Universal Bedsheet (PLUS)

Introduction
Public summary: 

Learn about the effects of gravity using a bed sheet?

See how your bed sheets have gravity.
Useful information
Kit List: 

Bed sheet
Clips for tension
Heavy Balls
Marbles

Packing Away: 

????
Bed sheet should live in misc probably and clips live in the tool box. Heavy balls...

Explanation
Explanation: 

Stretch the bed sheet between tables, you want a taught unsupported are in the middle. Using a few lengths of wood, two tables and several clips this is doable.

The sheet here models a 2D version of space, we get this from looking straight down from above the sheet. While it curves downwards in our view we can't see this from above (other than stretch marks in the sheet, we call this constriction). So when we put a mass (this is a planet) on the sheet we don't really see a change. What could we see on the sheet? Place a small ball bearing near by and it rolls towards it. From our 2D view the ball has moved for no reason. From everything else we know about movement there must be a force, we can't see this force but by experimenting it definitely exists. We call this force gravity.

One thing we look at in these situations is symmetry. Assuming these masses are all perfect spheres we'd expect the gravity around them to be the same in all directions, we call this spherically symmetric with no preferred direction. What if we spun the ball, could that have an effect? Who knows, but we should get symmetry around the axis it's spinning in.

You can also talk about the whole geometry of this situation, what is a straight line on the bed sheet? well that's interesting.

We can do some checks on this, one places super accurate clocks on different floors of sky scrapers, those further away from the earth experience fractionally more time.

You can also fling balls at different speeds and get an understanding of why we get orbits and collapse.

Black holes are commonly represented by singularities in this model, here the sheet has a really tiny, infinitely heavy mass on it causing anything near it to just fall into the hole.

There are issues with this demo, gravity is a curvature of space-time, which is a 4D thing, so we have two dimensions missing. It's also mainly a time dilation.

Risk Assessment
Risk Assessment: 

Gravity causes child to hit floor after tripping on dropped ball.

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