Crater Box

Age

Elementary School, Middle School, High School

Format

Hands-on

Materials

   Sandbox
   Sand of varying grain size
   Steel balls of varying sizes 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Safety glasses to prevent sand being kicked into eyes. Don’t let the kids throw the balls into the sand, only drop!

Preparation

Sand needs to be filled in boxes that have high enough walls to keep in most sand and from being kicked out into eyes

Demonstration

Drop varying sizes of balls into sandboxes with grains of varying sizes. Demonstrate that impacts form craters because the material is pushed aside by the projectile, often times throwing material upwards/outwards. Show that as the height increases, the diameter and depth don’t increase nearly as much as the height. Explain that this is due to the sand acting as a solid at the moment of impact.

For the latter part of the demonstration, repeat the experiment in a vacuum to demonstrate that the lack of air causes a the craters to look different. (Further research is needed to explain why).

What to Say

Start by discussing the craters of the moon and asking if they know where they originate. Introduce the concept of meteor impacts in the early solar system and how the moon and planets would have been bombarded by meteors early in their history. If asked why there were many meteors explain that when the solar system was formed there was lots of debris left over that formed many meteors.

Show the different sized balls and different boxes of sand and explain what they’ll be doing. After the kids drop some of the balls and made craters, explain the concept of transfer of energy and how it relates to the demo. Explain how the craters get larger when the ball is dropped from a greater height and touch on the notion that the sand acts like a solid at the moment of impact which causes the ball to lose energy and how that causes the crater size and depth to be not directly proportional to each other (meaning that if the height is doubled, the crater size/depth is not doubled).

Explain that there is no air in space and perform the demonstration in the vacuum tube and note the difference in structure. This would simulate how impacts affect the moon as opposed to the earth.

A demo might go something like this:

“How many of you have ever looked at pictures of the moon and seen all those craters all over it?”(likely they have)

“Well, where do all of those come from?” (likely no one will know)

“Well, when the solar system was first forming, there was all kinds of left over stuff floating around in space. This stuff was a bunch of meteors, which are really just rocks floating in space. All these meteors were constantly smashing into the planets and the moon. What we’re going to do is make our own craters!” (kids oo-ing and ah-ing)

“What we have here are sandboxes with different kinds of sand from fairly normal sand to really fine grain sand (point out the boxes to show the difference) and a bunch of balls to drop into the sand to make craters! What I want you to do is to try dropping different sized balls into the sand from different heights and see what happens.” (after kids have dropped several balls proceed)

“So what’s happening is that when the ball hits the sand it pushes the sand away and a lot of times kicks it into the air. This is because of something called “transfer of energy”. The ball is transferring the energy from its fall into the sand and because the sand is loose, it gets pushes it aside before stopping.”

“You may have noticed that when you dropped the balls from a really high place, the crater got bigger, but not much bigger. It gets bigger because the ball has lots more energy when it hits the sand when it starts from a higher spot (demonstrate to be sure they see it) but, when the ball hits the sand it’s going so fast that the sand acts sort of like a solid and doesn’t let the ball push it away so some of the energy it got from falling is taken away. Since it doesn’t have as much energy; it doesn’t push as much sand.” (This is quite a lot to say at once, try to include some demonstration in the speaking to break up the monotony and hold attention. It may be opening a can of worms to talk about how it acts like a solid at impact in this demo and would be accompanied well by the Outrageous Ooze demonstration so they can see how this works for themselves)

The comparison can be made to water where it’s easy to place your hand in the water, but if you jump from a high place it can hurt (belly flops).

“So this works pretty well here on earth, but on the moon there’s almost no air. So now were going to try this out in a vacuum tube. A vacuum tube is just a tube that we sucked the air out of.” (do the demonstration and show how it looks different than when it’s dropped in air)

“This is a lot like what the moon is like.”

“So why don't we see any craters on the earth like we do on the moon? (random explanations) Well, the earth has a lot of geological activity. Geological activity is something like wind blowing things around, water washing things away, earthquakes, tornadoes, rain, snow and any sort of weather that can change the way the land looks. So on the earth, all this activity covers up the craters.” (Demonstrate this by shaking the box)

“The moon doesn't have this geological activity, so the craters will stay there for a loooonng looonng time.”

“Any questions?”

Why It Is

Notice that the proportionality is not a one to one correspondence (doubling the height does not double the crater size or the depth). This is because at the moment of impact, the granular medium “seizes up” and reacts similar to a solid causing energy to be lost.

The above equations state, in simpler terms, that to double the diameter of the crater, the height must be increased by a factor of sixteen and to double the depth of the crater, the height needs to be increased by a factor of eight!

Real Life Examples

Impact craters on the moon, walking in the sand on a beach, water splashes, anything being dropped.