Falling from Trees
Technical & Teaching Notes
Related sheets
Coconuts; Parachuting fruits; Gliding fruits; Spinning fruits; One-winged fruits; Two-winged fruits
Introduction and context
Seed dispersal is an important aspect of the reproduction of plants. If they are not suitably
dispersed, seeds will germinate and baby seedlings grow very close to their parent plant. This
means the seedlings will compete with one another and the parent plant for space, light, water
and nutrients.
Falling from trees is more ‘physics’ than ‘biology’, but it establishes some fundamental ideas that
are important when thinking about the different methods of wind dispersal that have evolved.
In this session students explore the concepts of gravity and air resistance in the context of falling
leaves and falling fruits from trees, and seed dispersal. They investigate the effects of the mass,
size and shape of a falling object. Having investigated the way that leaves and fruits fall from
trees, students use the particle model to explain it.
Possible barriers to learning
The session allows a number of common misconceptions associated gravity and objects falling to
Earth. These include:

gravity = ‘downness’;

when an object is stationary, no forces are acting on it;

heavy objects fall faster than light objects;

something stops moving because the force has run out;

air doesn’t weigh anything;

particles are the same as visible grains as in rocks, for example.
Links to National curriculum for science in England at key stage 3
In biology pupils should be taught about reproduction, which includes:
 reproduction in plants, including flower structure, wind and insect pollination, fertilisation,
seed and fruit formation and dispersal, including quantitative investigation of some
dispersal mechanisms
In physics pupils should be taught about forces, pressure in fluids and forcers and motion. This
includes:
 forces: with pushing things out of the way; resistance to motion of air and water;
 non-contact forces: gravity forces acting at a distance on Earth and in space;
 atmospheric pressure, decreases with increase of height as weight of air above
decreases with height;
 pressure in liquids, increasing with depth; upthrust effects, floating and sinking;
 pressure measured by ratio of force over area – acting normal to any surface;
 forces being needed to cause objects to stop or start moving, or to change their speed or
direction of motion (qualitative only).
Science & Plants for Schools: www.saps.org.uk
Falling From Trees: p. 1
Safety Notes
The only ‘danger’ may come from bouncing balls that get out of control.
Equipment and materials
Each student or group will need:
 a table tennis ball, squash ball and golf ball
 leaves from a tree (all of similar size to one another)
 acorns (or suitable substitute) – anything of a similar size and mass
 2 sheets of A4 paper
Teaching Notes
Time could be spent discussing Newton’s observations. The documented evidence says that he
did observe apples falling and, from that, got his early idea about gravity. There is no evidence,
however, that he was ever hit on the head by an apple.
Further, time could be usefully spent introducing (or reminding) students of the importance of
seed dispersal in the reproductive process.
The first two activities are about investigating factors that affect how objects fall. The speed of
descent depends not on mass, but upon air resistance.
It is unlikely that the experiments will be carried out in a wind. It would be useful to point this out
(or prompt students to identify this), since wind dispersal depends on fruits falling slowly enough
to be blown by the wind.
Activity 1: Does mass matter?
Students may have come across this idea, but the use of falling fruits would provide a novel
context for the investigation. The idea of a fair test is introduced and used to explain why the
three balls are used rather than fruit. Weighing and measuring the diameters of the balls can
introduce or reinforce measuring techniques and the idea of errors and uncertainties in
measurement.
If time is short, students can be given these masses and diameters for the three balls used: table
tennis: 3 g, 4 cm; squash: 24 g, 4 cm; golf 46 g, 4.1 cm.
Answers
1. The test is about the effect of mass, so all other factors that might affect the experiment must
be eliminated. Using spheres with the same dimensions eliminates any effects that might be due
to shape and size.
2. No. They all land at the same time. There are many online videos that can be used to reinforce
this.
3. Gravity. The balls are pulled towards Earth’s centre by its gravitational force. You could tell
students about the smaller gravitational pull on the Moon which means that things fall more slowly
to its surface.
Science & Plants for Schools: www.saps.org.uk
Falling From Trees: p. 2
Activity 2: Do size and shape matter?
Again, leaves and fruits provide an alternative and relevant context for investigating the effect of
air resistance.
Answers
1. The two leaves might have slightly differing messes. However, the mass of both sheets of A4
paper is the same, so this is eliminated as a factor affecting descent time.
2. The more tightly crumpled ball reaches the ground first. The less crumpled one have a shape
and size somewhere between the tightly crumpled sheet and a flat sheet.
3. Shape and size, but not mass, affect descent time.
Activity 3: Using the particle model to explain
This part is aimed at helping students to understand the concept of air resistance and to use this
to explain their observations in parts 1 and 2.
They are likely to have used a simple particle model to explain the difference between solids,
liquids and gases, and changes of state.
Help students to grasp the concept of a solid object falling to the ground though a gas (air) whose
particles are moving randomly and rapidly in space. You can do a simple calculation to show how
far apart particles are in a solid or liquid compared to a gas.
1 cm3 of ice melts to give 1 cm 3 of water. The particles have not changed and they must be as
close to one another in water as they were in ice – otherwise the volume would have changed.
1 cm3 of water boils to give 1000 cm 3 of water vapour (a gas). So the particles, which are the
same in all three states (ice, water and water vapour) must be much further apart – in fact the
distance between two water particles must be 10 times greater in water vapour than in water.
The opportunity can also be taken to explain particles in gases are called molecules and are
made from two or more atoms (except the noble gases – Group 0 of the Periodic Table).
Answers
1. Any suitable diagrams.
2. Particles in a solid are very close together. They jiggle around (vibrate) in their fixed positions,
but cannot move from place to place. Particles in a gas are not close together and they move
randomly (chaotically) and rapidly from place to place.
3. As a solid object falls it must push its way through particles of air. There is friction between the
object and the air particles, and frictional forces between air particles and the moving object. This
is called air resistance – the resistance of air to an object falling freely through it.
4. The crumpled up paper has a smaller surface area than a sheet of paper and experiences less
air resistance. The same is true for an acorn and an oak leaf.
Science & Plants for Schools: www.saps.org.uk
Falling From Trees: p. 3