Free Fall Notes

Main ideas, concepts, and lessons

• Free fall definition

  • Free fall is motion influenced only by gravity (no other forces acting).
  • The course assumes no friction and no air resistance for most calculations.

• Assumption about air resistance

  • Air resistance is ignored in the standard free-fall problem set.
  • If a question includes air resistance:
    • Air resistance acts opposite the motion (it “resists” falling).
    • It causes a falling object to take longer than the ideal (no-air-resistance) prediction.
  • A video demonstration (NASA vacuum drop of a feather vs. a bowling ball) is mentioned to illustrate that without air, they fall similarly.

• Acceleration due to gravity

  • In free fall, there is a constant acceleration due to gravity:
    • (g = 9.80 \, \text{m/s}^2) (constant near Earth).
  • This constant is often written as “little g.”
  • The video notes unit expectations (meters per second squared; students may use feet per second squared in some engineering contexts).

• Terminal velocity concept (mentioned but not used in calculations)

  • Terminal velocity occurs when air resistance upward balances gravity downward, so acceleration becomes zero and speed becomes constant.
  • The class plans to ignore this effect for the problems shown.

• Key physics misconception addressed (tennis ball vs. ping-pong ball)

  • Students often think the heavier object falls faster.
  • Lesson: under the model of constant gravitational acceleration (ignoring air resistance), objects fall with the same acceleration regardless of mass, so they hit the ground at the same time in the ideal case.
  • Air resistance causes the real-world differences.

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Methodology / instructional framework (as presented)

1) Types of free-fall problems in the class

• Downward free fall

  • The object is dropped straight down (e.g., off a building).
  • Motion is only in one direction throughout the problem.

• Up-down free fall

  • The object is launched upward, reaches a maximum point, then falls back down.
  • Motion includes both upward and downward segments.

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2) Sign convention (direction matters)

• Velocity, acceleration, and displacement signs depend on the chosen positive direction.

• For both problem types:

  • Gravity acceleration points downward, so it can be positive or negative depending on the sign convention.

• Downward free fall

  • The instructor says you may often use all positive values:
    • Even though the real direction could be “down,” everything (velocity, acceleration, displacement) is in the same direction, so they can consistently be treated as positive relative to the chosen convention.

• Up-down free fall

  • The instructor sets:
    • Upward motion = positive
    • Downward motion = negative
  • Gravity acceleration (downward) is therefore negative in this convention.

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3) Applying the gravity sign in up-down free fall

• If upward is positive and downward is negative:
  • Acceleration due to gravity = (-9.80 \, \text{m/s}^2)

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4) Downward free fall: common parameter rules and pitfalls

• Initial velocity

  • If you drop the object from rest (no thrown velocity given):
    • (v_i = 0 \, \text{m/s})

• Final velocity

  • The instructor warns against a common mistake:
    • (v_f) is not zero just because the object started from rest.
    • (v_f = 0) only if the object stops in midair (not the typical case here).
  • The velocity just before hitting the ground will be a nonzero value (the ground would bring it to rest after impact).

• Height

  • Height given is treated as the distance from the starting point to the ground.
  • Because downward is consistent in direction for this problem type, signs can be handled consistently (often using all-positive values under the instructor’s approach).

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5) Up-down free fall: motion behavior along the path

• At the top of the motion

  • The object’s velocity becomes zero at the turning point:
    • It slows down while moving upward until (v = 0).

• Then it falls back down

  • It speeds up while moving downward.
  • Final velocity magnitude matches the initial velocity magnitude in the ideal case (same height, ignoring air resistance):
    • If launched upward with (+10 \, \text{m/s}), it returns downward with (-10 \, \text{m/s}).

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6) Time handling trick in up-down free fall

• At the top, you cannot use the total time for the whole trip if you are using conditions at the midpoint.
• If the problem gives total time in the air:
  • The top occurs at half the time.
  • Multiply by (1/2) when using top-of-trajectory conditions (like (v=0)).
• Conversely:
  • If using the negative velocity on the way down, time must be interpreted relative to the segment of motion being analyzed.

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Speakers / sources featured (as stated or identifiable)

• Speaker / instructor: Unnamed classroom instructor (the narration throughout).
• External source mentioned: NASA (vacuum/feather vs. bowling ball demonstration video).
• Video host mentioned: A British person (specific name not given).