There is a difference between the average speed that an
object moves at and the speed that an object is moving at any one instant
in time. To understand the difference think of a trip you might take to your
friend's house in a car. You leave your house and it takes you 15 minutes(.25hours)
to drive 3 miles across town. This would mean that your average speed was
3 miles/.25 hours or 12 miles per hour. Does this mean you drove 12 miles
per hour the entire time? The answer is no. You had to stop at a few stop
lights. You were able to go faster on the main streets in between green lights.
So at any one moment your speed was most likely different than the average
speed for the trip.
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If you passed a police officer on your trip, your first reaction was
to probably look at your speedometer. This would give you your speed
at that instant in time. This was your instantaneous speed.
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If the police office had a "radar gun" aimed at you, he would also
have gotten your instantaneous speed at that moment.
Hopefully you were going the speed limit or you will get a ticket.
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Why is it important to know the difference between instantaneous speed and
average speed? Well if your speed changes like on our drive we sometimes
want to know our speed at any one moment in time. This will be true for our
next topic of
uniform acceleration.
