Forces and Motion

Locating events

Teaching Guidance for 14-16 Supporting Physics Teaching

The world described in terms of events

Noticing where something is (its position) and when it's there (what the time on the clock is) are both right-now recordings of data.

So where something is and when it is there are both at an instant.

A record of a journey is a set of these pairs (d, clock time). These pairs are often referred to as events. A multiple exposure picture is a good depiction of such a sequence. Less valuable, but more accessible are successive frames on a movie clip of the motion.

You record a position and an instant to fix an event. A series of such recordings narrates the journey, as has become popular on websites where you're encouraged to share such stories by uploading your GPS tracks (which are just such a series of recordings of events).

Teacher: Let's look again at this motion on the movie clip. How long does each frame last?

Ed: A very short time – it depends on the video camera.

Teacher: What should we look for to get the best record?

Belinda: As short a time as possible.

Teacher: Good: as close to zero as possible.

Teacher: So when is the ball here [points]?

Carly: Just as the clock points to that time. Not before, not after.

Teacher: That's very clear. It's at that instant, and only that instant. At that clock time the positioninstantaneous is x. And it's only there right then, because the ball keeps moving.

appears in the relation F=ma a=dv/dt a=-(w^2)x
is used in analyses relating to Terminal Velocity
can be represented by Motion Graphs
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