This demo shows a curved line, the distance traveled by a freely falling object close to the Earth's surface. If we measure the slope of the line (speed of the object) by comparison with another point, we run into some problems. What happens when we change slider f, the interval over which we try to get an average speed? What value do we get for speed when f=0?

We want to know the [i]speed[/i] of the object at exactly 4 seconds. What happens when we change slider f, the interval over which we're calculating the average speed? What value for speed do we get when f=0?