Topic 1.5: Three Launches, Three Predictions (And Why the Last One Is the Hard One)
There's a big difference between solving a projectile motion problem on paper and actually predicting where a real object will land — before it launches. Working through the equations in advance and then testing the result against reality is one of the most honest ways to find out whether an understanding of physics actually works, or just looks right on paper. Today's lab does that three times in a row, each one a little harder than the last, using the same toy car and launcher for all three so the launch velocity itself can be tracked and refined along the way.
The first launch is the simplest: fire the car along a flat floor and time just the first half-meter of travel. That's not the same as the whole-run average measured back in Week 1 — it's a much better estimate of the car's true launch speed, since it's measured before rolling friction has had much chance to slow things down.
The second launch moves that same car to the edge of a table. Knowing the table's height, the vertical fall time can be calculated before the car ever leaves the launcher — height alone determines how long the fall takes, completely independent of how fast the car is moving forward (the same independence principle from Topic 1.5, Session 2). That means once the car actually lands, dividing the measured distance by that already-known time hands back an even more precise launch velocity than the flat-floor timing did — no rolling friction involved, just a fraction of a second in the air.
The third launch is the real test. The same car now launches from an angled, elevated stand — a known angle, but also a known height above the floor. Every other projectile problem this unit landed back at the same height it launched from, which let "time up equals time down" work as a shortcut. Not here. Since the landing point is lower than the launch point, that symmetry breaks completely, and the only way through is the same method used for a rooftop launch earlier in the unit: find the impact velocity first using the height and the vertical launch component, then use that to solve for time, and only then find the horizontal range. Three separate steps chained together, instead of one clean shortcut.
Videos
- Introduction to Projectile Motion — Flipping Physics (quick refresher on the vertical-first, horizontal-second strategy)
Try It Yourself
This was a lab and whole-unit review day, so instead of a written problem set, here are the same questions the class worked through with their own launch data:
- Why is timing just the first half-meter after launch a better estimate of launch speed than timing the whole rolling distance?
- For the table launch: once you know the height, why do you already know the fall time before the car ever leaves the launcher?
- For the angled elevated launch: why doesn't "time up equals time down" work here, when it worked for every other projectile problem this unit?
- Name one specific source of error that could make an object land short of its predicted distance, and explain the physical reason why.
- Looking back across this whole unit — distance and displacement, velocity, acceleration, motion graphs, reference frames, vectors, the kinematic equations, free fall, and projectile motion — pick the one idea you think is most important for understanding everything that came after it, and explain why.