A Ladder Will Fall Infinitely Fast when Pulled

Common sense tells us this can't possibly be true, but can you find the flaw in the following supposed "proof" of this claim?

• Step 1: As shown, let x denote the horizontal distance from the bottom of the ladder to the wall, at time t.

• Step 2: As shown, let y denote the height of the top of the ladder from the ground, at time t.

• Step 3: Since the ladder, the ground, and the wall form a right triangle, .

• Step 4: Therefore, .

• Step 5: Differentiating, and letting x' and y' (respectively) denote the derivatives of x and y with respect to t, we get that

  

• Step 6: Since the bottom of the ladder is being pulled with constant speed v, we have x' = v, and therefore

  

• Step 7: As x approaches L, the numerator in this expression for y' approaches -Lv which is nonzero, while the denominator approaches zero.

• Step 8: Therefore, y' approaches as x approaches L. In other words, the top of the ladder is falling infinitely fast by the time the bottom has been pulled a distance L away from the wall.

See if you can figure out in which step the fallacy lies. When you think you've figured it out, click on that step and the computer will tell you whether you are correct or not, and will give an additional explanation of why that step is or isn't valid.

See how many tries it takes you to correctly identify the fallacious step!