Vectors and Projectiles
Parabolic Motion of
Projectiles
A projectile is an object upon which the only force is gravity.
Gravity, being a downward force, causes a projectile to accelerate in
the downward direction. The force of gravity could never alter the
horizontal velocity of an object since perpendicular components of
motion are independent of each other; vertical force does not effect
a horizontal motion. The result of a vertical force acting upon a
horizontally-moving object is to cause the object to deviate from its
otherwise linear path. This is depicted in the animation below.
According to Newton's law of inertia, an object in motion in a
horizontal direction would continue in its horizontal motion with the
same horizontal speed and direction unless acted upon by an
unbalanced horizontal force. The animation above shows a green sphere
moving to the right at constant speed. The horizontal distance
traveled in each second is a constant value. The red sphere undergoes
an vertically-accelerated motion which is typical of an object upon
which only the force of gravity acts. If these two motions are
combined - vertical free fall motion and constant horizontal motion -
then the trajectory will be that of a parabola. An object which
begins with an initial horizontal velocity and is acted upon only by
the force of gravity will follow the path of the blue sphere. It will
travel the same horizontal distance in each consecutive second but
will fall vertically a greater distance in each consecutive second.
The result is a parabolic path as shown in the animation above.
For more information on physical descriptions of motion, visit
The Physics Classroom Tutorial.
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