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All lessons Mechanics28 min

Projectile Motion

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← Free FallProjectile Launched from a Height →
01
Hook
02
Explore
03
Formalize
04
Practice
05
Challenge
Interactive simulation
01

Hook

A sniper fires a bullet perfectly horizontally off a clifftop. At the same instant, an identical bullet simply drops straight down from the same height. The fired one screams 800 m sideways; the dropped one lands at your feet. Which hits the ground first?

02

Explore

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03

Formalize

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04

Practice

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05

Challenge

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Spoilers

Projectile Motion — summary and key formula

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The question

A sniper fires a bullet perfectly horizontally off a clifftop. At the same instant, an identical bullet simply drops straight down from the same height. The fired one screams 800 m sideways; the dropped one lands at your feet. Which hits the ground first?

They land at exactly the same moment — horizontal speed does nothing to fight gravity. See why, and you understand everything from a thrown football to a spacecraft trajectory.

The key idea

In projectile motion, horizontal and vertical motions are completely independent. Gravity affects only the vertical motion; the horizontal velocity stays constant throughout (ignoring air resistance). Analyse each axis separately, then combine.

Both the fired and dropped bullets feel g=9.8 m/s2g = 9.8\,\text{m/s}^2g=9.8m/s2 downward — horizontal speed is irrelevant to gravity, so they land together. Horizontal motion is uniform (no horizontal force); vertical motion is uniformly accelerated (constant downward pull). The 45° launch maximises range because R=v02⋅sin(2θ)/gR = v_0^2\cdot sin(2\theta )/gR=v02​⋅sin(2θ)/g and sin(90∘)=1sin(90^\circ) = 1sin(90∘)=1, the largest value sine takes — exactly why the sim's range peaked at 45°. **Limiting case:** θ=90°\theta = 90°θ=90° is pure free fall up-and-down (range zero); θ=45°\theta = 45°θ=45° maximises range; complementary angles (30°/60°30°/60°30°/60°) share a range because sin⁡2θ\sin 2\thetasin2θ matches. **Connect it:** a projectile is two old laws glued at right angles — constant velocity sideways (no horizontal force, first law) and free fall vertically (second law) — running simultaneously and ignoring each other.

The formula

x=v0cos⁡θ⋅ty=v0sin⁡θ⋅t−12gt2x = v_0\cos\theta\cdot t \qquad y = v_0\sin\theta\cdot t - \tfrac{1}{2}gt^2x=v0​cosθ⋅ty=v0​sinθ⋅t−21​gt2
  • ·x = horizontal distance (m)
  • ·y = height above launch (m)
  • ·v₀ = launch speed (m/s)
  • ·θ = launch angle from the horizontal
  • ·t = time (s)
  • ·g = 9.8 m/s². The horizontal component v₀cos(θ) never changes; the vertical component starts at v₀sin(θ) and is steadily reduced by gravity.

Common mistake

Thinking the horizontal and vertical motions affect each other — they are INDEPENDENT: gravity changes only the vertical motion while the horizontal velocity stays constant.

What to remember

  • ·Split any projectile into independent horizontal and vertical motions.
  • ·Horizontal: constant velocity. Vertical: free fall under g.
  • ·The vertical motion sets the time of flight; range = horizontal speed × that time.