The good, the bad and the ugly

22nd April 2005 at 01:00
Chris Holt encourages pupils to rate the physics in popular films

In James Cameron's science fiction film The Terminator, Arnold Schwarzenegger plays the title role of a humanoid machine sent from the future to present day Los Angeles to kill Sarah Connor. She has to be killed because she is destined to give birth to a son who will lead the battle against the machines in the future. The film is fast-moving, violent, and hugely popular. So why not make it the subject of a physics lesson?

The Terminator, like many Hollywood movies, contains a lot of physics - and it's not all bad. Modern physics doesn't rule out time travel. According to Einstein's theory of relativity, time slows down as you approach the speed of light, so travel into the future might be accomplished by travelling at high speed. Time travel into the past might be achieved if wormholes - tunnel-like structures caused by the deformation of space - turn out to be a reality.

Physicists spend a lot of time considering the paradoxes of time travel and, interestingly, The Terminator includes one. A human is sent back from the future to try to protect Sarah Connor from the terminator. The human time-traveller impregnates her, and the son who is born becomes the leader of the future battle against the machines, the leader whom the machines are attempting to destroy.

Here are some other film topics that have potential for discussion in physics lessons:

* KS12 In an episode of the television series Jonathan Creek, a man is murdered when he steps with bare feet on to a metal plate which is connected to mains electricity.

Electrocution would be possible if the victim was also touching an earthed conductor so that the current has a path through his body. But the victim is seen touching nothing and so, in practice, he would be unharmed. It is interesting to note that Hollywood got this right, at least, in the Sylvester Stallone movie Tango and Cash. The heroes are seen dangling from a powerline and are not electrocuted since they are not connected to earth.

* KS3 There's no such thing as magic, so when Mary Poppins floats in the air supported by an umbrella there must be a logical explanation.

Perhaps she is supported by a strong wind. Using Bernoulli's equation you can calculate what wind speed would be necessary by using assumed values for the area of the umbrella and weight of Mary Poppins.

You will find you will need a very high velocity. Your class may be pleased to discover that the physics in The Terminator is superior to that in Mary Poppins. This would be a good time to point out that James Cameron, the director of The Terminator, has a degree in physics from California State University.

* KS4 When people are shot with a bullet, in some films they are depicted as collapsing in a heap, while in others the victim is blown off those little wooden sidewalks and halfway across the street.

Which depiction is correct? You can do the calculations using conservation of momentum. If you assume the bullet weighs 25g and travels at 400ms and the victim weighs, say 80kg, then I calculate his recoil velocity to be 0.125ms (he collapses in a heap).

In the recent version of The Italian Job, a safe is opened while under water. If the safe were airtight would that really be possible? The depth looks like about 2m and if you assume the area of the door is one square metre then you can calculate the force holding the door shut. It's fairly large!

* Some of these items were suggested from the many websites which catalogue errors in the movies, including scientific errors. Have a look at www.intuitor.commoviephysics or www.nitpickers.com

Dr Chris Holt is a physicist and freelance science writer

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