Great Escapes?
What would you do if you were trapped in a freezer by an invisible Kevin Bacon? Would you build an electromagnet and effect one of Hollywood's great escapes?
Fizzling or fleeing?
Could a wetsuit save you from electricity, as in the Deep Blue Sea, or would Saffron Burrows have been frying tonight? Just one of the scientific puzzlers in Tricky Situations.
Bungee jump or suicide leap?
Could Bruce Willis have saved the world with a singlet and a hosepipe - or would he just have created a nasty way to Die Hard?
Plain sailing?
Related programme
Robert Llewellyn and Dr Jonathan Hare take on Hollywood Science, testing the science that filmgoers take for granted. Here they look at how well the science in the movie Speed stacks up
Speed was the 1994 smash hit film starring Keanu Reeves and Sandra Bullock. Keanu Reeves plays Jack Traven, an LAPD SWAT team specialist who is sent to defuse a bomb that a revenge-driven extortionist (Dennis Hopper) has planted on a crowded bus. Bullock has to maintain the velocity of the bus at 50mph (or greater) so that the bomb does not explode, killing all on board.
The film is a high-octane race of suspense, non-stop action and surprise twists. In a heart stopping, adrenalin pumping moment, the bus turns onto an uncompleted section of a freeway overpass, with a 50-foot gap in the road. Of course they leap the gap - this is Hollywood!
Our Hollywood Scientists, using their unique backyard technology, prove the flaws in the movie’s thesis. But what forces are at work in Speed, and how fast and at what angle does the bus have to be to make it? How could you work this out theoretically?
We decided to find out...
If you watch the film, the section of the freeway they are jumping appears to be flat. It does not look like there is any kind of ramp, an angled surface, to jump off. This is a problem. Why? Because vertical motion and horizontal motion are independent of each other. What does this mean? Let’s look at each one in turn.
Vertical Motion: objects falling due to gravity, or aeroplanes rising, overcoming gravity.
If we consider an object falling due to gravity, how long would it take to fall a set distance?
If an object falls from rest, then it will fall at approximately 10 ms-2, due to the constant pull of the earths gravity.
So how fast will it be falling after 5 seconds?
If speed = acceleration x time then
10 x 5 = 50 ms-1 or 180km/hr.
That is the final vertical speed, but what would the average speed have been? If you are accelerating at a constant rate (and when falling you usually are) then the average speed would be half of the final speed so it will be 25 ms-1 So how far would the object have fallen in 5 seconds? 25 x 5 = 125 metres.
Here come the equations!
This can be written as: 
So now we want to do two calculations, firstly, the time it takes to jump a 50ft hole in the freeway, and secondly how far the bus will fall in this time.
If the bus is travelling above 50mph, (let’s give them the benefit of the doubt and assume it is going about 75mph (33ms-1)).
To travel 50ft (15.24m which is 3.28ft to a metre), will take
15.24/33.5 = 0.454 seconds.
In that time the bus will have fallen - according to our formula s=0.5at2:
0.5 x 10 x 0.4542 = 1.03m
So does the film hold up scientifically?
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Content last updated: 02/04/2003
