Interview Question

Product Design Engineer Interview

-Cupertino, CA


If I have a solid rod and hollow rod with the same mass and I let them slide in a ramp. Which one reaches the bottom first and why?

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5 Answers


Since they've clearly told sliding. Acceleration along the ramp is independent of the mass or the moment of Inertia. Hence, both reaches down at the same time.

Amrith Rangan on


Solid bar, smaller moment of inertia allows it to spin faster

Ryan on


Solid reaches first

Anonymous on


Depends. If friction is not accounted for, then they reach at the same time. Yes, acceleration is the same, however, assuming both rods are made of the same material and uniform dimensions (aka same length and outer diameter, but one still is hollow), frictional force will be greater on solid rod, causing it to slow down more. If this is the case, then hollow rod will reach first.

Anonymous on


It depends. If materials are the same, the hollow rod's section, (diamter x length), is bigger, so the air drag is stronger. Or when the hollow rod is rolling, it will carry air inside then the total mass is higher than the other.

Y.C. on

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