Machine Design

# The Dangers of Falling Objects

Falling objects injured three different people in three separate incidents I recently investigated. The common thread: unsecured objects above the victims’ heads.

As in any falling-object case, most of the investigation involves simple physics. Conservation of energy lets us predict a falling object’s impact velocity, kinetic energy, and impact force based on distance and weight.

There is much more room for error than in your high-school physics laboratory. Victims are often uncertain exactly where they were struck, and witnesses can contradict each other.

To accurately calculate impact force, we also need to know the stopping distance of the object — that is, how far it continued to travel, penetrating or deforming the point of impact. Less penetration indicates higher impact force. And bounce-back indicates the impact force was even greater, enough to change the object’s momentum.

In one accident I investigated, a person at a nightclub was struck by a fiberglass statue falling from the club’s upper level. The statue was one of several, none of which were anchored to their bases, and no one knew if someone upstairs bumped into the statue by accident, intentionally knocked it over, or threw it down.

Although the statue weighed only 34 pounds, it seriously injured his shoulder, hip, and thigh. It was difficult to reconstruct the accident because no one knew exactly where the victim stood. I calculated the impact force for each of the three different locations where the victim could have been.

My findings — along with the fact that the club did not have enough personnel to control rowdy drinkers, who were still being served despite their inebriation, or keep track of the statues — have a good chance of settling the case out of court.

In a more cut-and-dried case, a man getting a soft drink from a vending machine was leaning toward the machine when one of its doors came loose and hit him. Medical personnel found a substantial bump on his head. Witnesses agreed the victim did nothing that made the door fall.

I weighed the door, measured the distance between the man’s head and the door’s center of gravity, and calculated the impact force on his head from the door. Once again, the results made a settlement likely.

A third incident involved a woman sitting at a restaurant booth. A full bottle of champagne fell from a display shelf directly behind and above her and struck her in the head. She sustained a concussion that developed into serious brain problems. Again, I was able to calculate the force of the impact using simple physics.

Anyone installing a display or machine part above where a person might stand or sit should think through such calculations and secure any item that has the potential to fall or come loose from its perch. MD

Lanny Berke is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].

Edited by Jessica Shapiro