Describing and Measuring Motion

End of Chapter Project• In lieu of a test, you are going to write a formal lab

report• A formal lab report is what scientists use to describe

their research in a reproducible manner• It contains the following information

– Title Page– Abstract– Introduction– Materials/Methods– Procedure– Results– Discussion– Conclusion– References/appendix

Your Goal

• To identify the motion of several common objects and calculate how fast each one moves

• To complete this project you must:

– Measure distance and time carefully

– Calculate the speed of each object using your data

– Prepare a formal lab report of your data, including diagrams, results and calculations

Key Concepts

• Determine when an object is in motion

• Calculate an objects speed and velocity

• Demonstrate how to graph motion

How do you know if you are moving?

• Have you ever been on a train?

• Sometimes you cannot always tell if the train is moving

• Looking at an outside building or surrounding area might help, but the building seems to move past the train—really its you passing the building

• However, sometimes you may see another train that appears to be moving. Is the other train really moving, or is your train moving? How can you tell?

Describing Motion

• Deciding of an object is moving isn’t as easy as you might think

• For example, you are sitting at a desk taking these notes, right?

• Are you moving?

• Well, parts of you might be. Your eyes blink, your chest moves up and down. But you would probably say that you are not moving.

Motion

• An object is in motion if it’s distance from another object is changing.

• Because your distance from your chair is not changing, you could say you are not in motion

Reference Points

• To decide if you are moving, you use your chair as a reference

• A reference point is 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

• Objects that we call stationary—such as a tree, a sign, or a building—make good reference points.

• From the point of view of this passenger, what could they use as a reference point?

• You probably know what happens if your reference point is moving.

• Have you ever been in a school bus parked next to another bus?

• Suddenly, you think your bus is moving backward. But, when you look out a window on the other side, you find that your bus isn’t moving at all—the other bus is moving forward!

• Your bus seems to move backward because you used the other bus

as a reference point

Relative Motion

• Are you moving as you read this powerpoint?

• The answer to that question depends on your reference point

– When your chair is your reference point, you are not moving

– But if you choose another reference point, you may be moving

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

• If you compare yourself to the sun, you are moving quite rapidly

• This is because you and your chair are on Earth, which moves around the sun. Earth moves about 30 km every second!– Going that fast, you could travel from New York

City to Los Angeles in 2 minutes.

Relative Motion From a Plane

• The plane does not appear to be moving

• The skydivers appear to be moving away

• A point on the ground appears to be moving away

Relative Motion From the Skydivers

• The plane appears to be moving away

• The skydivers do not appear to be moving

• The ground appears to be moving closer

Relative Motion From the Ground

• The plane appears to be moving across the sky

• The skydivers appear to be moving closer

• The ground does not appear to be moving

Measuring Distance

• You can use units of measurement to describe motion precisely.

– For example:

• 1 cup of milk for a recipe

• Run 2 miles for practice

• Buy 3 lbs of fruit at the store

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 milimeters (mm) are in 14.5 meters (m). Since there are 1000 mm in 1 meter, the conversion factor is

1,000 mm

1m

• Multiply 14.5 meters (m) by the conversion factor to find mm:

14.5 x 1,000 mm = 14,500 mm

How many centimeters are in 22.5 meters?

• Hint: 100 cm for every 1 meter

• 2,250

Measure the length and width of your index finger in mm and convert to cm.

1 Mile = 1.6 km

• If I travel 5.4 miles, how many kilometers?

• In Germany, they use km. If I traveled there and drove in my car for 17 km, how many miles is that?

• A 5 K run is equivalent to how many miles?

Average vs. Instantaneous Speed

• Average: Divide the total distance by the total speed

• Instantaneous: is the rate at which an object is moving at a given instant in time

Describing Velocity

• Knowing the speed at which something travels does not tell you everything about its motion

• To describe an object’s motion completely, you need to know the direction of its motion

• For example, suppose you hear that a thunderstorm is traveling at a speed of 25 km/hr.

• Should you prepare for the storm?

• Well, it depends on the direction of the storm’s motion

• Storms usually travel from west to east, so if your west of the storm, don’t worry, if your to the east…TAKE COVER!

• When you know both the speed and direction of an object’s motion, you know the velocity of the object

• Speed in a given direction is called velocity

– i.e: 25 km/hr eastward is the velocity of the storm

Review

• What is a conversion factor?

– A conversion factor is a fraction in which the numerator and denominator represent equal amounts in different units

Review

• 1 mile = 1.6 km; I travel 1.1 miles to work, how many km is that?

– 1.8 km

Review

• Determine if the following examples are speed or velocity.

– 78 mph

• speed

– 44 cm/s to the west

• velocity

– 56 nm/hr northward

• velocity

– 99 km/min

• speed

Graphing Motion

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

• Time is on the x-axis and distance on the y

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

• The slope tells you how fast one variable changes in relation to the other variable in the graph

Calculating Slope

Different Slopes

• Most objects do not move at a constant speed

• From the steepness of the slopes you can tell how fast or slow an object is moving.

Equation

• Divide the rise by the run (rise is y, run is x)

Review• How do you know if an object is moving?

• Why is it important to know if your reference point is moving?

• Suppose you are riding in a car. Describe your motion relative to the car, the road, and the sun.

• Why is it important to know velocity rather than speed?

• The rise of a line on a distance vs time graph is 600m and the run is 3 minutes. What is the slope?