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This United States Navy training film describes the principles of relative motion to be applied in naval maneuvers for ships at sea. It opens with a cartoon depicting a caveman hunting with a rock (:39). Without understanding relative motion, when he throws the rock at the deer, he misses every time as the deer is in motion (1:00). When begins to guess the two motions of the moving deer and the rock he begins to understand relative motion and becomes a skillful hunter (1:58). From the deer’s perspective it appears the rock is coming straight for him although it was actually thrown ahead of him (1:17). A forward pass is shown in a football game which also uses relative motion (1:52). On the road, relative motion is showed by two cars driving at 50 mph and when one speeds up or slows down it appears the cars are moving away from one another or closer to one another, respectively (2:25). In naval operations, relative motion problems could be plotted to receive accurate solutions (2:56). The definition is provided which is: relative motion is the apparent motion of one moving object to another moving object (3:15). A demonstration follows of two ships crossing one another (3:23). Ship A in the demonstration is to watch the apparent motion of ship B and vice versa (4:01). Ship A’s relative motion is parallel to the relative motion of ship B (4:33). A vector line is shown which represents the direction of ship A and another vector line which shows the direction of ship B (4:49) and when the two are joined they form the two sides of a vector triangle (5:24). The third side of the vector triangle represents the true course and speed of ship B. This vector diagram is the key to solving all relative motion issues (5:44). The direction and speed of it’s third side can be discovered if any of the other two sides are known (6:02). An example is provided of two ships moving on opposing courses (6:23) and when their vectors are added together, the relative speed between the two ships is determined (6:39). A destroyer is pointed to on the water (7:52) and the film sets up an example where this ship has just received word to proceed with maximum speed to join a convoy course. The convoy was to be north and moving east, therefore the destroyer was to head north east (8:13) which would mean it was leading the convoy until a vector diagram could be formed to discover the actual course. The first step would be to draw a relative plot showing the destroyer’s position as well as the convoy’s position (8:25). The line which represents the destroyer’s direction of relative motion is pointed to with the convoy as a reference (8:39), but if this line was used as a course, the destroyer would miss the convoy (8:47). The convoy course and its true speed combined form one side of the vector diagram (8:59). Next, the captain would plot the direction of relative motion (9:07). Without knowledge of the relative speed, the line is shown extended indefinitely (9:07). The third side is the destroyer’s true course and speed (9:20). The diagram is completed when the vector line is swung from the starting point until touching the direction of relative motion (9:33). From here, as the definite length for the direction of relative motion has been acquired, the length can be measured and the speed of relative motion can be deduced (10:02). Another example of a problem with relative motion follows, which is usually solved with only the seamen’s eye (10:22). A cruiser ship is to move alongside of an oiler (10:26). The oiler is moving ahead of the cruiser which meant the cruiser would have to move towards it and slightly to the right so that the direction of relative motion will go towards the oiler (10:42). His experience will be used in order to determine how much of an increase of speed is necessary, how far to the right he should move and when he was to return to his base course (10:53). This film begins to wrap up with a run through of what was learned (11:13) and it was of the Sea Power for Security films (12:09). We encourage viewers to add comments and, especially, to provide additional information about our videos by adding a comment! See something interesting? Tell people what it is and what they can see by writing something for example like: “01:00:12:00 -- President Roosevelt is seen meeting with Winston Churchill at the Quebec Conference.“ This film is part of the Periscope Film LLC archive, one of the largest historic military, transportation, and aviation stock footage collections in the USA. Entirely film backed, this material is available for licensing in 24p HD and 2k. For more information visit
