Skip to main content

Ask a Physicist: Workouts and Wingspan

Do long arms give you a disadvantage at the gym?

Bernie, from San Francisco, wrote in yesterday:
My co-worker and I are having an discussion about weight lifting. The basic question is: does it require more force (strength) to bench press a given mass if your arms are longer? Holding everything else is the same does the increase in distance (longer arms) give that person a disadvantage over a person with shorter arms?
Bernie,
Thanks for writing in! Being a stereotypically scrawny physics-type, I wouldn't have thought of the question, but it's a great one. The short answer to your question is "yes", although there's a bit of terminology we ought to get straight here—the force exerted by each person would be the same, but the work would be different.

In physics, force is defined by Newton's second law: mass multiplied by acceleration, or F = m x a. The same way that the unit of distance is a meter and the unit of time is seconds, the standard unit of force is Newtons. And since Earth's gravity pulls mass down with a constant acceleration (g = 9.81 m/s^2), we can describe an object's weight in Newtons as well—the object's mass times Earth's gravity. A one kilogram object exerts a downward force of 9.81 Newtons, and requires that same force to be lifted. No matter how far you're lifting it, you're still applying 9.81 Newtons of force—even if you're just holding the object steady at a certain height.

Work, on the other hand, is defined as the integral of force with respect to distance. It's important to note that holding an object at a certain height and then moving horizontally with it isn't work in the physics sense—it's only work if you're changing the object's potential energy, i.e. lifting it up against Earth's gravity.

That means that the amount of work done, i.e. the potential energy added to the weights you're bench-pressing, is proportional to your arm's length. A quick measurement revealed my arms to be 63 centimeters long, from armpit to the center of a closed fist, while the arms of my colleague James (who happens to be able to bench 300+ lbs) are 56 centimeters long. That means that, for each rep, James would only be doing about 89% the work I'd do, if we're each lifting to full extension.

"Proportionally," said James, "that means you should be able to bench, what, 270 lbs?"

Yeah, right.

Stephen Skolnick

Comments

Popular Posts

How 4,000 Physicists Gave a Vegas Casino its Worst Week Ever

What happens when several thousand distinguished physicists, researchers, and students descend on the nation’s gambling capital for a conference? The answer is "a bad week for the casino"—but you'd never guess why.

Ask a Physicist: Phone Flash Sharpie Shock!

Lexie and Xavier, from Orlando, FL want to know:
"What's going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!"

The Science of Ice Cream: Part One

Even though it's been a warm couple of months already, it's officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

(We've since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)

Over at Physics@Home there's an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?