Eleventh Grade · Science · week 7

Projectile Motion: Horizontal Launch

Analyze horizontally launched projectile motion by treating the horizontal and vertical directions as independent.

A projectile is any object moving through the air under gravity alone after being launched -- a thrown ball, a rolling marble that leaves a table edge, a bullet. The single most powerful idea in this unit is that horizontal motion and vertical motion happen completely independently of each other. Horizontally, with no horizontal forces acting (ignoring air resistance), velocity stays constant the whole flight. Vertically, the object is in ordinary free fall, accelerating at 9.8 m/s^2 downward, exactly like last week -- gravity does not know or care that the object is also moving sideways.

For an object launched purely horizontally (no initial vertical velocity), this independence means: the TIME spent in the air is decided completely by the vertical drop (how long free fall from that height takes), while the horizontal distance traveled (the range) is just that same time multiplied by the constant horizontal velocity. A ball rolling fast off a table lands at the same time as one that barely rolls off -- only how far it travels sideways changes.

📋 For the teacher
Teach it (10–15 min):
  • This week's signature lab: roll a marble or small ball off the edge of a table at a measured height, and measure how far it lands from the table's base. Before measuring, predict the landing distance using the fall height and an estimated launch speed, then compare prediction to reality.
  • Do the "two balls off the table" try-item as a live demonstration if you can manage releasing both at once (a ruler flicked to launch one while dropping the other from the same height works) -- hearing them land at the same instant is far more convincing than being told it.
  • Keep the two halves of the problem visually separate on paper -- a "vertical column" (height, g, time) and a "horizontal column" (velocity, time, range) -- so the independence of the two directions is a habit, not just a rule.
  • Point out that the ONLY quantity shared between the two columns is time -- it is the bridge that lets you solve the vertical part first, then hand that same t over to the horizontal part.
Talk about it:
  • Why does a projectile's horizontal speed have no effect on how long it stays in the air?
  • What determines the time of flight for a horizontally launched projectile?
  • If you doubled the horizontal launch speed but kept the same height, what would happen to the range? What would NOT change?

Mastery looks like: They correctly split a horizontal-launch projectile problem into an independent vertical (time-of-flight) part and horizontal (range) part, using the shared time to connect them.

Common stumbles: Trying to use a single kinematics equation that mixes horizontal and vertical quantities together; assuming a faster horizontal launch means a longer time in the air.

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