http://www.ck12.org Chapter 4. Motion and Forces
speed=120 mi
3 h
=40 mi/hQ:Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster
speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri^14 hour to make the round trip.
What was her average speed for the entire trip? (Hint: The round-trip distance is 6 km.)
A:Terri’s speed can be calculated as follows:
speed=
6 km
0 .25 h
=24 km/hInstantaneous Speed
When you travel by car, you usually don’t move at a constant speed. Instead you go faster or slower depending on
speed limits, traffic lights, the number of vehicles on the road, and other factors. For example, you might travel 65
miles per hour on a highway but only 20 miles per hour on a city street (see the pictures in theFigure4.5.) You
might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars.
Therefore, your speed at any given instant, or your instantaneous speed, may be very different than your speed at
other times. Instantaneous speed is much more difficult to calculate than average speed. If you want to learn more
about calculating speed, watch the video at this URL:
http://www.youtube.com/watch?v=a8tIBrj84II
FIGURE 4.5
Cars race by in a blur of motion on an
open highway but crawl at a snail’s pace
when they hit city traffic.Calculating Distance or Time from Speed
If you know the average speed of a moving object, you can calculate the distance it will travel in a given period of
time or the time it will take to travel a given distance. To calculate distance from speed and time, use this version of
the average speed formula given above:
distance = speed×time
For example, if a car travels at an average speed of 60 km/h for 5 hours, then the distance it travels is:
distance = 60 km/h×5 h = 300 km
To calculate time from speed and distance, use this version of the formula: