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 metres per second. Some are kilometres 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 kilometres and hours instead of metres 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 metres	1.8 seconds	about 2.9 metres per second
A walk to a friend's house	205 metres	about 146 seconds	1.40 metres per second
A round trip to a store	6 kilometres	30 minutes	12 kilometres per hour
A car trip	150 kilometres	3.2 hours	about 47 kilometres per hour

USE THE TABLE

Read the question.
Tap your answer.

Which row has the highest speed in metres per second?

The round trip to the store covered 6 kilometres in 30 minutes. Why is the speed 12 kilometres 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 metres per second. Kilometres 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 kilometres, but you end where you started.

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

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

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

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

Which unit of speed is the SI unit: metres per second or kilometres per hour? Type the first word.

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

SLOWEST TO FASTEST, IN METRES 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 metres 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.