Projectile symmetry

The flight is symmetric: equal heights have equal speeds; the path is a mirror around the peak.

Projectile symmetry0 min · Free lecture

In this lesson

  • The flight is symmetric: equal heights have equal speeds; the path is a mirror around the peak.

What goes up must come down — at the same speeds, same heights, same times.

Around the peak, t and v_y mirror: same height ⇒ |v_y| equal, vₓ equal ⇒ same speed. Time up to a height equals time down from it.

Use symmetry to halve the work: compute up to the peak, double for the total, mirror for the descent.

vup(y)=vdown(y)v_{up}(y) = v_{down}(y)

Speed symmetry at equal heights

Worked example

For a fixed launch speed, the horizontal range of a projectile is maximum when the launch angle is:

  1. R = v0² sin 2θ / g
  2. R is maximum when sin 2θ = 1, i.e. 2θ = 90°, so θ = 45°

Answer: 45°

Transcript (0 min)
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Projectile symmetry — FemtoLearn.
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The flight is symmetric: equal heights have equal speeds; the path is a mirror around the peak.
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Practice

Free · 4 questions with full solutions
  1. Q1 · Numerical · difficulty 2/5

    A projectile is launched with speed 15 m/s at an angle 70° above the horizontal. Taking g = 10 m/s², find its horizontal range.

    R=v02sin⁡2θgR = \frac{v_0^2 \sin 2\theta}{g}
  2. Q2 · Numerical · difficulty 2/5

    A projectile is launched with speed 15 m/s at an angle 25°. Taking g = 10 m/s², find its maximum height above the launch point.

    H=v02sin⁡2θ2gH = \frac{v_0^2 \sin^2\theta}{2g}
  3. Q3 · Numerical · difficulty 2/5

    A projectile is launched with speed 25 m/s at an angle 15°. Taking g = 9.8 m/s², find its total time of flight.

  4. Q4 · MCQ · difficulty 1/5

    For a fixed launch speed, the horizontal range of a projectile is maximum when the launch angle is:

    • 30°
    • 45°
    • 60°
    • 90°