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,

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:

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:

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