# Craters in the classroom

Introduction

In this activity, objects of differing densities and sizes (marbles, ball bearings and golf balls) will be dropped from a known height onto a surface of flour and cocoa. Once dropped, the kinetic energy of these objects will blast a crater into the surface, sending out rays (ejecta rays) around the object. Students will note the shape/extent of these rays, and once the object is removed from the crater, they can also measure its diameter. Results of this investigation can be presented graphically or verbally, and conclusions drawn regarding the nature of impact craters on Earth. Any improvements that can be made on the experiment can then be discussed.

This activity is best done in groups of at least 3 students, one to drop the impact object, one to time, and one to collect the results. Students should be encouraged to discuss what they think are the main factors affecting the sizes of impact craters, and write down their predictions for any trends in their results i.e. larger impact objects will create larger craters etc.

Getting Started

1. Lay down the newspaper and put the saucepan/container in the middle.
2. Fill the container to a depth of about 10cm with the flour.
3. Sprinkle the surface of the flour with a thin layer of cocoa powder. Make sure it is evenly spread and flat.
4. Note what the test field looks like.
5. Measure the mass of each impact object and note its mass in kg (1g=0.001 kg or 1x10-3kg).
6. Measure the diameter of each impact object in metres.  This can be done most easily by holding up two rulers either side of the marble, and using a third ruler to measure the distance between them. Note the diameter and therefore, radius measurement in m.
7. Using the formula,

density= massvolume
calculate the density of each impact object used.

Method

1. Hold the impact object directly above the container of flour/cocoa. Measure the distance to the container. (Note: since the time taken for the impact object to hit the flour/cocoa is to be timed, this distance should be made as large as possible to minimise timing errors).
2. Drop the impact object from this height, starting the stopwatch as this is done. The stopwatch must be stopped once the impact object has hit the flour/cocoa.
3. Before removing the impact object from the container, look at the ejecta rays that have formed. Sketch them and make any comments regarding their shape/extent etc.
4. Remove the impact object and measure the crater diameter and ejecta ray diameter. Make a note of these values.
5. Flatten the flour/cocoa surface once more, and repeat the experiment twice more with the same impact object.
6. Using impact objects of different size/density (choose one or the other), repeat steps 1-5, noting the results throughout the investigation.

Results

The results can be noted down in table form, using the example below as a guide.

 Time taken (s) Average time taken (s) Crater diameter (m) Average crater diameter (m) Object 1 Radius ________m Density ________kg/m3 1st drop 2nd drop 3rd drop Object 2 Radius _______m Density _______kg/m3 1st drop 2nd drop 3rd drop Object 3 Radius _______m Density _______kg/m3 1st drop 2nd drop 3rd drop

1. Using the two equations below, calculate the K.E. of each impact object as it hit the surface of the flour/cocoa.

The kinetic energy (K.E) of each impact object dropped can be found using the formula:

K.E= 12 mv2

where m = mass of the object

v = velocity of the object

The velocity (speed in this case) of the object can be found by using:

speed= distancetime

2. Plot the results of the investigation on a scatter plot i.e. impact object density vs. crater diameter or impact object diameter vs. crater diameter.

Discussion

After carrying out the above activity, you can think about and discuss the following:

a) how did the size of the impact object affect the size of the crater? How did it affect the ejecta rays?
b) how did the density of the impact object affect the size of the crater? Did this affect the ejecta rays?
c) do the bigger craters have more rays around them?
d) how do the diameters of the craters compare to the diameters of the impact objects? Are they bigger/smaller/same size?
e) What happened to the cocoa as the impact object was dropped?
f) Was the flour visible at any time during the investigation i.e. in some impacts, or all impacts or none?
g) What does this investigation tell us about craters on the surfaces of planets?
h) How could this investigation be improved?
i) What were the main sources of error in the investigation? How can these be minimised?
j) Does K.E. affect the size of the craters made? If so, how?
k) Were the results as expected? Did they match any predictions you made prior to carrying out the investigation?

(Main image courtesy of McDonald Observatory - http://mcdonaldobservatory.org/astroday09/)

Objectives:
After carrying out this activity, students will be able to recognise and describe how impact craters are formed on Earth.

## Next:

Planning:

For this activity you will need the following apparatus:

• Marbles of different sizes
• Golf Balls
• Stainless steel ball bearings
• Saucepans/containers large enough for objects to be dropped in
• Cocoa
• Flour
• Rulers
• Newspaper (unless you are doing the activity outside)
• Electronic measuring scales
• Stopwatch