## Wednesday, April 16, 2008

### Some fun...some science toys

Smart Camel...
or Just Physics?

Remember this toy...it will move towards the edge of a table and stop--why? Force vectors.

Here's the story...
Here is a mechanical toy that knows when to stop marching as it approaches the edge of a table. How does it know? This is a good illustration of force vectors. An explanation was also given by Heidi Strahm Black in the Physics Teacher, Vol. 36, Sept. 1998 issue, p.375. This type of toy can be purchased in many toy shops, for instance, the American Science Surplus at www.sciplus.com for about \$2.50 for a set of four. Some care should be given in the initial setup. The string that pulls the animal toy should be straight ahead. Because the friction depends on the surface, the weight which pulls the string has to be adjusted first to start the marching and then to get the intended effect that the toy will stop at the very edge of the table.

Physics Explanation

Tsing Bardin...

The force that is exerted on the toy is the tension of the string, which makes an angle to the horizontal direction.

Only the horizontal component of this force is driving the horizontal motion. The vertical component actually adds to the weight of the toy, and therefore increases the friction force. If the horizontal pull equals the friction force, then the toy has zero net force, and it will move at a constant velocity. However, since the angle of the string becomes steeper and steeper as the toy marches closer to the edge of the table top, the horizontal pull decreases but the vertical component of the force as well as the frictional force increases. The toy will therefore decrease its velocity until at the edge of the table the force is vertical , and there is no horizontal component to pull on the toy. The toy will then stop if its kinetic energy is small enough to be dissipated as work against the friction. The actual calculation is complicated, particularly with uncertainties in the response time of the toy and in the friction of the table edge against the string, and requires calculus as both the friction and the horizontal pulling force vary with the angle of the string. In practice you may have to try several surfaces or to modify the weight which pulls on the string. The minimum force required to get the toy started is simply calculated by equating the horizontal component of the force with the static friction. The string makes an angle A with the horizontal direction. We can estimate the friction coefficient of the surface by putting the toy without the string on an inclined plane with the same surface material. Tilt the surface, say with an angle B, until the toy starts to move.

From figure 2. the coefficient of friction = tan B. The minimum force F is calculated as follows:
F cos A = coefficient of friction (F sin A + weight of the toy) = tan B (F sin A + weight of the toy). Where A is the angle between the string and the horizontal direction and B is the angle of the slope.

Beats me. Rub a simple stick on the notches and the propellor will rotate...reverse the procedure and the propellor will change directions.

If you know what makes it function...write.

This isn't quite a toy but nevertheless a fascinating instrument illustrating Michael Faraday's law of induction and providing unlimited light merely by shaking the instrument. No batteries; no batteries to recharge. The item is available in just about any general retail market. The fundamental idea is quite simple: The flashlight uses the "Faraday Principle" which states that when an electric conductor is moved through a magnetic field an electric current will flow in the conductor. The flashlight will then store the charge in a capacitor where the energy is used to illuminate a bright LED light.

In 1820 Oersted discovered that a current traveling along a conductor has a magnetic effect on its surroundings. This connection between electricity and magnetism interested scientists of the day and after many failures Michael Faraday made the great discovery of Electromagnetic Induction on August 29 1831, in which he obtained an electric current from a magnetic field.

The main ingredients of the Faraday Principle are shown above in a diagram representing a set-up to illustrate the basic principle. Let's take a coil with a large number of turns, and connect it to a sensitive meter called a galvanometer. If you move the permanent magnet towards the coil the needle on the meter will move in one direction indicating an electric current. Now if the magnet is moved away from the coil the needle deflects in the opposite direction than before. If no motion of the magnet is present, the needle returns to the original position indicating no induced current. The faster the movement, the greater the voltage induced in the coil system.

The exact same Faraday principle is adopted in the probes, the coil is replaced by water (also a conductor), the induced voltage appearing across the two electrodes. As discussed above, the faster the velocity, the greater the induced voltage, easy!

Of Frogmen and Submarines...
of Cabbages and Kings

Dive, Dive, Dive

Nearly fifty years ago these toys were extremely popular...and are still available today. Back then they could be purchased for 25 cents and a boxtop from a Kellog's cereal. Benjamin & Henry Hirsch/Hirsch labs, a small cosmetic manufacturer, was the supplier for the frogmen and submarine [U.S.S. Nautilus]. Tons of money was made and tons of cereal was sold. What is involved here is a cool blend of chemistry and physics. The chemistry part involves "baking powder" [sodium bicarbonate (NaHCO3)]. But there is more than just the sodium bicarbonate: Cream of tartar [potassium bitartrate KC4H5O6] which is the acidic part that reacts with the alkaline sodium bicarbonate releasing the gas carbon dioxide [CO2]. The carbon dioxide is released and the phenomena turns into the physics of buoyancy. Often the substitute for for potassium bitartrate is a combination of sodium aluminum phosphate [Na3Al2H15(PO4)8] and calcium dihydrogen phosphate [Ca(H2PO4)2]. Using the submarine as an example there is an area inside that holds baking powder. This area closes up completely except for a small hole or series of holes on the bottom. When placed into water, the submarine sinks. After several seconds the baking powder reacts with water to produce carbon dioxide bubbles. As a result, a small air bubble is formed underneath the submarine which gives it just enough buoyancy to rise to the top of the water level. Once the submarine hits the top of the water level, the submarine tilts to the side and the carbon dioxide bubble is released from underneath the submarine--the sub sinks back down. As it goes down it continues to produce more carbon dioxide and once again the submarine rises.

Buoyancy is the upward force on an object produced by the surrounding fluid in which it is fully or partially immersed, due to the pressure difference of the fluid between the top and bottom of the object. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least to seem lighter. Buoyancy acts against the force of gravity and so makes objects seem lighter with respect to gravity.

PUTT, PUTT, PUTT, PUTT

I remember them. Mine was home made from a thin copper sheet resembling a crude boat or tub. Dad substituted the candle with a different flame source: A screw cap cylinder with a small hole in the lid in which was placed a small quantity of water and a chunk of "Carbide" [carbon disulfide]--acetylene. PUTT, PUTT, PUTT, PUTT for sure. Wish I still had it.

"... celebrating the joys of the pop-pop boat"

"Propulsion of the Putt-Putt Boat – I"