Click the play button to see the motion of the circle and the square.

In the initial view, the square moves to the right at constant speed, and the circle moves upward and to the right at constant speed.

Both the circle and the square leave tracks behind at equal time intervals. These tracks make it possible to determine if the path of each is a straight line and if the speed is changing. If the speed is not changing, the track marks will be equally spaced.

Are the square and circle moving with constant velocity? How can you tell?

Using Newton's first law, is there a net force acting on the square?

You can view the motion of the circle from the perspective of the square. To do this, click on the "View from Square" button and then run the simulation.

Notice how the motion of the circle appears different. The path is now up and to the left.

Viewed from the perspective of the square, is the circle moving at constant velocity?

Now jump to the perspective of the circle by clicking the "View from Circle" button. Run the simulation. Viewed from the perspective of the circle, is the square moving at constant velocity?

Jump back to the initial view by clicking the "Original view" button. You can control the speed of the circle as well as its acceleration. The acceleration was initially set to zero. Set the acceleration to 0.4 m/s2, which is like applying a force to the circle. Run the simulation from the original view, and note how the circle accelerates, as shown by the increasing space between the circle's tracks.

How will the motion of the square appear from the perspective of the accelerating circle? Make a prediction and then select the circle view. Run the simulation to check your prediction. Is the new circle perspective an inertial reference frame? That is, does Newton's first law hold in the accelerated circle's frame?