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Does physics make sense? Feel the force!

Sometimes people can be tripped up in understanding the world by a basic bit of science. Yet if we can overcome that misunderstanding, suddenly an awful lot becomes clearer. One good example of this is the basic operation of forces acting on a simple ball you throw in the air. Take a moment to get answers to these three questions before reading on (ignore air resistance, as pointed out below):
 
 Don't read on until you've mentally answered each question.

No cheating - get those answers straight in your mind.

When this little test was given to secondary school science teachers in the UK, the majority got it wrong (so don't worry if you did). If that sounds bad, bear in mind most UK science teachers aren't physicists.

The answers? In each case, exactly the same. Just one force, downwards. The force due to gravity. Once the ball has left the thower's hand it has nothing acting on it but gravity. The acceleration is always downwards.

Apart from being a useful little exercise in understanding of physics, I think there's a wider implication on taking a scientific viewpoint here. It's always useful, whether you are assessing the value of a homeopathic remedy, the dangers of WiFi radiation or the flight of a ball to ask 'Just what is acting? How is it having an effect? What will the result be?' There's a lot of knowledge about the world that can quite simply be gained if we take this approach more often, rather than leaping in with assumptions and 'what everybody knows.'
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Comments

  1. Adam Creen1 February 2010 at 11:56

    fair point, Brian, and as a Mechanics teacher I use similar questions to root out misconceptions. But in the 1st & 3rd picture, there's also air resistance opposing motion. Even in a simple explanation we must mention it tho it may be difficult to model.

    ReplyDelete
  2. Brian Clegg1 February 2010 at 13:53

    Fair point - yes, it is important to mention air resistance too (the original did, I just over-condensed it for the blog).

    But the way most people get it wrong is to think that there is an upward force on the ball in picture 1, and balancing forces up and down in picture 2.

    At risk of being wicked, I refer you to this http://www.stanford.edu/~pgbovine/geek-behaviors.htm

    ReplyDelete

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