ǻx = displacement

ǻt = elapsed time

A plane flies Acme to Bend in 2.0

hrs, then straight back to Cote in

1.0 hr. What is its average

velocity for the trip in km/h?

2.5 - Instantaneous velocity

Instantaneous velocity: Velocity at a specific

moment.

Objects can speed up or slow down, or they can change direction. In other words, their

velocity can change. For example, if you drop an egg off a 40-story building, the egg’s

velocity will change: It will move faster as it falls. Someone on the building’s 39th floor

would see it pass by with a different velocity than would someone on the 30th.

When we use the word “instantaneous,” we describe an object’s velocity at a particular

instant. In Concept 1, you see a snapshot of a toy mouse car at an instant when it has a

velocity of positive six meters per second.

The fable of the tortoise and the hare provides a classic example of instantaneous

versus average velocity. As you may recall, the hare seemed faster because it could

achieve a greater instantaneous velocity than could the tortoise. But the hare’s long

naps meant that its average velocity was less than that of the tortoise, so the tortoise won the race.

When the average velocity of an object is measured over a very short elapsed time, the result is close to the instantaneous velocity. The

shorter the elapsed time, the closer the average and instantaneous velocities. Imagine the egg falling past the 39th floor window in the example

Instantaneous velocity

Velocity at a specific moment

(^28) Copyright 2000-2007 Kinetic Books Co. Chapter 02