Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Saving an old lady

You are trying to pull an old lady out of the way of an oncoming truck. You are able to give her an acceleration of 20 m/s2. Starting from rest, how much time is required in order to move her 2 m?

First we organize our thoughts:

Variables given: Δx, a, vo

Variables desired: Δt

Irrelevant variables: vf

Consulting the triangular chart above, the equation we need is clearly Δx = voΔt + 1/2 aΔt2, since it has the four variables of interest and omits the irrelevant one. Eliminating the vo term and solving for Δt gives

Last Update: 2010-11-11