How Fast is that? Ice skaters 1 Video
By Matt Vonk
1. Watch the video all the
way through.
Displacement:
Displacement is the
difference between the
ending position and the
starting position of an object.
2.
What is the
displacement of the
people (and dog) as they
move from the line on the
right to the line on the
left?
5m
3. Displacement is different than distance, for example, watch the motion of the left glove of the
third skater as it travels between the two lines. Notice that the glove moves back and forth in
such a way that the total distance it travels is greater than the 5 m distance between the two
lines. However, since displacement is just the difference between the final and initial positions
of the object, the third skater's glove will have the same displacement as the first skater's boot
or the dog’s nose.
Average Velocity:
Average Velocity = displacement / time.
4. Measure the displacement of the left boot of the skater in a blue jacket as the boot travels
between the two lines.
5m
5. Measure the number of frames the tip of the boot requires to travel between the two lines.
Final frame = 618, Initial frame = 424.
6. Convert frames to seconds.
618-424=194 frames
Here's an example: 18 frames x (1 second/ 240 frames/s) = 0.075 s
194 frames x (1 second / 240 frames/s) = .808 s
7. Calculate the average velocity of the boot. Express your answer in m/s and mph.
5 m / 0.808 s = 6.2 m/s
6.2 m/s x (100 cm/1 m)x(3600 s/1 hour)x(1 inch/2.54 cm)x(1 foot/12 inches)x(1 mile/5280 feet)
= 14 mph
Instantaneous Velocity:
Instantaneous velocity is how fast something is moving in a particular direction at any moment in
time.
8. Now find the average velocity of the left glove of the skater who falls.
Final frame = 648, Initial frame = 369.
648-369=279 frames
279 frames x (1 second / 240 frames/s) = 1.16 s
5 m / 1.16 s = 4.3 m/s = 9.6 mph
9. Notice that although this is the glove's average velocity there are times when it moves faster
and slower.
10. Between which frames does the glove move the slowest?
It looks stationary between frames 369 and 407
11. Between which frames does the glove move the fastest?
It looks pretty fast between frames 462 and 491
Direction:
In this class, it will be very important to decide which
direction you'd like to call positive. Since the motion in this
video is to the left, most of you probably chose that as your
positive direction without even realizing it. If you did then
the displacements and velocities that
Excuse me, is that
you found above would be positive.
positive or
12. In the video, find something that
moves in the negative direction for a
short time.
negative?
It was surprisingly hard for me to find a single
thing in the video that was moving backward.
Although a lot of the gloves are moving backward
relative to the people who are carrying them, it’s not
clear to me that they are actually moving backward
relative to the ice (since the people are moving
forward as fast or faster than they are swinging
their arms back). The clearest exception that I
could find was the left glove of the fallen skater as
Khmer Temple Cambodia
he tries to get up between frames 934 and 957.
Photo by Matt Vonk