4PH1

Movement & Position

Forces & Motion · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 12% of your exam marks.

stable
High
Stable12%

Distance-time graphs, speed calculations and velocity appear in nearly every series.

Aim

  • Measure the of an everyday falling object by timing how long it takes to fall through a known distance
  • Suitable objects: a paper cone, a ping-pong ball, a small ball of cotton wool. The object must be light enough that air resistance keeps the speed slow enough to time by hand

Variables

  • Independent variable: drop height, d (in m)
  • Dependent variable: time taken to fall, t (in s)
  • Control variables: the same object used in every drop, released from rest, dropped from the same starting position with the room's air kept still

Apparatus

EquipmentPurposeResolution
Metre rule (or longer tape)Mark out the drop heights1 mm
StopwatchTime each fall0.01 s
Falling object (paper cone or ball)The motion to be investigated—
Apparatus for investigating motion: a hand releases a paper cone from a marked height while another hand holds a vertical tape measure beside it, a stop clock reading 00:00 sits below, and a double-headed arrow labelled "distance, d" marks the drop height
Source: Core Practical: Investigating Motion by Save My Exams

Method

  1. Mark a starting drop height of 0.5 m above the landing point using a metre rule clamped to a retort stand
  2. Hold the object level with the mark, with the stopwatch ready in your other hand
  3. Release the object from rest and start the stopwatch the instant it leaves your fingers
  4. Stop the stopwatch as soon as the object touches the mat; record the time
  5. Repeat the drop a further two times from the same height; take the mean of the three readings
  6. Repeat steps 1–5 from heights of 0.9 m, 1.3 m, 1.7 m and 2.0 m

Analysis

  • For each drop height, calculate the average from:

average speed = drop height / mean time

  • A taller drop gives a longer fall time and a larger average speed, because gravity has more distance over which to speed the object up
  • Plotting average speed against drop height shows whether the relationship is linear or curved within the experimental range

Sources of error

  • Systematic error, reaction time. A human takes roughly 0.25 s to start the stopwatch and another 0.25 s to stop it; that is a sizeable fraction of a 0.5 s fall. Reduce this either by using longer drops (so reaction time is a smaller percentage of the total) or by replacing the stopwatch with light gates wired to a trapdoor release, which starts and stops the timer automatically
  • Systematic error, parallax. When you set each drop height, line your eye up squarely with the mark so the reading is not skewed by viewing the scale from above or below
  • Random error, air movement. Even a small draught will deflect a paper cone or cotton ball mid-fall; close doors and windows and ensure nobody is walking past during a drop

Safety

  • Place a soft mat or padded tray at the landing point so the falling object does not bounce, break or roll into walkways