Table of Contents

Chapter 9 Motion and Energy

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9.1 Describing Motion

9.2 Speed and Velocity

9.3 Acceleration

9.4 Energy

Section 1:Describing Motion

When is an object in motion?

Chapter 9 Motion and Energy

Relative MotionWhether or not an object is in motion depends on the reference point you choose.

Chapter 9 Motion and Energy - Describing Motion

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.

Chapter 9 Motion and Energy

Section 2:Speed and Velocity

How do you calculate speed?

Chapter 9 Motion and Energy

Calculating Speed

If you know the distance an object travels in a certain amount of time, you can calculate the speed of the object.

Chapter 9 Motion and Energy

Average & Instantaneous SpeedMost 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.

Chapter 9 Motion and Energy

SpeedClick the Video button to watch a movie about speed.

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.

Chapter 9 Motion and Energy

Graphing Motion

You can use distance-versus-time graphs to interpret motion.

Chapter 9 Motion and Energy

Graphing Motion

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.

Chapter 9 Motion and Energy

Section 3:Acceleration

What kind of motion does acceleration refer to?

Chapter 9 Motion and Energy

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 AccelerationTo 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.

Chapter 9 Motion and Energy

Calculating AccelerationAs 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

Chapter 9 Motion and Energy

Calculating AccelerationAs 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 SolveWhat 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)/TimePerform the calculation.

Acceleration = (22 m/s - 4 m/s)/3 s = 18 m/s/3 sAcceleration = 6 m/s2

The acceleration is 6 m/s2 down the slope .

Chapter 9 Motion and Energy

Calculating AccelerationAs 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.

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.

Chapter 9 Motion and Energy