Motion

Describing and Measuring Motion

Slow Motion on Planet Earth

Acceleration

Table of Contents

Motion - Describing and Measuring Motion

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

Motion - Describing and Measuring Motion

Measuring Distance

You can measure distances shorter than 1 meter in centimeters. The wingspan of the butterfly is 7 cm.

Motion

Converting Units Use a conversion factor to convert one metric unit to another. A conversion factor is a fraction in which the numerator and denominator represent equal amounts in different units. Multiply the number you want to convert by the conversion factor.

Suppose you want to know how many millimeters (mm) are in 15.5 meters (m). Since there are 1,000 millimeters in 1 meter, the conversion factor is:

1,000 mm/1 m

Multiply 14.5 meters by the conversion factor to find millimeters.

14.5 m X 1,000 mm/1 m

= 14.5 X 1,000 mm

= 14,500 mm

- Describing and Measuring Motion

Motion

Converting Units

Practice Problem

How many centimeters are in 22.5 meters?

22.5 m = 2,250 cm

- Describing and Measuring Motion

Motion - Describing and Measuring Motion

Calculating Speed

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

Motion - Describing and Measuring Motion

Graphing Motion

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

Motion

Graphing Motion Activity

Click the Active Art button to open a browser window and access Active Art about graphing motion.

- Describing and Measuring Motion

Motion

What You Know

What You Learned

Using Prior KnowledgeBefore you read, write what you know about motion in a graphic organizer like the one below. As you read, write what you learn.

1. A moving object changes position.2. Objects move at different speeds.

1. Motion is compared to a reference point.2. The SI unit of length is the meter.

- Describing and Measuring Motion

Motion

Velocity

Click the Video button to watch a movie about velocity.

- Describing and Measuring Motion

Motion

Data Sharing Lab

Click the PHSchool.com button for an activity about sharing data for the Skills Lab Inclined to Roll.

- Describing and Measuring Motion

Motion

End of Section:Describing and

Measuring Motion

Motion - Acceleration

Calculating Acceleration

To determine the acceleration of an object moving in a straight line, you must calculate the change in speed per unit of time.

Motion

Calculating Acceleration

As a roller-coaster car starts down a slope, its speed is 4 m/s. But 3 seconds later, at the bottom, its speed is 22 m/s. What is its average acceleration?

Read and Understand

What information have you been given?

Initial speed = 4 m/s

Final Speed = 22 m/s

Time = 3 s

- Acceleration

Motion

Calculating Acceleration

As a roller-coaster car starts down a slope, its speed is 4 m/s. But 3 seconds later, at the bottom, its speed is 22 m/s. What is its average acceleration?Plan and SolveWhat quantity are you trying to calculate?

The average acceleration of the roller-coaster car = __What formula contains the given quantities and the unknown quantity?

Acceleration = (Final speed – Initial speed)/TimePerform the calculation.

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

The roller-coaster car’s average acceleration is 6 m/s2.

- Acceleration

Motion

Calculating Acceleration

As a roller-coaster car starts down a slope, its speed is 4 m/s. But 3 seconds later, at the bottom, its speed is 22 m/s. What is its average acceleration?

Look Back and Check

Does your answer make sense?

The answer is reasonable. If the car’s speed increases by 6 m/s each second, its speed will be 10 m/s after 1 second, 16 m/s after 2 seconds, and 22 m/s after 3 seconds.

- Acceleration

Motion

Calculating Acceleration

Practice Problem

A falling raindrop accelerates from 10 m/s to 30 m/s in 2 seconds. What is the raindrop’s average acceleration?

(30 m/s – 10 m/s) ÷ 2 seconds = 10 m/s2

- Acceleration

Motion

Calculating Acceleration

Practice Problem

A certain car can accelerate from rest to 2.7 m/s in 9 seconds. Find the car’s average acceleration.

(27 m/s – 0 m/s) ÷ 9 s = 27 m/s ÷ 9 s = 3 m/s2

- Acceleration

Motion

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.

- Acceleration

Motion

Main Idea

Detail Detail Detail

Identifying Main IdeasAs you read the section “What is Acceleration?”, write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.

In science, acceleration refers to...

Increasing speed Decreasing speed

Changing direction

- Acceleration

Motion

Links on Acceleration

Click the SciLinks button for links on acceleration.

- Acceleration

Motion

End of Section:Acceleration

Motion

Graphic Organizer

Reference point

Motion

is described relative to a

Distance ÷ Time

Speed Velocity

is measured by

equals in a given direction is called

Motion

End of Section:Graphic Organizer