WEST LAFAYETTE, Ind. (AP) _ Nearly half the physics professors at Purdue University have pledged not to accept money for research on President Reagan's ''Star Wars'' defense proposal.

In May, the Pentagon announced that Purdue was one of five institutions selected to share $4 million to study electronic systems for Star Wars.

But university president Steven C. Beering said Wednesday that the school was not engaged in any Star Wars research, although some of its work may be applicable to the technology.

The 29 professors who signed the pledge believe the plan, formally known as the Strategic Defense Initiative, is ''a system that won't work,'' Louis A.P. Balazs, one of three faculty members who circulated the pledge, said Wednesday.

''Some believe it is a dangerous thing and increases the possibility of a nuclear war while weakening the nation,'' Balazs said.

In addition, the plan to shoot down missiles from space would be very expensive and would divert funds from research on conventional weapons, he said.

Beering called the pledge ''a symbol promoted as a matter of world peace.''