Bucket Swing

Swinging Tray

Difficulty: 4 out of 10

Materials:

·        Pizza tray

·        Plastic cups filled with water

Procedure/ observation:

1.     Fill the cups with water and place them evenly on the tray

2.     carefully begin to swing the tray in a circular fashion. (Sway it left and right first and then while gaining speed make a full circle) Remarkably, the water is not spilled! As an aside, it is actually quite easy to swing the tray without spilling the water, the difficulty arises when the demonstrator tries to stop swinging the tray.

3.     BE VERY CAREFUL WHEN STOPPING THE SPINING

Explanation:

F_c = mv²/r, where F_c = centrifugal force, m = mass, v = speed, and r = radius.

ESIMATE:

cup has mass of 0.35 kg, 
radius of the circle is 0.35 meters
rotational speed is 6 meters per second

During this step, the water in the cup is experiencing a force downward due to gravity.
Fg= ma
F_g = 0.35 (kg) * 9.8 (m/s²) = 3.43 (N)

When the demonstrator swings the tray in a circle, there is still a force downward due to gravity. We calculated this force is 3.43 Newton’s. During this step, when the tray is upside down, the cup is no longer being supported by the tray. The water does not fall, however, because it is experiencing an upward force due to its circular motion. The centrifugal force is calculated (see calculation below) to be 6 Newton’s. This force is more than great enough to cancel out the force due to gravity. The fact that the centrifugal force is much greater than the force due to gravity is not surprising. This additional force creates tension on the string which is felt by the demonstrator and allows him to control the swinging motion.

F_c = {0.35 (kg) * {6.0 (m/s)}²}/ 0.35 (m) = 6 (N)

Standing on Light Bulbs

Standing on a light bulb

Difficulty: 8 out of a possible 10 advise that you explain it and not actually do it if your short on time!

Materials:

·        Triangular piece of wood designed with wires and inlets to allow 3 light bulbs to be screwed into the wood (equally apart).

·        An electrical cord allows the light bulbs to be plugged in.

·        The triangle is an equilateral triangle with sides of length 20 inches.

·        3 light bulbs (NOT fluorescent)

Procedure:

1.     The light bulbs are put into the piece of wood which is then turned upside down so that the wood is supported by the light bulbs

2.     The wires are connected to the light bulb and then the cord plugged into the outlet

3.     Once step two is done the light bulbs should turn on! (this adds a dramatic effect to the demonstration!)

4.     A demonstrator carefully places all their weight onto the piece of wood.

5.     If done correctly the light bulbs should be able to withstand the weight of the demonstrator.

Explanation:

The catenary shape of the light bulb allows it to withstand the force of gravity and weight that the demonstrator applies on it. Because of the reinforcing nature of this shape, it is very strong and can support much more weight than other round shapes. The exact physics behind the catenary are very complex, but the general idea is that each small portion of the shape is reinforcing the other portions.

Additional information:

Calculating the force applied on the light bulb:

Assume the demonstrators weight is 50 kg
Force of Gravity: 9.8

Fg= Force applied

Fg=ma

Fg= 50 X (9.8)

Fg= 490 N
Therefore the light bulbs are holding 490 Newton’s of force

12 Nails Balance on 1

Balance Nails

Difficulty: 2 out of a possible 10 very easy!

Materials:

• A block of wood with one nail already securely in place
• 12 identical nails with heads (the nails should be 10 penny size or larger)

Procedure:

1.     Putting one nail into the wooden block

2.     Place the wood block flat on a desk or table.

3.     None of the eleven nails should touch the wood block, the desk or table, or anything else that might help hold them up. No additional equipment other than the wood block and the nails may be used.

4.     Put on nail down horizontally, and line four facing toward you on one side and four facing away from you on the other.

5.     Put the reminder nail on top

6.     Slowly move the now touching nails on top of the one nail which is secured in the wooden block.

Observation/ Explanation:

If the demonstration went right, 10 nails would be balanced on one. The nails seem as though they are defying gravity but in reality their center of gravity is equal to the other side.

Gravity pulls an object down as if all of its weight were concentrated at one point called the "center of gravity." Objects fall over when their center of gravity is not supported. For symmetrical objects like a ball or a meter stick, the center of gravity is exactly in the middle of the object. For objects that are not symmetrical, like a baseball bat, the center of gravity is closer to the heavier end. The stability of the nails depends on their center of gravity being right at or directly below the point where they rest on the bottom nail. Add too many nails to the left or right and they become unstable and fall off. Since the nails have equal weight ratio on all sides the nails do not fall. All in all, due to center of gravity and equal forces acting on the nails causes the nails to appear as though they are defying gravity. (Amazing song my I add)