You’ve probably heard the story about Newton. It goes something along the lines: young Newton was sitting under an apple tree. An apple fell, hit him on the head and he hit upon the idea of gravity.
Flowers, cones and seeds
Many trees produce flowers. When a flower is fertilised, ovules in the pistil become seeds and the flower becomes a fruit in which the seeds contained. Some trees such as conifers produce cones and not flowers. Fertilisation happens when wind carries pollen from a male cone to a female cone. Ovules in the female cone become seeds. Hard scales protect the seeds.
Every seed has a protective coat inside which is an embryo and nutrients essential for its growth. When released from its fruit, a seed germinates (embryo starts to grow) and produces a seedling which grows into a mature plant.But how did Sir Isaac Newton tell the story? In 1726 he told hit story to William Stukely, who included it in his biography of Newton. Stukely wrote, "After dinner, the weather being warm, we went into the garden & drank thea [tea] under the shade of some apple tree; only he & myself. Amid other discourse, he told me, he was just in the same situation, as when formerly the notion of gravitation came into his mind. Why sh[oul]d that apple always descend perpendicularly to the ground, thought he to himself; occasion'd by the fall of an apple, as he sat in contemplative mood.”
So it seems the story is true, except that there is no mention of an apple landing on him. Newton was well known to be irritable and grumpy, so he would probably have said if it had happened.
Many fruits fall from trees straight down to the ground. Some are carried by the wind in various ways. Depending on their shapes, they may drift, flutter, glide, spin or rotate. To understand why fruits behave differently when they drop from the tree, there are three ideas to keep in mind:
The force of gravity pulls objects towards the centre of the Earth;
Anything dropping to the ground falls through air;
On a windy day the air is moving.
So to understand how things fall you need to know about gravity, air and the shape of objects that are falling. And that includes fruits and leaves from trees.
Fruits and the seeds in them spread away (disperse) in various ways.
Rolling: Some fruits fall straight to the ground, but roll away.
Animals: Some fruits are eaten by animals and their seeds dispersed in the animals’ droppings. Others hook to an animal’s fur or fleece and are knocked or pulled off later.
Wind: Some fruits dry out. They fall off, but not straight to the ground. Instead they are carried away by the wind.
Water: Some fruits float and are carried in streams and oceans.
Activity 1: Does mass matter?
Nuts are fruits
Sounds odd? Perhaps, but it’s true. Nuts are simply fruits that have developed a very hard shell.Fruits such as apples, oranges, coconuts and acorns fall from trees straight to the ground.
Do heavy fruits fall faster than lighter ones? A group of students decided to investigate.
To make it a fair test they agreed that fruits with the same size and shape, but different masses should be used. But these are not easy to find.
Mass and weight
Weight is gravitational force between an object and the Earth. The more mass an object has the greater its weight. An object with mass 1 kilogram has a weight of 10 newtons.
So instead they decided to model fruits falling using different balls. They chose a table tennis ball, a squash ball and a golf ball. All have the same diameter, but a different mass.
Carry out your own investigation using a table tennis ball, a squash ball and a golf ball.
1. Measure and record the mass (in grams) and diameter (in centimetres) of each ball.
2. Drop them from the same height. Do the same from a greater height and then again from an even greater height. In each case, record what happened.
1. Explain why using the three balls rather than three similar sized fruits made it a fairer test.
2. How does the mass of an object affect how long it takes to reach the ground?
3. Why do the balls fall to the ground when they are dropped?
Activity 2: Do size and shape matter?
The students concluded that mass did not affect how quickly objects fall. Do you agree with them?
Next they wondered about size. They dropped a football and a tennis ball from the same height. Again, both reached the ground at the same time. It seemed that size didn’t matter either.
Try it yourself.
But what about objects that have very different sizes and shapes? There was an oak tree in the students’ neighbourhood and they collected some leaves and acorns. Just by holding them, the students could tell that an acorn had more mass than an oak leaf.
They experimented and found that an oak leaf fell more slowly than an acorn. The acorn has a greater mass than the leaf, but the students had already found that mass does not matter. One of them noticed that while the acorn fell in a straight line to the ground the oak leaf fluttered down.
Try it yourself.
They began to wonder if their previous idea that mass doesn’t matter had been wrong. Was it the greater mass of the acorn that made it fall more quickly than the oak leaf?
Then they hit upon an idea. They found two oak leaves of the same size and dropped them from the same height. Both fluttered to the ground at the same time. Now they scrunched up one leaf into a ball and repeated the experiment by dropping the two leaves – one un-scrunched and one scrunched up – at the same time. The scrunched up leaf reached the ground first. One student also noticed that the scrunched up leaf seemed to slow down and flutter as it began to ‘un-scrunch’.
Try it yourself. You could use any pair of leaves provided that have near enough the same size and shape. You could also try using two sheets of A4 paper. Crumple one sheet into a tight ball and compare it with the un-crumpled sheet.
How can we explain this behaviour? Well, you need to think about air. After all, the ball, leaves and pieces of paper do not fall through empty space. They fall through air.
1. Why might comparing a sheet of A4 paper with a crumpled up ball of A4 paper be a fairer test than comparing an un-scrunched leaf with a scrunched up leaf?
2. One sheet of A4 paper was crumpled one up very tightly into a small ball. Another sheet was also crumpled but less tightly to bigger ‘ball’. What would happen if both balls of paper were dropped from the same height at the same time?
3. What have you learned about factors that affect how quickly things falling from trees?
Activity 3: Using the particle model to explain
You should be familiar with the particle model of solids, liquids and gases. It can be used to explain how object fall through air.
Air is a gas. It’s a mixture of nitrogen and oxygen, with much smaller quantities of other substances. The particles are nitrogen molecules, each made from two nitrogen atoms, and oxygen molecules, each made from two oxygen atoms. We can represent these particles as N2 and O2.
Paper, oak leaves and acorns are solids. They are mixtures of many different compounds. The particles of these compounds are made from atoms of carbon, hydrogen and oxygen. In some of the compounds atoms of other elements are also used, but in very much smaller quantities.
1. Sketch two diagrams to show the arrangement the particles (a) in air, (b) in a solid.
2. Describe the how the movement of particles in a solid differs from particles in a gas.
Air resistance is caused by the frictional forces of air against a moving object. The faster the object moves the greater the air resistance.
3. Use the particle model of matter to describe what happens when a solid object falls through air.
Hint: Look at the description of air resistance. What are ‘the frictional forces’?
4. Explain why a crumpled sheet of paper falls faster than sheet of paper and why an acorn falls faster than an oak leaf.