0.10 Phy1070: motion -- uniform and relative velocity  (Page 6/13)

The magnitude of the average velocity vector, identified by the variable named vecVelMag , is compute by dividing the magnitude of the displacement vector by the total time. This results in a value havingunits of feet/second as shown by the comments.

The direction of the average velocity vector

The direction of the average velocity vector, identified by the variable named vecVelAng , is recognized as being the same as the direction of the overall displacement vector with units of degrees.

Display the results

Finally, the document.write method is called several times in succession to display theoutput text shown in Figure 4 .

Multiple concurrent velocities

When a body has multiple concurrent velocities, the overall velocity of the body is equal to the vector sum of the individual velocities. You can computethat vector sum in any way that works for you, including the parallelogram rule, a tail-to-head vector diagram, or the mathematical techniques that we will usehere.

Relative velocity

In this section, we will also be dealing with a topic called relative velocity . For example, suppose we see a man walking down the aisle in a passenger car of a train that is moving slowly at a uniform velocity along astraight track. How fast is the man moving?

The frame of reference

The answer to that question depends on the frame of reference of the observer. For example, to another passenger in the same rail car, it may appear that the man is moving at about3 feet per second, which is a reasonably comfortable walking speed.

However, to someone standing on the ground outside of the passenger car (pretend that the side of the car is transparent), it may appearthat the man is moving much faster or much slower than 3 feet per second, depending on which direction the man is moving relative to the motion of thetrain.

It could even turn out that insofar as the outside observer is concerned, the man isn't moving at all, or is moving backwards.

Exercise #1 for a man on a train

A man is walking along the aisle in a passenger car of a train that is moving at 1 mile per hour toward the east. The man is walking in the same direction thatthe train is moving. The man is walking at a uniform velocity of 2.933 feet per second. (You will see shortly why I chose such a strange walking speed for the man.) What is the man's overall velocitywith reference to the ground?

Tactile graphics

An svg file named Phy1070b1.svg is provided for the creation of a tactile velocity vector diagram for this scenario. The table of key-value pairs for thisfile is provided in Figure 5 .

The image contained in this file is shown in Figure 6 for the benefit of your assistant who will manually emboss the diagram. Anon-mirror-image version is shown in Figure 15 .

The units for both velocity vectors must be the same. Therefore, the length ofthe velocity vector for the man is based on a conversion from 2.933 feet per second to 2 miles per hour.