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
Equipment
Purpose
Resolution
Metre rule (or longer tape)
Mark out the drop heights
1 mm
Stopwatch
Time each fall
0.01 s
Falling object (paper cone or ball)
The motion to be investigated
—
Method
Mark a starting drop height of 0.5 m above the landing point using a metre rule clamped to a retort stand
Hold the object level with the mark, with the stopwatch ready in your other hand
Release the object from rest and start the stopwatch the instant it leaves your fingers
Stop the stopwatch as soon as the object touches the mat; record the time
Repeat the drop a further two times from the same height; take the mean of the three readings
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