An object may appear to have one motion to one observer

and a different motion to a second observer,

depending on how the two observers are moving with respect to one another.

This java applet let you view objects from different frame of reference.

Imaging there is a river in the center of the screen (yelow dots are moving with water)

a red boat is also moving with respect to the river,

There is a person(blue) walks close to one side of the river, (S)he can swim across the river.

You can easily change frame of reference by moving your mouse to different regions.

e.g. If you move your mouse within the river,

YOU will becomes an observer moving with water.

If you move your mouse into the boat region,

You becomes on the boat watching other objects moving.

You can walk along with that person, or just standing there on the ground.

Try to move the mouse at different regions!

We will do something more complicate later!

frame of reference	Ground	Person	River	Boat
color code	Gray	Blue	dark Green	Red

2. If you click show checkbox , velocity vectors for your frame of reference

will be shown with numbers. Its color is determined by the above color codes.

3. You can press the mouse button to suspend the animation.

If you press with left mouse button, animation will resume when yopu release it.

If you press with right mouse button, you need to click it again.

How to change the velocity vectors?

Just click near the tip of the arrow, and drag it left and right.

Please try it first! Then, we will do something more interesting!

3. There are two number near the person.

Those are vertical and horizontal speed of that person at your frame of reference.

While the animation is suspended :

Click near the left leg of that person and drag the mouse up and down,

you are changing her (his) vertical speed.

Click with right mouse button to resume the animation,

That person will move toward the river, then, swim across the river.

(The horizontal speed with respect to ground will increase due to river current)

Now, try it! And watch (her)him at different frame of references.

Here is a demo (picture) of the program.

Suggestion from Surendranath Reddy.B.:

add a square powered moving object,

and a smll ball(yellow) moving between object's two ends.

You can click show checkbox for more information (velocity and position),

click and drag the tip of velocity vector (while animation is suspended)

to change it.

In order to demonstrate 2D relative motion, a 2D checkbox were added.

Click the 2D checkbox and a ball will be thrown upward from the above moving object.

The trajectory of the ball(blue) will be shown for you.

I hope you enjoy playing with it.

Those numbers at the left side are object's horizontal coordinates,

and you also know the timing.

Why not copy those numbers (while animation is suspended) and make a plot out of it.

Are you going to do it?

Your suggestions are highly appreciated! Please click hwang@phy03.phy.ntnu.edu.tw

Author：Fu-Kwun Hwang, Dept. of physics, National Taiwan Normal University

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