Mechanics: Mechanical Energy to Thermal Energy

PIRA Class: non-PIRA

Purpose:
To demonstrate the conversion of mechanical energy to thermal energy.

Description:
Have a volunteer hold a piece of paper while you slam the balls together so that they strike the paper from both sides. After the strike you will see a hole burned in the paper from the strike and may be able to smell singed paper.

NOTE: Point out that the balls are smooth spheres with no protrusions thus the hole in the paper must be due to combustion not an object puncturing the paper.

VARIATION: Replacing the paper with aluminum foil will result in a ripple-like pattern of concentric circles in the foil rather than a burnt hole.

From the reference below: "... frictional forces between paper and steel spheres and inside the paper, resulting from the paper expulsion from the area between the spheres, can increase local temperatures within the contact area to greater than 233 oC (451 oF), resulting in the auto-ignition point of paper."

The “ripples” in aluminum are not waves in liquified aluminum but are likely due to high strain plastic flow of aluminum out of the region of contact between the spheres. This outflow of aluminum pushes on surrounding aluminum resulting in buckling.

Equipment:
Mechanical Energy to Thermal Energy balls and a piece of paper, optional aluminum foil

Location:
Shelf 132, aluminum foil is in the cabinet across from PHYS-102 next to the drawer of tapes and glues

Extras:
Ripples in an Aluminum Pool?