The Physics of Coconuts

Students’ Sheet

Coconuts are the fruit of the cocoa palm, Cocos nucifera. The coconut that we buy is the “kernel” of the fruit – the coconut that falls from the tree is much larger than this because it consists of the “kernel” surrounded by a thick fibrous layer inside a waterproof outer case.

These seeds have evolved to be spread by water although now they are usually spread by people. A coconut can move up to 10m from its parent tree just by bouncing and rolling after falling to the ground from a height of 10 to 20m. Coconuts can also survive up to 120 days afloat in the sea and then germinate when they are washed up on land. After germination the milky water inside the seed will provide enough moisture to sustain the seedling, even in sandy conditions.

The coconut has a mass of 285g, but the whole fruit is 1.4kg

European sailors who discovered this fruit in the 16th century noticed that it appeared to have a face so they named it coconut after the Spanish word coco meaning “a grinning face”. When the coconut germinates the root will push out through one of three dark patches.

Falling coconuts are a hazard to people in tropical areas and there are rare cases of people being killed by falling coconuts.

In this activity you will be investigating the physics of floating and falling coconuts, using the coconut “kernel”.

Activity 1 – The floating coconut

1. Measure the mass of a coconut.

Mass of coconut = ………………………………g

2. Measure the volume of your coconut.

One way to do this is to fill a jug of water to the brim and to place a large beaker underneath the spout. Carefully place the coconut in the jug until it is just submerged. The volume of water that has been displaced is the volume of the coconut.

Volume of coconut = …………………………………cm3

3. Calculate the density of the coconut using the equation

density = mass/volume

Density of coconut =………………………………………………g/cm3

4. Will it float in fresh water? Is there a way to decide this without putting the coconut in water? (Hint: the density of pure water is 1.0g/cm3)

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………….

5. Would the coconut float in salt water?

……………………………………………………………………………………

Extension:

Explain any differences in how the coconut would float in salt water compared to fresh water.

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

Activity 2 – The falling coconut

Imagine a coconut falling 20m from a tree onto a beach. Assume the mass of the coconut is 1.5kg

1. What force pulls the coconut towards the ground? ......

2. Calculate the potential energy of the coconut in the tree before it falls.

……………………………………………………………………………………………………………………………………………………………………………………

3. What is the kinetic energy of the coconut just before it hits the ground?

4. Calculate the velocity of the coconut as it hits the ground.

…………………………………………………………………………………………………………………………………………………………………………………….

5. Calculate the momentum of the coconut just before impact.

Extension:

Calculate the time taken for a coconut to fall from a tree 20m high.

Sketch a velocity-time graph for the falling coconut

Practical or Demonstration

Try dropping the coconut from a suitable height onto a tray of sand

(SAFETY - make sure that no one is in the vicinity).

Now drop the coconut onto a hard surface – does it survive the impact?

6. Explain why the coconut is more likely to break open when landing on a hard surface than sand, referring to momentum change in your answer.

Equations

Potential energy = mass x g x height

Kinetic energy = ½ x mass x velocity2

Momentum = mass x velocity

Acceleration = change in velocity / time

Acceleration due to gravity, g = 10 m/s2

Questions

1. Why has the seed evolved to have a waterproof outer casing?

2. Why do you think there is a thick layer of fibres between the outer casing and the kernel?

3. Why has the coconut evolved an air space inside the kernel?

Science & Plants for Schools:

The Physics of Coconuts – student notes: p. 1

This document may be photocopied for educational use in any institution taking part in the SAPS programme.

It may not be photocopied for any other purpose. Revised 2012.