Yet, after the credits role, and the movie goers make their way
home, the images that stick with them are not those of kissing,
lunacy, or explosions, but rather the exciting set-pieces involving
mechanics that are continuously on display. From the elevator on which
the film opens, to the bus, where the majority of it takes place, to the subway
on which it concludes, it feels like a 116-minute mechanics course,
albeit an entertaining one. I don't know this for a fact, but I would
suspect that director Jan de Bont took a physics class as a kid and enjoyed it
immensely.
I suppose it is not surprising that the movie features mechanics,
as its title is a key term of kinematics - speed is defined as
the magnitude of velocity. And, when an ex-cop turned
psycho attaches a bomb to a city bus, he programs it with this kinematic
parameter in mind: the bomb is armed once the bus surpasses a speed of 50 mph,
and is set to blow should it ever fall below this value again.
If there were more class time in the Mechanics course that I teach,
I would actually show Speed in class. And, after each
action sequence, I would pause the film to discuss the key concepts of
mechanics on display, and even solve explicitly for some of the unknown
parameters. As this exercise is quite time-consuming, I simply encourage
my students to try this activity on their own.
In the first scene alone, many aspects of mechanics are highlighted when
an elevator filled with innocent people threatens to plummet to the
ground. The periods of free fall experienced by both the
elevator and those inside begs several questions, like "Should
the passengers float upwards?" and "Would they increase their
likelihood of survival if they jumped just before the cabin hits the
ground?" I'll leave readers to consider these on their own.
When the cabin and its contents are supported by a single rope, how much
tension manifests inside it? Does the rope extend, and if so, by how
much? Why does the supporting crane above break? I'll answer this
last one: the tension in the cord creates a large moment (or torque) about the
support structure. The bending moment leads to a local stress that is
larger than the ultimate stress value of the material making up the structure.
We could go on and dissect the mechanics of the entire film in this
fashion, but instead, I would like to focus on two particular action sequences
in some detail. These two sequences occur on the fast-moving bus, and I
always discuss them with my mechanics classes.
