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A purple ball undergoes projectile motion as shown in the animation (position is
given in meters and time is given in seconds). The blue and red objects
illustrate the x and y components of the ball's motion. Ghost images are
placed on the screen every second. To understand
projectile motion, you must first understand the ball's motion in the x and y
directions separately (any multidimensional motion can be resolved into
Consider the x direction. Notice that the x coordinate of the projectile (purple) is identical
to the x coordinate of the blue object at every instant. What do you notice about the spacing
between blue images? You should notice that the displacement between successive
images is constant. So what does this tell you about the x velocity of the projectile? What does it tell you about the x acceleration of the projectile?
This should tell you that the object moves with a constant velocity in this
direction (which is also depicted on the left graph).
Now consider the y direction. Notice that the y coordinate of the projectile (purple) is identical
to the y coordinate of the red object at every instant. What do you notice about the spacing
between successive images for the red object? You should notice that the displacement
between successive images gets smaller as the object rises and gets larger as the object
falls. This means that it has a downward acceleration. By studying the
right graph, we can also see that the y acceleration is constant.
A particularly important point to understand for the motion of a projectile is
what happens at the peak. What is the velocity of the projectile at the peak? This is a tricky question because
you have a good idea that the y velocity is zero. However, does this mean that the velocity is
zero? Remember that velocity has two components, vx and vy. At the peak,
vx is not zero. Therefore, the velocity at the peak is not zero.
to view the velocity and acceleration vectors.
Physlet authored by Aaron Titus with support by the National Science Foundation under
Grant No. DUE-9952323 and placed in the public domain. It can also be found in the book Physlet Physics by Wolgang Christian and Mario Belloni.
are scriptable Java Applets written by Wolfgang Christian of Davison College.