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9.1 Describing Motion 9.2 Speed and Velocity 9.3 Acceleration 9.4 Energy

Section 1: Describing Motion
Chapter 9 Motion and Energy Section 1: Describing Motion When is an object in motion?

Relative Motion Chapter 9 Motion and Energy - Describing Motion
Whether or not an object is in motion depends on the reference point you choose.

Distance and Displacement
Chapter 9 Motion and Energy Distance and Displacement Distance is the total length of the actual path between two points. Displacement is the length and direction of a straight line between starting and ending points. A quantity that consists of both magnitude and a direction is called a vector.

Section 2: Speed and Velocity
Chapter 9 Motion and Energy Section 2: Speed and Velocity How do you calculate speed?

Calculating Speed Chapter 9 Motion and Energy
If you know the distance an object travels in a certain amount of time, you can calculate the speed of the object.

Average & Instantaneous Speed
Chapter 9 Motion and Energy Average & Instantaneous Speed Most things do not travel at a constant speed, therefore average speed is usually calculated. This is done by dividing the total distance traveled by the total time. Instantaneous speed is the rate at which an object is moving at a given instant in time.

Click the Video button to watch a movie about speed.
Chapter 9 Motion and Energy Speed Click the Video button to watch a movie about speed.

Section 2: Speed and Velocity
Chapter 9 Motion and Energy Section 2: Speed and Velocity How can you describe changes in velocity? Velocity (v) is speed in a given direction. Changes in velocity may be due to changes in speed, changes in direction, or both.

Graphing Motion Chapter 9 Motion and Energy
You can use distance-versus-time graphs to interpret motion.

Graphing Motion Chapter 9 Motion and Energy
Slope is the steepness of a line on a graph and tells you how fast one variable changes in relation to the other variable. The slope of a distance-versus-time graph represents speed, that is, the rate that distance changes in relation to time.

Section 3: Acceleration
Chapter 9 Motion and Energy Section 3: Acceleration What kind of motion does acceleration refer to?

Acceleration is the rate at which velocity changes.
Recall that velocity has two components, direction and speed. Any time the speed of an object changes, the object experiences acceleration. That change can be an increase or decrease. A decrease in speed is sometimes called deceleration, or negative acceleration. An object that is changing direction is also accelerating, even if it is moving at a constant speed. The simplest example of this type of motion is circular motion, or motion along a circular path, like the moon around the Earth. A car moving around a curve or changing lanes at a constant speed is accelerating because it is changing direction.

Calculating Acceleration
Chapter 9 Motion and Energy Calculating Acceleration To determine the acceleration of an object, you must calculate its change in velocity per unit of time. Acceleration = Final Velocity – Initial Velocity Time If velocity is measured in meters/second and time is measured in seconds the unit of acceleration is in meters per second per second, which is written as m/s2.

Calculating Acceleration
Chapter 9 Motion and Energy Calculating Acceleration As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration? Read and Understand What information have you been given? Initial velocity = 4 m/s Final velocity = 22 m/s Time = 3 s

Calculating Acceleration
Chapter 9 Motion and Energy Calculating Acceleration As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration? Plan and Solve What quantity are you trying to calculate? The acceleration of the roller-coaster car = __ What formula contains the given quantities and the unknown quantity? Acceleration = (Final velocity - Initial velocity)/Time Perform the calculation. Acceleration = (22 m/s - 4 m/s)/3 s = 18 m/s/3 s Acceleration = 6 m/s2 The acceleration is 6 m/s2 down the slope .

Calculating Acceleration
Chapter 9 Motion and Energy Calculating Acceleration As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration? Look Back and Check Does your answer make sense? The answer is reasonable. If the car’s velocity increases by 6 m/s each second, its velocity will be 10 m/s after 1 second, 16 m/s after 2 seconds, and 22 m/s after 3 seconds.

Graphing Acceleration
Chapter 9 Motion and Energy Graphing Acceleration You can use both a speed-versus-time graph and a distance-versus-time graph to analyze the motion of an accelerating object.