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17 September 2010

Most of you missed the point of #1 last night...

I assigned the question above for homework one night last week.  Students were asked to justify their answer.  I'd say that 2/3 of the class got this wrong.  So, here's what I posted that night to the class folder.  Use the problem and/or the discussion, if you wish.

The graph shows something going 20 m in 4 s at constant speed, stopping for a couple of seconds, then continuing at the same speed for another 3 s.

That means an average speed of 5 m/s in the first 4 s. That's like 12 mph (to convert from m/s to mph, just mutiply by 2 and a bit.)

So, youall are telling me that a baby can crawl at 12 mph? Maybe young Jor-El... or, are you telling me that a car on the freeway is going 12 mph? He'll be rear-ended, or, hopefully, pulled over for being either drunk or stupid.

Point is, you do need to be aware of the meaning of speeds. 10 m/s isn't just some old number. 10 m/s is like 24 mph, like a car at Woodberry or on a small street. But only the fastest sprinters like Usain Bolt can run this fast.

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