Leap of Faith

Before you try stunt-jumping your car over the nearest chasm, read up on the physics of why you're doomed to fail

By Adam Weiner Posted 03.18.2009 at 1:22 pm 10 Comments

This video puts some perspective on the action-movie high-speed car chase jump phenomenon. Notice how close this car comes to wrecking when launched off of a little teeny two-foot-high ramp and moving at a relatively slow velocity.

In fact, just for fun, let's do a rough estimate of the takeoff velocity. We approximate that the car lands about 10 meters from its takeoff point and is in the air for close to one second. Applying this information we can do a simple calculation to determine its horizontal component of velocity on takeoff:

v_x = Δx/ t = 10 m/1s = 10 m/s

Using a little vector addition we can also determine the net velocity off the ramp based on the ramp angle. We'll leave this as an exercise for anyone so inclined (no pun intended), but because the take off angle is pretty small (we estimated 17 degrees) the net velocity is still only approximately 10.5 m/s or 23 mi/hr -- not really a high-speed stunt.

Fun, games and calculations aside, one of many problems any "would-be" stunt car driver is going to face on attempting a jump, is that the car is generally going to follow the standard parabolic trajectory of a projectile. Let's assume for the moment that the car has its center of mass relatively near its geometric center. In that case the upward angle of the car at any given height on its ascent should be the same as the downward angle at the same height during the descent. If, as is more likely, the car has its center of mass located towards the front half of the car, it will cause it to nose down even more steeply. In fact, in the video we can see the car landing at an alarmingly steep angle. They were lucky to pull that one out.

If you've ever seen an actual stunt jump then you know that they put ramps at both ends of the jump. That's for good reason. (They also have specially designed suspension systems.) Try sticking the landing when your car impacts the ground at an angle of 25 degrees below horizontal, at a speed of say 80 miles per hour. The hood is going to crumple and you're going to need rehab before the tires ever make contact. Seriously, don't try this at home!

So next time you're watching Gone in Sixty Seconds, Speed, The Fast and the Furious or any movie with some spectacular cinematic leaps, recall this little video and contemplate just how far the fantasy of the action film is divorced from any semblance of reality. (We're not suggesting that you don't already know that, but this gives a pretty vivid demonstration of what kind of limitations we might be dealing with.)

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.

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10 Comments

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bws28

03/18/09 at 3:51 pm

Basically just put a bunch of lead bricks in the back of your car then jump the ramp with the center of mass toward the trunk.

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JimmyD

03/18/09 at 4:22 pm

In the movies though, the car will be showroom new again in the next scene...
They destroyed over 300 1968 and 1969 Dodge Chargers while filming "The Dukes of Hazard" TV series. About 2 per episode...

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DWPittell1

03/20/09 at 6:37 am

Actually the front-heavy weight of the car (due to the engine) is almost irrelevant to its tendency to land nose-down. Heavy objects fall faster than light objects only insofar as air friction does less to retard the free-fall of the former. (And the vertical components of velocity, and thus of air friction, are quite low here.) The reason the car lands nose down is because the rear wheel is still being forced up by contact with the ramp, while the front of the car, having cleared the ramp, is already accelerating downward due to gravity (i.e., it is going up less slowly). The somersaulting motion of the car can persist even beyond the point where it is nose down, provided there is enough time in the air (e.g., if the ramp is on the top of a cliff).

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CCTProjectionist

03/20/09 at 10:41 am

The cars in your basic action thriller movie are stripped of their interior pieces and then equipped with a heavy roll cage. Weights are then added into the trunk to counteract the front weight bias.

The kids in the amateur vid are very lucky that car didn't imitate a lawn dart. In addition to not being prepped out, the Mitsubishi/Dodge they're using has a strong front weight bias, due to its FWD layout.

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CCTProjectionist

03/20/09 at 10:42 am

The cars in your basic action thriller movie are stripped of their interior pieces and then equipped with a heavy roll cage. Weights are then added into the trunk to counteract the front weight bias.

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CCTProjectionist

03/20/09 at 10:42 am

The cars in your basic action thriller movie are stripped of their interior pieces and then equipped with a heavy roll cage. Weights are then added into the trunk to counteract the front weight bias.

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Jacquo

03/25/09 at 5:15 pm

A few years ago I saw a home video broadcast on TV shot by a teen in Quebec where his two buddies tried to jump a dip in the road. They were doing over sixty mph when they leaped their old full size car into the air. The car nosed down to the pavement then tumbled about a dozen times before landing on its top in the field. The two teens in the car were not wearing their seat belts not suprisingly were killed in the stunt. The video was released to the media by the father of one of the teens killed as a warning to others that would think to try such a foolish stunt.

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andyv_15

04/15/09 at 10:18 pm

If using a rear or Mid engine car you would have a more balanced tragectory....It may be a lil more spendy though.
~Andrew

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Semra

08/14/09 at 9:16 pm

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JasonBrigman

09/22/09 at 12:28 pm

If it is the bad guys in the car though - in the movies half of the time the car will explode also - lol
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