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Science 7–10 · Year 10

Gravity acts on every mass alike: dropping a heavy and a light ball together

Physical sciences — Waves and motion: motion and Newton's laws (NSW Stage 5 focus area)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Objects of different mass fall with the same acceleration when air resistance is negligible, because the gravitational force on each is proportional to its mass, so by a = F / m every mass gains speed at the same rate.

What you need

  • golf ball, 1
  • table-tennis ball, 1
  • steel ball bearing 25 mm, 1
  • sheet of A4 paper, 2
  • tape measure 3 m, 1
  • smartphone with the phyphox app (acoustic stopwatch) or a slow-motion camera, 1
  • shallow cardboard box about 300 mm square and 100 mm deep with a plywood or hardboard sheet in its base, as the landing surface, 1
  • masking tape to mark a landing square on the floor, 1 roll

How to do it

  1. Predict which ball lands first when the golf ball and table-tennis ball are released together from 2.00 m. Write the prediction and the reason.
  2. Mark a landing square on the floor with the tape and stand the box in it. One learner holds both balls at exactly 2.00 m above the board in the box (measured with the tape) and releases them at the same instant; two others stand back from the square, watch, and listen for the two landings on the board. A foam mat absorbs the impact and silences the light ball, so the board is used instead. Repeat five times.
  3. Repeat with the steel ball and the golf ball, then the steel ball and a crumpled sheet of paper, then the steel ball and a flat sheet of paper.
  4. Time the steel ball's fall from 2.00 m with the phyphox Acoustic Stopwatch: rest the ball on a ruler held level at 2.00 m above the board and strike the ruler sharply sideways, so the strike is the first sound and the landing on the board is the second. Take five timings and average them. Slow-motion video beside the tape, counting frames at the frame rate the phone reports, is an alternative.
  5. Find the steel ball's acceleration from the average time with the Stage 5 formulas: average speed = 2.00 m / time; the speed rises steadily from zero, so on a velocity-time graph the distance fallen is the area of a triangle and the landing speed is twice the average speed; then acceleration = change in speed / time.

What you should see

The golf ball and table-tennis ball land almost together: without air resistance both take 0.639 s from 2.00 m, and the drag model below gives the table-tennis ball 0.669 s against the golf ball's 0.641 s, a lag of about 0.03 s. Two impacts 0.03 s apart are usually heard as a close double click rather than one sound, and the lag comes from air resistance on the much lighter ball, not from gravity. The steel ball and golf ball land together. The flat sheet of paper takes far longer and flutters, while the crumpled sheet lands close to the ball. The measured fall time of the steel ball is close to 0.639 s, which gives an average speed of 3.13 m/s, a landing speed of 6.26 m/s and an acceleration of 9.8 m/s^2, and the spread of the five timings shows the timing uncertainty.

What changes

What you change
mass and shape of the dropped object
What you measure
time to fall 2.00 m (s) and order of landing
What you keep the same
  • release height 2.00 m
  • simultaneous release from rest
  • same landing surface

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Heavier objects fall faster in proportion to their weight.
  • Falling objects move at a constant speed.
  • Air resistance is a property of the object rather than a force that depends on speed and shape.

Safety card

Low riskLearners carry it out

Hazards

  • steel ball landing on a foot
  • steel ball bouncing out of the landing box
  • learner standing on a chair to reach 2 m

Controls

  • drop into the box on the marked square, with everyone's feet outside the square
  • the box walls catch the bounce and one learner stops any ball that escapes
  • release from a step stool with a spotter, not a chair

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/faafd3c3df
  3. spark.iop.org/measurement-g-using-electronic-timer
  4. spark.iop.org/collections/acceleration-due-gravity
  5. phyphox.org/experiment/acoustic-stopwatch
  6. phyphox.org/wiki/index.php/Experiment:_Acoustic_Stopwatch

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