Friction - "Marbles"

Standards -NSES Content Standard B -Physical Science: Motions and forces -Grades 9-12

Between any two charged particles, electric force is vastly greater than the gravitational force. Most observable forces such as those exerted by a coiled spring or friction may be traced to electric forces acting between atoms and molecules.

Misconceptions

• Friction is a force like gravity and magnetism

• Friction always hinders motion, thus reducing friction is always desired

• Sliding objects come to a stop because of an absence of applied force

• Friction cannot act in the direction of motion

• Only active agents exert force

• Motion implies active force while no motion implies no force2

• Mass makes things stop2

• Frictional forces are only due to irregularities in surfaces moving past one another

Magic

The second illusion is “Marbles” by Isaac Louie (Magicity.com, product number 11-1883R, $11.99). It is another card illusion with a different angle. A volunteer selects a card from a deck of cards and returns it to the top. The magician places the deck of cards on the table and gives a marble to the volunteer. The volunteer then rolls the marble at the deck as if they were bowling. When the marble hits the deck, the cards separate at the previously chosen card.

The illusion works because the magician has “dusted” the chosen card with a fine powder that reduces the friction between the cards. As the volunteers chosen card is returned to the deck (on top), the magician taps the top card with their finger proclaiming the deck to now be magic (and delivers a small amount of powder to the card at the same time). The cards are split, moving the “marked” card into the deck, and placed on the table. It is ready for the marble. Once they separate, the chosen card has magically appeared as the top card when the deck split.

It's not magic, it's science!

This "trick" focuses students' attention on the ideas that friction can be changed with changes in surface properties, sometimes reducing friction is good but not always, friction is not a constant, and that friction is produced only by objects moving past one another. Additionally, further discussion could be generated about Newton’s laws of motion, inertia, and energy (to name a few).