another rates question

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mathguy

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Jun 3, 2011
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Jason's mom drove 160 km to a stampede at 100 km/h. Then she drove back home at 90 km/h. What was the time difference between both trips.

This is what I think so far:

If she she drove to the stampede at 160 km and drove at 100 km/h, the way there took 1.6 hours because

speed = distance/time
100 km/h = 160 km/time
time (100 km/h) = 160 km
time = 160 km/h / 100 km = 1.6 hours


Going back took her 1.8 hours

speed = distance/time
90 km/h = 160 km/time
time (90 km/h) = 160 km
time = 1.8 hours


Would time difference be 1.8 - 1.6 = 0.2 hours and then that would be 60 (0.2) = 12 minutes?

The book says approximately 11 minutes, did I go wrong somewhere? Thanks!!! :D
 
Yes, as my physics lecturer said, never do any rounding until the very final answer....
 
One MUST worry about significant digits, but I like to ignore that at least once per problem, just to see how far off everything can be. I can often be found working a problem three times, once with as little rounding as possible, once with whatever method will create the greatest result, and one last time to create the least possible result.

Volume of a Right Circular Cyllinder.

r = 1.25
h = 4.00

pi * 1.25^2 * 4.00 = 19.64

3.142 * 1.255^2 * 4.005 = 19.82

3.141 * 1.245^2 * 3.995 = 19.45

This gives some idea how far off things might be and maybe how symmetric the expectation of error might be.

19.82 - 19.64 = 0.18
19.64 - 19.45 = 0.19
 
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