Direction of
Acceleration and Velocity
A GIF Animation
Consider the motion of a Hot Wheels car down an incline,
across a level, straight section of track, around a 180-degree curve,
and finally along a final straight section of track. Such a motion is
depicted in the animation below. The car gains speed while moving
down the incline - that is, it accelerates. Along the straight
sections of track, the car slows down slightly (due to air resistance
forces); again the car could be described as having an acceleration
(or perhaps you prefer deceleration). Finally, along the
180-degree curve, the car is changing its direction; once more the
car is said to have an acceleration due to the change in the
direction. Accelerating objects have a changing velocity - either due
to a speed change (speeding up or slowing down) or a direction
change.
The simple animation above depicts some additional information
about the car's motion. The velocity and acceleration of the car are
depicted by vector arrows. The direction of these arrows are
representative of the direction of the velocity and acceleration
vectors. Note that the velocity vector is always directed in the same
direction which the car is moving. That is, a car moving eastward
would be described as having an eastward velocity; and a car moving
westward would be described as having a westward velocity.
The direction of the acceleration vector is not so easily
determined. As shown in the animation, an eastward heading car can
have a westward directed acceleration vector. And a westward heading
car can have an eastward directed acceleration vector. So how can the
direction of the acceleration vector be determined? A simple rule
of thumb for determing the direction of the acceleration is that
an object which is slowing down will have an acceleration directed in
the direction opposite of its motion. Applying this rule of
thumb would lead us to conclude that an eastward heading car can
have a westward directed acceleration vector only if it is slowing
down.
So be careful when discussing the direction of the
acceleration of an object; slow down, apply some thought and use the
rule of thumb.
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.
This page last updated on 6/30/97.