Newton's First Law of Motion Newton's first law of motion states that an object continues in a state of rest or uniform motion in a straight line unless it is acted upon by external unbalanced forces. A "state of rest" means that an object is not moving. "Uniform motion" means that an object continues moving in the same direction at constant speed. All objects resist changes to their states of motion. This is known as inertia . If an object is at rest, it will stay at rest until a force causes it to move. If an object is moving, it will continue moving in the same direction at the same speed until a force causes it to change speed and/or direction. Newton's first law is often referred to as "the law of inertia."

Inertia :

Inertia Inertia is an important concept in physics. It is important to remember that inertia is an object's resistance to changes in motion. In Newton's time, most people believed that all moving objects will naturally come to rest. They believed that forces were needed to keep objects moving. Newton's first law disputed these beliefs. Newton's first law states that forces are not needed to keep objects moving; rather, they are needed to make objects come to rest. All objects have inertia. Do some objects have more inertia that others?

Inertia and Mass :

Inertia and Mass Do some objects have more inertia than others? Yes! Inertia varies with mass. Objects with more mass have a greater resistance to changes in motion. They have more inertia. The more mass an object has, the more inertia it has. Look at the two cubes below. How can you determine which cube has the most inertia?

Inertia and Mass (cont.) :

Inertia and Mass (cont.) Which cube has more inertia? At first glance, the cubes appear to be equal. However, you cannot determine from which material each cube is made just by observing them. In order to tell which cube has the most inertia, you must determine which cube has the most mass. You could give each cube a push. The cube that requires more force to move is the one with the most inertia, meaning it also has the most mass. You could also weigh each cube, which would tell you the force of gravity acting on each cube. Gravity has to exert a larger force on the heavier cube because the heavier cube has more inertia; thus, the heavier cube has the most mass.

What You Have Learned :

What You Have Learned You learned about Newton's first law of motion. - Newton's first law states that an object continues in a state of rest or uniform motion in a straight line unless it is acted upon by external unbalanced forces. - Newton's first law is often called the "law of inertia." - Inertia is an object's resistance to changes in its state of motion.

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