Lesson 1: Newton's First Law of Motion

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State of Motion

Inertia is the tendency of an object to resist changes in its state of motion. But what is meant by the phrase "state of motion?" The state of motion of an object is defined by its velocity -the speed with a direction. Thus, inertia could be redefined as follows:

Inertia = tendency of an object to resist changes in its velocity.

An object at rest has zero velocity - and (in the absence of an unbalanced force) will remain with a zero velocity; it will not change its state of motion (i.e., velocity). An object in motion with a velocity of 2 m/s, East will (in the absence of an unbalanced force) remain in motion with a velocity of 2 m/s, East; it will not change its state of motion (i.e., velocity). Objects resist changes in their velocity.

An object which is not changing its velocity is said to have an acceleration of 0 m/s/s. Thus, we could provide an alternative means of defining inertia:

Inertia = tendency of an object to resist accelerations.

Test Your Understanding

Mr.Tracy and Benny Kaplinski are taking some time off for a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Benny Kaplinski guides a golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?

Depress mouse to see answer. The answer is 2. Once leaving the rim, the ball will follow an "inertial path" (i.e., a straight line). At the instant shown in the diagram, the ball is moving to the right; once leaving the rim, there is no more unbalanced forces to change its state of motion. Paths 1 and 3 both show the ball continually changing its direction once leaving the rim.

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Lesson 1: Newton's First Law of Motion

• Newton's First Law
• Inertia and Mass
• State of Motion
• Balanced and Unbalanced Forces
  

Go to Lesson 2