The Truck and The
Ball
A GIF Animation
Imagine a pickup truck moving with a constant speed along a
city street. In the course of its motion, a ball is projected
straight upwards by a launcher located in the bed of the truck.
Imagine as well that the ball does not encounter a significant amount
of air resistance. What will be the path of the ball and where will
it be located with respect to the pickup truck? How can the motion of
the ball be described? And where will the ball land with respect to
the truck? The animation below depicts such a situation. The path of
the ball and the truck 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 ball follows a
parabolic path and remains directly above the launcher at all times.
As the ball rises towards its peak, it undergoes a downward
acceleration. An upwardly moving ball which is slowing down is said
to be undergoing a downward acceleration. As the ball falls, it also
undergoes a downward acceleration. A downwardly-moving ball which is
gaining speed is said to have a downward acceleration. This downward
acceleration is attributed to the downward force of gravity which
acts upon the ball. If the ball'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 ball
would be located directly above the launcher from which it is
projected.
Many would insist that there is a horizontal force acting upon
the ball since it has a horizontal motion. Yet this is simply not the
case. The horizontal motion of the ball is the result of its own
inertia. When projected from the truck, the ball 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:
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.