Week 01 · Thursday: Measuring Motion (Sports Physics 6-8)

Thursday: Reading a Speed Table

Raven spreads a page of worked problems across the picnic table. "These are from a physics text," she says.

"A marble, a walk to a friend's house, a car trip, a round trip to a store," Rocket reads. "All with numbers."

"What do you notice about the units?" Raven asks.

Rocket runs a finger down the page. "Some are meters per second. Some are kilometers per hour. That is confusing."

Nova hovers over the car row. "Same idea, different unit. Distance divided by time, every time."

"So the marble and my walk can go in the same table," Rocket says. "If we keep the units straight."

Raven nods. "Then let us build it."

Speeds from a physics text

The OpenStax high school physics text works several speed problems. Here are their numbers, gathered into one table.

Each speed is distance divided by time. Two rows use kilometers and hours instead of meters and seconds.

Read the units column before you compare any two rows.

Motion in the text	Distance	Time	Average speed
A rolling marble	5.2 meters	1.8 seconds	about 2.9 meters per second
A walk to a friend's house	205 meters	about 146 seconds	1.40 meters per second
A round trip to a store	6 kilometers	30 minutes	12 kilometers per hour
A car trip	150 kilometers	3.2 hours	about 47 kilometers per hour

USE THE TABLE

Read the question.
Tap your answer.

Which row has the highest speed in meters per second?

The round trip to the store covered 6 kilometers in 30 minutes. Why is the speed 12 kilometers per hour?

Which two rows can be compared without changing units?

What the units tell you

OpenStax notes that the SI unit of speed is meters per second. Kilometers per hour and miles per hour are also used.

A speed in one unit cannot be compared directly with a speed in another. Match the units first.

The text also separates average speed from instantaneous speed. A car speedometer shows instantaneous speed, the speed at that moment.

Your Field times give average speeds. The jog probably started slow and sped up, but the average smooths that out.

The text adds one more idea. For a round trip, the distance is 6 kilometers, but you end where you started.

The marble in the table traveled 5.2 meters in ____ seconds.

The car trip covered 150 kilometers in 3.2 hours, about ____ kilometers per hour.

Statement	True or false?
A speedometer shows instantaneous speed.	?
Your Field times give instantaneous speeds.	?
Kilometers per hour and meters per second can be compared without changing units.	?
The SI unit of speed is meters per second.	?

WHY THIS EXERCISEAverage and instantaneous speed are both useful, but they answer different questions.

The SI unit of speed is meters per ____. Type one word.

WHY THIS EXERCISEYour Field Log uses the SI unit so your numbers match the physics text.

SLOWEST TO FASTEST, IN METERS PER SECOND

Tap a card.
Then tap its spot.

1Slowest

2Next

3Fastest

Try it

Find your own walking speed in yesterday's table. Write it beside the 1.40 from the text.
Were you faster or slower than the walk in the text? Write one sentence about it.

Draw a bar chart with four bars: the marble, the text walk, your walk and your jog, all in meters per second.

Which row in the table surprised you? Say why.Explain to a younger child why a speedometer and a stopwatch answer different questions.Pick one row of the table. Check its speed by dividing the distance by the time yourself.

Excellent data reading. Tomorrow you will learn why "moving" always means moving compared to something.