Chapter 8 Lesson 3Describing and Measuring Motion

When is an Object in Motion?SC.6.N.2.2., SC.6.N.2.3, SC.6.P.12.1

Vocabulary

• Motion• Reference point• International System of Units• distance

When is an Object in Motion?

An object is in motion if its position changes relative to another object.

To decide if you are moving, you use a reference point, a place or object used for comparison to determine if something is in motion.

An object is in motion if it changes position relative to a reference point.

Relative Motion

• If you use your chair as a reference point as you sit and read, you are not moving.

• Suppose you use the sun as a reference point instead of your chair.

• If you compare your position to the sun, you are moving because you & your chair are on the Earth which revolves around the sun.

Measuring Distance To describe motion, you need to use units of

measurement. Scientists use a system of measurement called

the International System of Units (SI) Distance is the length of the path between 2

points, the SI unit for length is the meter (m).

Chapter 8 Lesson 4

Speed and VelocityHow Do You Calculate Speed?

How Do You Describe Velocity?How Do You Graph Motion?

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Vocabulary• Speed• Average speed• Instantaneous speed• Velocity• slope

How do you calculate speed?

• Speed is the distance an object moves per unit of time.

• Formula for speed: speed = distance timeExample: If an athlete bikes a distance of 50 kilometers in 2 hours, what is the athlete’s speed? 50 km = 25 km/hr. 2 hrs.

Average Speed

Average Speed = total distance divided by total time.Example: A triathlete swims a distance of 3 km in 1 hour, bikes a distance of 50 km in 3 hours and runs a distance of 12km in 1 hour. What is the athlete’s average speed?Total distance = 3km + 50km + 12km = 65kmTotal time = 1h + 3h + 1h = 5 hAverage speed = 65 km = 13km/h 5h

Instantaneous Speed

• Instantaneous speed is the speed at which an object is moving at a given instant in time.

What is the instantaneous speed of the speedometer on the left?

Velocity

Velocity is the speed and direction of an object’s motion. For example if a train is traveling west at 100 mph, its velocity is 100mph west.

How do you graph motion?

• You can show the motion of an object on a line graph in which you plot distance versus time.

• The steepness of a line on a graph is called slope.

You can calculate the slope of a line by dividing the rise by the run. Rise is the vertical difference between any 2 points on the line. Run is the horizontal difference between the same 2 points.

Chapter 8 Lesson 5 Acceleration

What is Acceleration?How Do You Graph Acceleration?

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Vocabulary

Acceleration is the rate at which velocity changes. In everyday language, acceleration means “the

process of speeding up”.

In Science, acceleration refers to increasing speed, decreasing speed, or changing direction.

CHANGING SPEED

• Whenever an object’s speed changes, the object accelerates.

• A car that begins to move from a stopped position or speeds up to pass another car is accelerating.

• A car can also slow down, this change in speed is sometimes called deceleration or negative acceleration.

CHANGING DIRECTION

Even an object that is traveling at a constant speed can be accelerating when they change direction. A car accelerates as it follows a gentle curve in the road or changes lanes.

Calculating Acceleration

Acceleration = Final speed – Initial speed TimeExample: A small airplane is moving down a runway. It accelerates from a speed of 0 m/s to a speed of 24 m/s in 3 seconds. What is the airplane’s average acceleration?Final speed = 24m/s 24m/s – 0m/s = 24m/s = 8 m/s²Initial speed = 0 m/s 3 s 3 sTime = 3 s

Graphing Acceleration• You can use a speed-versus-time graph and a

distance-versus-time graph to analyze the motion of an accelerating object.

Distance-Versus-Time GraphUse the time and distance data in the table to plot a line graph. Plot time on the horizontal axis. Plot distance on the vertical axis. What would be a good title for the graph?

Speed-Versus-Time GraphUse the time and speed data in the table to plot a line graph. Plot time on the horizontal axis. Plot speed on the vertical axis. What would be a good title for the graph?