A ladybug is moving in a straight line. You use a clock to measure the time. You take a multiple-exposure photogragh to produce a still image of the moving ladybug. The image shows its position at diffirent times. If you know both the time and position, you can calculate the average speed.

Taking the wagon as an isolated system, you study its motion. You identify that there are four forces acting on the wagon: the gravitational force; the normal force of the ground; frictional force and the force on handle exerted by the man. These four forces act on the same object, the wagon, and they can be summed as a net force (interactive exercise).

Center of mass is the point where the weight acts. A wrench has an irregular shape and its center of mass is not at the geometric center. The realistic motion of a thrown wrench is a combination of rotational and translational motions.

The concept of torque can be used to study the stability of an object. This animation illustrates the situations of unstable equilibrium. If the support base is small, a slight lean to the left or right puts the center of mass past the support point and creates a torque. The torque produces a rotation in the same direction as the initial lean, making the situation worse.

A motion may be viewed rather differently from different reference systems. In this animation, the van is moving at a constant velocity relative to you. People in the van take the van as their reference system and your reference system is the ground.