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MCAT Physics Question — Doorknobs

  • by John
  • Nov 27, 2014
  • MCAT Blog, MCAT Physics, MCAT Question of the Day

Why are the doorknobs of most doors placed on the edge of the door, opposite the hinge where the door is attached to the doorframe?

 

A) Placing the doorknob as far as possible from the door’s center of gravity helps minimize the frictional force between the door and the doorframe during opening.

B) Maximizing the distance between the knob and the hinge allows the door to be opened with less force, because torque and distance of the lever arm are directly related.

C) Increasing the distance between the knob and the hinge reduces the acceleration produced when opening the door, which is advantageous for general safety concerns.

D) There is no physical explanation to justify this choice, and it is likely an historical artifact.

 

Explanation

 

Most doors open by rotating on a hinge. As such, they are opened through the application of torque. To make it as easy as possible to open the door, the knobs are placed as far from the hinge as is feasible. This maximizes the lever arm, reducing the amount of force needed to open the door.

 

A: Frictional force between the door and the doorframe should already be near zero for a properly hung door. Ff depends on the normal force, and since a door doesn’t rest on the doorframe itself, there shouldn’t be any frictional force.

 

C: A larger lever arm increases torque. This doesn’t directly increase or decrease acceleration, as that depends on how massive the door is and how much force is applied.

 

D: Incorrect, as explained in choice (B).

 

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