.) How does the description of an object’s position depend on a reference point? Describing Position (cont.) An object’s position is described by a reference direction, such as toward the slide. Describing Position (cont.) The reference direction is * Describing Position in Two Dimensions • When you describe a position using two directions, you are using two dimensions. • To find a position in two dimensions you must choose a *, specify reference directions, and then determine the distance along each reference direction. Describing Position in Two Dimensions (cont.) How can you describe the position of an object in two dimensions? Describing Changes in Position Motion * motion from Latin motere, means “to move” The man in the boat is not in motion compared to his fishing pole. He is in motion compared to the buoy. Describing Changes in Position (cont.) • Displacement * • Distance depends on the path taken. • Displacement depends only on the initial and final positions. Describing Changes in Position (cont.) Distance and displacement are equal only if the motion is in one direction. What is the difference between distance and displacement? • A reference point, a reference direction, and distance are needed to describe the position of an object. • An object is in motion if its position changes relative to a reference point. • The distance an object moves and the object’s displacement are not always the same. Speed and Velocity • What is speed? • How can you use a distance-time graph to calculate average speed? • What are ways velocity can change? Speed and Velocity • speed • constant speed • instantaneous speed • average speed • velocity What is speed? Speed * What is speed? What is speed? (cont.) You can calculate speed by dividing the distance traveled by the time it takes to go that distance. What is speed? (cont.) Constant speed * What is speed? (cont.) When the car’s speed changes, it moves a different distance each period of time. What is speed? (cont.) • Instantaneous speed * • Average speed * Distance-Time Graphs • Graphs that show comparisons between distance and time are called distance-time graphs. • Constant speed is shown as a straight line on a distancetime graph. Distance-Time Graphs (cont.) • You can use distance-time graphs to compare the motion of two different objects. • The steeper line indicates a faster speed. You can use distance-time graphs to calculate the average speed of an object. Distance-Time Graphs (cont.) How can you use a distancetime graph to calculate average speed? Distance-Time Graphs (cont.) If the speed of an object changes instead of being constant, its motion on a distance-time graph is a curved line. Velocity Velocity * velocity from Latin velocitas, means “swiftness, speed” Velocity (cont.) • The velocity of an object can be represented by an arrow. • A greater speed is shown by a longer arrow. • The arrow points in the direction of the object’s movement. Velocity (cont.) Velocity changes when the speed of an object changes, when the direction that the object moves changes, or when both the speed and the direction change. Velocity (cont.) How can velocity change? • Speed is a measure of the distance an object travels in a unit of time. You can describe an object’s constant speed, instantaneous speed, or average speed. • A distance-time graph shows the speed of an object. • Velocity includes both the * Acceleration • What are three ways an object can accelerate? • What does a speed-time graph indicate about an object’s motion? Acceleration • acceleration Acceleration—Changes in Velocity • Acceleration * • An object accelerates when its velocity changes as a result of increasing speed, decreasing speed, or a change in direction. Acceleration—Changes in Velocity (cont.) Acceleration has a direction and can be represented by an arrow. Acceleration—Changes in Velocity (cont.) What are three ways an object can accelerate? Calculating Acceleration Acceleration is a change in * Calculating Acceleration (cont.) Acceleration is in the direction of motion if it is positive and opposite the direction of motion if it is negative. Speed-Time Graphs • A speed-time graph shows how speed changes over time. • A speed-time graph has time on the horizontal axis—the x-axis, and speed on the vertical axis—the y axis. Speed-Time Graphs (cont.) horizontal from Greek horizein, means “limit, divide, separate” vertical from Latin verticalis, means “overhead” Speed-Time Graphs (cont.) An object at rest is not moving, so its speed is always zero. Speed-Time Graphs (cont.) For an object moving at constant speed, the speed-time graph is a horizontal line. Speed-Time Graphs (cont.) The line on the speed-time graph for an object that is speeding up has an upward slope. Speed-Time Graphs (cont.) The line on the speed-time graph for an object that is slowing down has a downward slope. Speed-Time Graphs (cont.) What does a speed-time graph show about the motion of an object? Summarizing Motion • There are several ways to describe motion. • You can describe position by direction and distance from a reference point. • You can compare distance and displacement to find average speed. • You can describe velocity by speed and direction. • An object accelerates if it speeds up, slows down, or changes direction. • Acceleration in a straight line can be calculated by dividing the change in speed by the change in time. • A speed-time graph shows how an object’s speed changes over time. The motion of an object can be described by the object’s position, velocity, and acceleration. Lesson 1: Position and Motion • An object’s position is its distance in a certain direction from a reference point. • The position of an object in two dimensions can be described by choosing a reference point and two reference directions, and then stating the distance along each reference direction. • The distance an object moves is the actual length of its path. Its displacement is the difference its initial position and its final position. Lesson 2: Speed and Velocity • Speed is the distance an object moves in a unit of time. • An object moving the same distance each second is moving at a constant speed. The speed of an object at a certain moment is its instantaneous speed. • You can calculate an object’s average speed from a distance-time graph by dividing the distance the object travels by the total time it takes to travel that distance. • Velocity changes when speed, direction, or both speed and direction change. Lesson 3: Acceleration • Acceleration is a change in velocity over time. An object accelerates when it speeds up, slows down, or changes direction. • A speed-time graph shows the relationship between speed and time and can be used to determine information about the acceleration of an object.
