Whachu Know Bout Physics!!!: March 2010

The movie Hot Rod is an action packed comedy full of high flying stunts, falls and fall off your seat comedy that only a select few have had a chance to fall in love with. Hot Rod is a movie about a guy, named Rod played by Andy Sandburg, and his quest to gain the respect of his father. The only way he can achieve this is by doing stunts in order to show his father that he is a true man, that can stare death in the face, yet he is thwarted by the judgmental attitude of his stepfather. Eventually, Rod is presented with the opportunity to prove this to his stepfather by doing one big stunt, jumping 11 busses, so that he can raise money to pay for his heart surgery.

Because this movie is full of stunts, I have the opportunity to study some of the physics of the CG action scenes where wire tricks and green screens are used to mimic the laws of physics, yet are flawed by the need for story and comedy. First, we have a training scene in which Rod takes up street luge, in order to train himself to withstand g-forces. In this scene he impacts a trailer home at a seemingly high rate of speed, yet in doing so there are a few flaws in the physics, which can only be explained by the use of real stuntmen and the directors need to keep them alive. Another scene in which the physics are flawed: is a scene in which Rod in hit with a Van at a high speed, this is done in a very starling, yet physically impossible manner. And finally, the Big Jump in this scene the director uses a CG bike and wires to portray Rod flying through the air, over 11 school buses, only to fail and wipe out in front of the entire viewing audience, in doing so; the laws of physics are broken for the sake of story telling.

In the street luge scene, Rod lays flat on his back on a modified skateboard, and is then pushed down a hill with a very steep grade of road and at the bottom of the hill is a trailer home, in which he impacts; damaging it and himself in the process. At the beginning of the stunt, Rod is pushed down the hill by two of his friends. Mid-stunt we get a down shot of Rod traveling at about 22 inches per or 25 to 30 miles per hour, as indicated by the length of his head inside the helmet (being about 1 foot), yet at the end of the stunt his speed is decreased substantially to about 10 mph, which is 7 inches per frame. Rod then impacts the trailer smashing out the windows on the other side of the trailer traveling at 5 mph. Due to the law of acceleration he should have at least stayed at a constant speed before impacting the trailer, and should have impacted the trailer at a higher rate of speed than he started off with and ultimately tearing through the side wall of the trailer and putting the life of the stuntman in danger, but ultimately adding to the jolting slapstick comedy of the scene. In the end, unless through friction the stuntman was allowed to sow his speed using his shoes as breaks he should have impacted the trailer harder.

After being told that his real father was not really a stuntman, yet a victim of pie murder (He choked on a piece of pie during an eating contest) Rod rejects his life as a stuntman and goes to the liquor store to buy “groceries”. When confronted by his friends, he walks across the street with his shopping cart and is impacted by the teams van, at a speed and stopping power that just doesn’t seem to make any sense. It seems as though the van is traveling at a high rate of speed at the beginning of it line of action, then it abruptly stops at the impact of rod, and the stuntman is then flung at a slower rate of speed then the van began with. Although at the amount of acceleration the van was picking up, it could have never been able to stop it the amount of time it did. As the van enters screen left it was tracked at about 15 to 22 inches from frame to frame, using the vans wheel width as a point of measurement, which means that the van was traveling at about the speed of 20 to 30mph and stops dead in its path of action, within about one to two feet from the impact. So the action of the van moving was not equal to the reaction of the breaks and the momentum built up at the speed it was once traveling. Another observation is that the stuntman is flung just after impact, at a distance of 7 to 15 inches per frame, which means that the van only translated about 10 to 20 mph of speed into the its victim. Throughout vigorous observation I have come to notice that the van might have come to a stop due to some sort of cable restraint, so as to not let it get in the way of the character of Rod whom is obviously green screened into the picture and also yanked off screen by a cable system. A truly unsuspectingly funny scene created by surprise and exaggerated physical instances.

The final scene I chose to explore was: the final stunt Rod decides to take on in order to save the life of his father. In this stunt Rod decides to jump 11 school buses which is, about 396 feet (Every bus being about 35 inches wide and giving each bus about a foot of parking clearance) a huge feat for any stuntman including Evel Knivel himself. This is why the movie directors decided to employ a stunt team along with various cables pulleys and a green screen, in order to create the pseudo jump done by Rod. This intern creates flaws in the physics of the stunt, but lends itself to the story. Let’s begin with the take off; from the start Rod hits the ramp going at a speed of about 25 mph with what looks to be the perfect pop at the lip of the ramp. The director also chose a wide shot, which helped me to see the parabolic arch created by the jump. The arch looked to be just enough to get him to the landing ramp, yet in the middle of the jump everything goes wrong. Rod, in mid jump, takes the time to look at the crowd and in his excitement take one hand off of the handle bars in order to give everyone a thumbs up, in doing so; he unknowingly loses his grasp on the motorcycle and begins his painful decent to the ground. It is here that the cables and pulleys take effect, for he is then thrown off of his original path of action and is somehow lifted higher then the motorcycle, separating the two by about 2 yards. All of this action is shown through a medium shot in front of a green screen. After this shot we are shown a wide shot of the both Rod and the motorcycles decent, it is here that the bike begins to fall faster, lower and with an awkward rotation. This rotation takes many different forms from shot to shot; the main one is this during the wide shot when it then begins to wobble left to right mid air without any real rotation or tumble during its decent. If it were in true free fall it would carry through with the rotation in way or the other. In another shot the bike goes in a full rotation end over end, which is probably when the bike is truly falling sans cables and pulleys. After this is the impact, in this section of the fall the parabolic arch of the bike is still intact, except when the bike suddenly decides to make a turn in mid-air to screen left, propelling itself into the stage (Where a band was once playing) about 10 to 15 yards to the right of the landing platform. So, not only does the bike impact first it decides to fall to the right of its path of action, if the physics of this jump were correct the bike would have not only landed at the same time as the stuntman due to the law that states that the distance that an object falls does not depend on its weight as long as the force of air resistance is minimal, but since their arcs were set on the same path from the beginning the bike would have fallen onto the landing platform along with the stuntman. Although flawed, the scene adds wonderful drama and suspense to the movie, which leads to a very funny comedic dream sequence.

The opportunity to study some of the physics of the CG action scenes with wire tricks and green screens, allowed me to truly study some of the made up actions which allowed for story and comedy to be shared with the viewing audiences. The luge scene, the van scene and the Big stunt all lead up great moments in the movie, yet they were all mutually flawed in the aspects of physics.