4 . The steady speed over 200 s (a bit over 3 minutes) is 0.25 m/s, or 25 cm/s, or about a foot per
second.
A cockroach crawls steadily along the school’s running track, searching for food. The cockroach
starts near the 50 yard line of the football field; around three minutes later, the cockroach reaches the
goal line and, having found nothing of interest, turns around and crawls at the same speed back toward
his starting point.
5 . The maximum speed here is 50 m/s, or over a hundred mph, changing speed dramatically in only 5 or
10 s. So:
A small airplane is coming in for a landing. Upon touching the ground, the pilot puts the engines in
reverse, slowing the plane. But wait! The engine throttle is stuck! So, although the plane comes to rest
in 5 s, the engines are still on. The plane starts speeding up backwards! Oops ...
6 . This thing covers 5 meters in 3 seconds, speeding up the whole time.
An 8-year-old gets on his dad’s bike. The boy is not really strong enough to work the pedals easily, so
he starts off with difficulty. But, after a few seconds he’s managed to speed the bike up to a reasonable
clip.
7 . Though this thing moves quickly—while moving, the speed is 1 m/s—the total distance covered is 1
mm forward, and 1 mm back; the whole process takes 5 ms, which is less than the minimum time
interval indicated by a typical stopwatch. So we’ll have to be a bit creative:In the Discworld novels by Terry Pratchett, wizards have developed a computer in which living ants
in tubes, rather than electrons in wires and transistors, carry information. (Electricity has not been
harnessed on the Discworld.) In performing a calculation, one of these ants moves forward a distance
of 1 mm; stays in place for 3 ms; and returns to the original position. If this ant’s motion represents
two typical “operations” performed by the computer, then this computer has an approximate
processing speed of 400 Hz times the total number of ants inside.
8 . Though this graph looks like #7, this one is a velocity–time graph, and so indicates completely
different motion.
A small child pretends he is a bulldozer. Making a “brm-brm-brm” noise with his lips, he speeds up
from rest to a slow walk. He walks for three more seconds, then slows back down to rest. He moved
forward the entire time, traveling a total distance (found from the area under the graph) of 4 m.
9 . This stuff moves 300 million meters in 1 s at a constant speed. There’s only one possibility here:
electromagnetic waves in a vacuum.
Light (or electromagnetic radiation of any frequency) is emitted from the surface of the moon. In 1 s,
the light has covered about half the distance to Earth.10 . Be careful about axis labels: this is an acceleration –time graph. Something is accelerating at 1000
cm/s^2 for a few seconds. 1000 cm/s^2 = 10 m/s^2 , about Earth’s gravitational acceleration. Using
kinematics, we calculate that if we drop something from rest near Earth, after 4 s the thing has
dropped 80 m.
One way to simulate the effects of zero gravity is to drop an experiment from the top of a high tower.
Then, because everything that was dropped is speeding up at the same rate, the effect is just as if the
experiment were done in the Space Shuttle—at least until everything hits the ground. In this case, an