Safety and health newsletter for the Oregon construction industry

November 17, 2014

 

The terminal velocity of a tape measure and other facts about falling objects

terminal velocity calculation

Earlier this month, a New Jersey man was killed by a one-pound tape measure that a construction worker dropped from the top of a building 400 feet above him. Apparently, he had parked his vehicle at the site to deliver wallboard. The plummeting tape measure ricocheted off a piece of metal and struck him in the head as he was getting out of his vehicle. The blow knocked him unconscious and he died later at a hospital. He was not wearing a hard hat.

Falling objects are serious hazards at high-rise construction sites, which is why they must be secured when they are not used and why workers must wear hardhats.

How fast was that tape measure moving before it struck its unfortunate victim?

In a vacuum, objects accelerate to 9.8 meters per second per second thanks to gravity. But the tape measure was not falling in a vacuum. At a construction site (and the rest of the real world), the air resistance acting on a falling tape measure counters the force of gravity and its speed levels off. Exactly what that speed is – its terminal velocity – depends on the tape measure's shape, size, and mass.

Because we don't know what kind of tape measure was involved in the mishap, let's assume it was a typical 35-foot power lock tape measure with the following specs:

We also need to know the drag coefficient for the tape measure, the density of the air through which the tape measure was falling, and gravitational pull acting on it. That's not easy data to come by, so let's just estimate it.

Terminal velocity calculation graphic

Here is all the data we need to calculate the terminal velocity of a typical 35-foot tape measure:

Now all we have to do is plug those numbers into our handy terminal velocity calculator and we get a terminal velocity of 38.21 mph (or 56.05 ft/sec).

What has been falling at construction sites in Oregon

In 2013, 32 Oregon construction workers filed (accepted) disabling claims for injuries from falling parts, materials, tools, and equipment. Here is a summary:

Parts and materials:

Falling object Accepted disabling claims
Dimensional lumber 4
Structural metal materials 3
Concrete blocks, cinder blocks 2
Metal pipes, tubing 2
Plywood, wood paneling 2
Stone, marble, granite slabs 2
Wood, lumber 2
Wooden beams 2
Beams, unattached metal 1
Dies, molds, patterns 1
Drums, pulleys, sheaves 1
Metal plates, metal panels 1
Sheet flooring 1
Sheet metal 1
Structural stones or slabs 1
Wallboard, drywall, sheetrock 1

Tools and equipment

Falling object Accepted disabling claims
Box cutter 1
Non-powered saw 1
Crowbar 1
Caliper 1
Extension ladder 1
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