The Plane and The Package

A GIF Animation

Consider a plane moving with a constant speed at an elevated height above the Earth's surface. In the course of its flight, the plane drops a package from its luggage compartment. What will be the path of the package and where will it be with respect to the plane? And how can the motion of the package be described? The animation below depicts such a situation. The path of the plane and the package are shown; additionally, the velocity components (horizontal and vertical) are represented by arrows in the animation.

As can be seen from the above animation, the package follows a parabolic path and remains directly below the plane at all times. As the package falls, it undergoes a vertical acceleration; that is, there is a change in its vertical velocity. This vertical acceleration is attributed to the downward force of gravity which acts upon the package. If the package's motion can be approximated as projectile motion (that is, if the influence of air resistance can be assumed negligible), then there will be no horizontal acceleration. In the absence of horizontal forces, there would be a constant velocity in the horizontal direction. This explains why the package would be located directly under the plane from which it is dropped.

Many would insist that there is a horizontal force acting upon the package since it has a horizontal motion. Yet this is simply not the case. The horizontal motion of the package is the result of its own inertia. When dropped from the plane, the package already possessed a horizontal motion, and thus will maintain this state of horizontal motion unless acted upon by a horizontal force. An object in motion will continue in motion with the same speed and in the same direction ... (Newton's first law). Remind your self continuously: forces do not cause motion; rather, forces cause accelerations.

For more information on physical descriptions of motion, visit The Physics Classroom. Specific information is available there on the following topics:

Other animations can be seen at the Multimedia Physics Studios. Other useful resources regarding the physics of motion is available through the Glenbrook South Physics Home Page.

This page was created by Tom Henderson of Glenbrook South High School.

Comments and suggestions can be sent by e-mail to Tom Henderson.

A hearty thanks is due to lab assistant Bryce Mautner for her

assistance with the graphics and GIF animation.

This page last updated on 3/17/97.