The man with the wire-frame glasses, the greying hair and the rumpled blue shirt makes for an unlikely superstar.

"Today we will talk exclusively about work and energy," he says, turning from his audience, chalk in hand, to spend the best part of an hour adorning a blackboard with mathematical symbols and equations.

But in the 47th minute of lecture 11 in the classical mechanics course at the renowned Massachusetts Institute of Technology, Professor Walter Lewin, a 71-year-old Dutch physics professor, does something that has brought him legions of fans across the world. He picks up a prop and carries out an experiment upon a willing subject: himself.

"I have here a pendulum," he declares, dangling a wrecking ball from a line. "I am such a strong believer in the cultivation of mechanical energy that I am willing to put my life on the line."

A murmur arises from the audience of students as Lewin stands against a wall with the ball touching his chin, ready to release it in the expectation that on its return it will not swing up any higher and break his nose.

Like any good showman, Lewin raises the stakes. "This is no joke. If I don't succeed ... this will be my last lecture."

He releases the ball. It swings back, perilously close to his chin, but does not rise any higher. The audience laughs and cheers.

"Physics works and I'm still alive," Lewin proclaims. "See you Wednesday."

Performances such as lesson 11 have helped turn Lewin into one of the digital age's more unpredictable phenomena: the internet celebrity. Since MIT put Lewin's lectures online he has topped the most downloaded list at iTunes U, an educational adjunct to the Apple digital music service, and has attracted fan mail from around the world through MIT's site.

But Lewin is not alone. Top of the academic pops this week on iTunes U is Berkeley professor Hubert Dreyfus's philosophy lecture What is Existentialism? Just below him is a contemplation of quantum mechanics by Leonard Susskind of Stanford University.

But Lewin is the true showman of the group. In one lecture he rides a tricycle propelled by a fire extinguisher to demonstrate how a rocket lifts off; in another he beats a student with cat fur to show the effects of electrostatics. One video has him dressed in a safari outfit as he fires a golf ball from a cannon at a stuffed monkey wearing a bulletproof vest. That was to demonstrate the trajectories of objects in freefall.

Such is Lewin's appeal, he is able to win hearts and minds in the most difficult battlefield of all.

"You are now my Scientific Father," wrote one online fan, a physics teacher from Iraq. "In spite of the bad occupation and war against my lovely IRAQ, you made me love USA because you are there and MIT is there."