Rocket sits in the wagon while Raven pulls it slowly along the driveway. He tosses a tennis ball straight up and catches it.
"From in here the ball went straight up and down," he reports.
Raven stops. "I saw it curve forward. What do you notice? We both watched the same ball."
Nova circles the wagon. "Who was moving, Rocket, you or the ball?"
Rocket pauses. "Compared to the wagon, I was still. Compared to the driveway, we were both moving."
"So every motion is measured against something," Raven says. "A reference frame."
Rocket grins. "Then the Field is our reference frame. The tape lines never move."
OpenStax explains that the location of an object at any time is its position, measured relative to a reference frame.
Earth is often used as a reference frame. Your Field, with its tape lines, is a piece of Earth that stays put.
The text describes a classroom. The walls do not move, so your walk to the door can be measured against them.
A reference frame can also move. From a moving wagon, Rocket measured the ball against the wagon, not the driveway.
OpenStax says there is not one correct reference frame. All reference frames are equally valid, but you must say which one you used.
The text tells of Galileo, who studied motion in the 1600s. He imagined a windowless ship moving steadily on a calm sea.
A person inside cannot tell the ship is moving. Only by looking at the shore can they describe their motion.
A person on shore would say the ship moves past. A person on the ship would say the shore moves past.
Galileo concluded that a description of motion is only meaningful if you specify a reference frame.
A player jogging beside a rolling ball sees the ball as nearly still. A fan in the stands sees both fly past.
A swimmer in a moving current measures strokes against the water, while a judge measures against the pool edge.
OpenStax describes a lab with a bouncing ball. Standing still, you see a walking partner's bouncing ball zigzag.
Walking beside the partner, you see the same ball go straight up and down.
When you report a speed, name the reference frame and the units. Then anyone can repeat your measurement.
| Week review | True or false? |
|---|---|
| Average speed is distance divided by time. | ? |
| The SI unit of length is the metre. | ? |
| There is exactly one correct reference frame for every motion. | ? |
| A description of motion needs a reference frame to be meaningful. | ? |
| A speedometer shows average speed. | ? |
Great week of measuring. Tomorrow is Game Day: family speed trials at the Field.