Table of Contents

Forces & Motion Banner

Spandex Gravity Table:

Einstein's general theory of relativity is one of humankind's greatest achievements, and the theory passed its first test in what is now part of Broome's lawn bowls club in far north-west Australia.

A Spandex & PVC pipe General relativity interactive at Instructables.

Fig 1. Spandex Gravity Table

Parts

PVC tube 12.5mm

Spandex on EBay

NOTE RE SPANDEX: A few common fibersn have different names in the ISO standard. For example, the name viscose for the predominant form of rayon, and elastane for spandex. You may use either name. Lycra is Dupont's trademark name. Spandex is the common US term, and Elastane is the term used in the rest of the world.

An 80/20 Nylon/Elastane mix is common in dance fabrics

Spandex Experiments

Bending Space-time - Cavendish Experiment:

Source: Most of the following is a redacted version of a project devised in 1997 by John Walker that is available in full here.

The content has been simplified to help make it more suitable for primary school students.

Introduction

We spend our entire lives under the influence of the Earth's gravity, yet rarely, if ever, do we experience the universal nature of gravitation.

Although we know that things fall towards the earth when dropped, it takes a tremendous leap in understanding to go from “stuff falls” to “everything in the universe attracts everything else”.

This page presents a “basement science” experiment which reveals the universality of gravitation by demonstrating the gravitational attraction between ordinary objects on the human scale.

The experiment deliberately uses only the crudest and most commonplace materials, permitting anybody who's so inclined to perform it.

The Theory

Einstein's 1915 theory of General Relativity explains gravitation as spacetime curvature created by matter and energy. This curvature causes objects to change direction and gives rise to what we commonly refer to as 'gravitational effects'

The principle of space-time curvature can be demonstrated by rolling objects around on a stretchable, trampoline-like surface.

This experiment goes one step further by demonstrating how every object in the universe attracts everything else - we're really bending space-time using real objects!

Our first problem is that the mass of any items we have around us is swamped by the mass of the earth. What we have to do is create a device that will cancel the Earth's gravity so that the much smaller gravitational forces between objects that we will use in our experiment become evident.

One of the great all-purpose sledgehammers in the toolbox of physicists and engineers is differential measurement: In other words, don't worry about the absolute value of something, but only the difference between things.

What we're looking for, then, is a device which responds only to differences in gravitational attraction, cancelling out the much stronger constant gravitational attraction of the Earth

We, however, aren't going to measure anything–we're only interested in observing universal gravitation. This allows simplifying the torsion balance to something we can set up in the basement.

The principle of the torsion balance is extremely simple. Suspend a horizontal balance arm from a vertical elastic fibre. At each end of the balance arm are masses, much denser than material of the arm, which respond to the gravitational force. Once the suspending fibre, balance arm, and weights are set up and brought into balance, the downward force of gravitation acts equally on every component. The balance arm is then free to rotate without any hindrance from the Earth's gravity. It is constrained only by air friction and the torsional strength of the support fibre–its resistance to being twisted. We can then place test masses near the ends of the balance arm and observe whether the gravitational attraction between them and the masses on the arm causes the balance arm to move. When measuring the gravitational constant one must precisely calibrate the torsional strength of the fibre, but to simply observe gravitation we need only make sure the fibre is sufficiently limp to allow the gravitational force to overcome its resistance to twisting.

Torsion Balance

In practice, the balance arm is so free to move that once any force sets it into motion, it oscillates for a long period, spinning round and round if free or bouncing back and forth off the stops if constrained. To avoid this we need to damp the system so kinetic energy acquired by the bar is more rapidly dissipated. Well, nothing's more damp than water, so we add a water brake to the arm which turns in a fixed reservoir. The resulting drag as the balance arm moves is much greater than air resistance and frictional losses in the fibre, and reduces the oscillations to a tolerable degree.

Gravitational Balance

An aluminium ladder serves as the support from which the balance arm is suspended. Nylon monofilament fishing line, as shown above, is knotted to the middle of the third cross-beam at the back of the ladder, one above the brace bearing the little white box, about which more later. Using a ladder or similar movable support frame allows setting up the balance in the middle of the room. This is important because we are bending spacetime in the basement, in this case an underground storage room at Fourmilab. Ground level is about even with the ceiling of this room, about 45 cm above the top of the ventilation window at the upper right of the picture. An underground room is ideal because it minimises temperature variations and vibration which might perturb the balance arm. Both walls shown in this picture are sunk into solid limestone rock–if you set up the balance near one of these walls, the gravitational field from all that rock will mask that of the test masses, and the balance will assume a “gravity gradient” position with one of the ends of the bar pointing toward the wall, and will budge only slightly under the influence of the test masses. With the bar in the middle of the room, the tidal influence of the mass of the wall and the rock behind it is reduced to a negligible value. The pipe on the wall at the right is part of the serpentine pressurised hot water heating system; it was disabled to prevent air currents from disrupting the balance arm. In fact, since the room is underground, the heating system is rarely engaged, and only in the depths of winter, never in June.

The Balance Arm and Cradle

Detail of balance arm The balance arm is a 5 × 5 × 30 cm bar of plastic foam, hacked from a 5 cm thick slab of packing material with a Swiss Army knife. The bar is suspended in a cradle made of insulated telephone wire. The bar is held in its cradle by friction and the indentation made in the soft plastic foam due to the weights at either end of the bar; it's easier to adjust the bar for proper alignment this way than if it were glued to the cradle.

The Support Fibre

The nylon monofilament that suspends the cradle is barely visible at the top of the picture–it is fastened by a knot to a loop formed into the cradle wires by twisting them. The monofilament is a very fine “six pound test” (about 3 kg capacity) fishing line manufactured in Japan; a 300 metre spool of it costs about US$9. The masses which cause the bar to turn when a gravitational force acts upon them are lead “sinkers” used by fishermen, each weighing 169 grams. Two are placed on each end of the balance beam, giving it a total weight of 676 grams. Be sure to place the weights on both ends of the beam simultaneously so it doesn't topple, then adjust the placement so the beam is horizontal. Nylon monofilament is very elastic: when you put the weights on the beam the support line will stretch and the beam will end up closer to the ground. You may have to adjust the attachment of the line to the ladder (or other support) or, as I did, twist the cradle wires to restore the beam to the desired height. Finally, when you first hang the beam, it may take some time to release stresses in the fibre remaining from the manufacturing process and from its having been rolled onto a spool. It's best to let the arm hang for a couple of days, free to turn, to allow these initial stresses to equalise before attempting any experiments with gravitation.

The Water Brake

Closeup of beam and water brake The height of the beam is important because of the need for it to fit properly with the water brake. If the beam is allowed to swing freely, it will be terribly underdamped–once it starts to swing, only air friction and the minuscule losses in the fibre will act to stop it. This causes the beam to bounce around incessantly, masking the steady influence of gravitation. The water brake dissipates the energy of these unwanted oscillations precisely as an automotive shock absorber does; the flap's motion does work on a viscous fluid, water in this case, and deposits its energy in heating it.

Closeup of water brake flap The water brake consists of a flap which projects downward from the balance arm (in this case, a piece of aluminium cut with scissors from the tray of a “heat and eat” meal, fixed with white glue into a slot cut into the bottom of the balance beam). The flap projects into a reservoir (a tuna fish can) filled with water. A more viscous fluid such as salad oil would provide greater damping and less bouncing than water, but I opted for water since it's less icky to clean up when the inevitable spill occurs and can be disposed of when the experiment's done without a visit to the village recycling barrel.

If I were rebuilding the balance beam, I would use a longer and narrower flap and/or a larger and deeper water reservoir. If the flap is only slightly smaller than the inside diameter of the reservoir, you have to be very careful that the flap and reservoir are centred on the beam. Otherwise, the flap will touch the edge of the reservoir and freeze the beam in place, as that frictional force is many orders of magnitude greater than the gravitational force we wish the beam to respond to. The water reservoir can be as large as you like, as long as it doesn't interfere with placing the test masses; the larger it is, the less you have to worry about its being precisely centred.

Test Masses and Supports

Blocks of plastic foam support the test masses so their centre of gravity is at the same height as the masses at the ends of the balance beam, maximising the attraction. The foam also keeps the balls from tending to roll away. The black rectangle, actually an inverted mouse pad, serves as a background for the time display superimposed by the video camera, rendering it more readable when images are reduced in scale so movies download more rapidly.

Use the densest objects you can obtain for the ends of the balance beam and as test masses: lead sinkers, steel balls, plutonium hemispheres, etc. Density is important because the gravitational force varies as the inverse square of the distance between the centres of mass of two objects. With a dense substance, the centre of mass is closer to the surface, so you can get the centres of mass closer together and enhance the gravitational force. For example, consider two pairs of one-kilogram spheres, the first made of lead (density 11.3 g/cm³), the second of pine wood (density about 0.43 g/cm³), placed so the surfaces of each pair of spheres are 1 cm apart. A one kilogram lead sphere has a radius of 2.76 cm, so the centres of mass are separated by 1+2.76×2, or 6.52 cm. A one kilogram sphere of pine has a radius of 8.22 cm, by comparison, so the centres of mass of the two pine spheres are 1+8.22×2 = 17.44 cm apart. Taking the square of the ratio of these distances shows that the gravitational force between the lead spheres is more than 7 times that of the pine spheres. Since attraction is linear by mass but inverse square in distance, you're better off with a modest mass of high-density material than a large mass of a substance with lesser density.

It's best to use a nonmagnetic material like lead for the weights on the ends of the balance arm. The forces we're working with are so small that if you use, for example, steel ball bearings on the arms, you may end up accidentally reinventing the compass instead of detecting the force of gravity.

webcam

Popping into the room where the experiment's running is a no-no–air currents from opening and closing the door, not to mention walking around in the room could seriously disrupt things. The BSR camera and accompanying 13 cm (diagonal) monitor allows keeping tabs on what's happening in a non-intrusive manner.

Gravity in action

The following time-lapse movies (about 30 seconds per frame) show the torsion balance responding to the gravitational field generated by two 740 gram competition pétanque balls. The picture at left shows the camera angle employed in both movies.

In each, the movie begins with the bar stationary, in contact with one of the balls or the foam supporting it. The balls are then shifted to the opposite corners, where they attract the lead weights on the ends of the bar. The bar then turns, slowly at first and then with increasing speed as it is accelerated by the gravitational force growing as the inverse square of the decreasing distance between the masses.

The bar bounces when it hits the stop on the other end, and finally, after a series of smaller and smaller bounces as the water brake dissipates its kinetic energy, comes to rest in contact with the closer ball or support. This is the lowest energy state, at which the bar will always arrive at the end of the experiment

Summary

Miscellaneous Gravity/Force Experiments

Water Weightlessness

Materials:

Procedure:

Explanation:

Source: http://www.metrofamilymagazine.com/July-2014/Simple-Science-Experiments-Gravity-Water-Drop/

Broom Balance

Have some challenging fun trying to make a broom stand up on its own. Steps

1. Find a smooth, flat hard floor.

2. Take the broom by the long handle and position it so that it is standing up right with the bristles on the floor.

3. Try to get the broom to stand all by itself by making slight movements back and forwards with the handle.

4. You may want to pick the broom up and place it back on the floor several times try to adjust the balance and weight equally.

This can take several attempts and patience. You may even want to try using a different broom if you find it a real struggle to get it to become balanced.

This is an amazing force of gravity experiment. I could not believe that a broom could actually stand up on its own. The trick is to get the balance just right, which is called the center of gravity, and the broom will stand by itself. I left one broom standing over night and came back the next morning to find that it was still standing.

Gravity Lift Experiment:

Spinning with Gravity -Materials you will need: • Small rubber ball • Flat table top surface • Medium size canning jar (wide mouth) or a jar with a lip/neck (large mayonnaise jar)

Steps

1. Place the ball on top of the table 2. Place the jar over the ball so that the ball is inside the mouth of the canning jar.

Now see if you can lift the ball up from the table top without touching the ball or tipping over the jar.

Were you able to do it?

Try steps 3 and 4 to see if you can lift the ball now:

3. Start spinning the jar around in a circular motion (keeping it on the table). 4. Once the ball starts spinning inside the jar lift it from the table top. 5. The ball is lifted from the table and will continue to spin inside the jar until it loses is speed.

This works because the ball spinning inside the jar is trying to escape but the jar itself forces the ball to stay inside the wall of the jar.

Due to the force of the spin or speed the ball will continue to spin until it loses its speed and gravity will pull it back to earth and the ball will fall from the jar.

Gravity Questions

If you don't know what you are talking about, then 
best not try to explain what you do not understand.

See image: http://www.diyphysics.com/2012/06/01/if-people-would-just-understand-what-they-say/

Weight Vs Inertia

Can you hammer a nail in zero gravity?

Objects with a large inertia are difficult to move and so although you can hold up a heavy object (such as a hammer) in the spacecraft, it may still take a large force to move it across the spacecraft.

You can use a hammer in the spacecraft but it will be just as difficult as it is on Earth. The lack of effective gravity there has no effect on the inertia of the hammer or the nail, or on the resistance of the wooden board.

http://www.schoolphysics.co.uk/age14-16/Mechanics/Forces%20in%20motion/text/Zero_gravity/index.html

General relativity is a consequence of both special relativity and the equivalence principle, which says that an object's gravitational mass and inertial mass are equal.

Shortest Distance

What is the shortest path between two points? I bet most of you said a line, and in a lot of circumstances you would be correct. The problem is that this is only true if you are using a flat space like a sheet of paper. When your space begins to curve, you need to become more creative. Let’s take the cities of New York and Tokyo as an example. The shortest distance between them would be a straight line going through the Earth, but that’s no help to planes that need to stay above the ground. So airlines need to figure out a more complex path to make the journey as efficient as possible. This path is called a great circle! For our New York to Tokyo flight you need to travel north almost past Alaska to travel on the great circle. Here is a fun site that you can use to map great circles connecting airports around the world: The Great Circle Mapper

Now you might think that outer space would be an escape from these silly curved geometries, but you would be very wrong. Einstein’s theory of General Relativity showed us that space is very far from flat. Any object with mass will warp space much like a weight will warp a trampoline. The heavier the object, the greater the warping. This is the basic principle of gravity! One of the interesting effects of this curving of space is that light will behave like our airplane and always follow the shortest path between two points, which often isn’t a line. That is what our vortex table is meant to show. The marbles are trying to go from one side of the metal ball to the other. If the ground was flat, they could simply go right next to it, but in our curved fabric space, the shortest path is a nice even circle several inches from the metal ball.

http://astrocampschool.org/tag/gravity/

Great Circle Mapper: http://www.greatcirclemapper.net/en/great-circle-mapper/route/RJAA-KJFK.html

So-called Gravity 'Facts'

SpaceFoundation - Gravity Facts:

Understanding Gravity - Assessment

The purpose of these assessment tasks is to motivate students to improve their understanding of gravity by helping identify concepts that are not well understood.

The Science Education Assessment Resources (SEAR) provide a wide range of assessment resources (or 'tasks') suitable for use across the compulsory years of schooling. The tasks have been indexed to six levels as described in the scientific literacy progress map.

SEAR also contains general information about science assessment in About assessment; and links to key teacher resources including the various State and Territory curriculum frameworks in Resource links.

The SEAR assessment tasks include items that can be used for diagnostic, formative and summative purposes are supported by rich marking keys and are linked to a scientific literacy progress map and scale that connects with the OECD PISA assessments for 15-year olds the national primary science assessments for Year 6 students

Finding an appropriate task

You can search for resources appropriate to your students in three ways:

About SEAR: http://cms.curriculum.edu.au/sear/newcms/view_page.asp?page_id=3526#whatis Assessment Tasks: http://cms.curriculum.edu.au/sear/newcms/view_page.asp?page_id=3526#tasks Search Tasks: http://cms.curriculum.edu.au/sear/newcms/view_page.asp?page_id=3526#find

Conceptual strand/context (EC), Scientific literacy level (5), Assessment purpose (diag), Task type (open), Learning outcome focus (concept)

Pre-test - Gravity Concepts:

  1. Earth – 4EB074 : The purpose of this task is to diagnose Year 5–10 students' misconceptions about the shape of the Earth, where we live on it and of objects falling down towards the centre of the Earth. This task requires students to select a diagram representing the true situation from six drawn by children.
  2. Gravity – 4EB076 : The purpose of this task is to diagnose Year 7–10 students’ misconceptions about gravity on planets . This task requires students to consider whether distance from the Sun and size of planet affect gravity on that planet.
  3. The Moon – 4EB077 : The purpose of this task is to diagnose Year 4–9 students’ misconceptions about gravity on the Moon. This task asks students whether an object will float, fall or rise when released by an astronaut on the Moon.
  4. Force – 4EC122 : The purpose of this task is to diagnose Year 7–10 students’ misconceptions about forces acting on a golf ball in flight.
  5. Pushing the car – 4EC124 : The purpose of this task is to diagnose Year 7–10 students’ misconceptions about forces acting on a car being pushed up a slope.
  6. Throwing a ball – 4EC125 : The purpose of this task is to diagnose Year 7–10 students’ misconceptions about forces acting on a ball that has been thrown up into the air.
  7. Pulley – 5EC123 : The purpose of this task is to diagnose Year 8–10 students’ misconceptions about forces acting in a pulley system.
  8. It's raining – 4EB075 : The purpose of this task is to diagnose Year 4–10 students’ misconceptions about the shape of the Earth, our position on Earth and objects falling to the centre of the Earth. This task requires students to select a diagram representing how they believe it to be, from five drawn by children.
  9. Gravity on other planets – 5EC120 : This open-ended task requires students to research the magnitude (size) of gravity on the surface of other planets. Students then use this information to compare some aspects of ‘living’ on other planets.

Video Tools:

Record My Desktop: http://recordmydesktop.sourceforge.net/about.php GUI back-end: GTK-RecordMyDesktop http://sourceforge.net/p/recordmydesktop/wiki/Home/

Desktop webcam thumbnail: http://guvcview.sourceforge.net/

Linux : https://www.youtube.com/watch?v=0AzMrvQgJ-g

A common destination for video tutorials is YouTube, and as explained by @JanetRichard in this article, and combined with what you have natively in Ubuntu, you might be deciding for these: 

  Video format/CODEC: MPEG-4
  Aspect ratio: 16:9
  Resolution: 1280 x 720 (16×9 HD)
  Audio CODEC: MP3 or OGA (from OGG)
  Audio bitrate: 44kHz
  Channels: 2

Citizen Science Australia