It's not often that pre-University syllabuses explicitly expect pupils to show that any object moving freely in two dimensions under the influence of a single constant force moves in a parabolic trajectory, but they should be able to follow a proof somewhat like this one:

Projectile derivation.pdf

Once you've done this, it's lovely to be able to give them some reasonable evidence that this works in practice. For example, you could throw a tennis ball through the air, capture the motion on camera and then analyse the motion, but it's quite time-consuming and could seem rather abstract for many pupils. For a quicker solution, try this:

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First, print out some simple parabolas onto suitable transparent acetate - hopefully you'll still have some lying around from the days when overhead projectors were used more! This Excel file might save you some effort:

Inverted parabolas for projectile motion.xlsx

Then, connect a rubber tube (spare Bunsen burner tubing works nicely) to a lab water tap and lightly clamp it to produce a nice curved stream through the air, like this:

The water pressure in the taps I use is rather weedy - if yours is better, this will look rather more convincing! If you want an excuse to go al fresco, you could easily do this outdoors with a hosepipe.

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Now, get your pupils to hold up their transparent printed parabolas and adjust the stream's path until it matches one of the parabolas, like this:

Further adjustment of the water flow, angle, or position of the acetate should make it possible to match each of the other parabolas in turn.