Newton’s First Law

Newton’s First Law of Motion: The Law of Inertia copy Although Galileo discovered the notion of ‘inertia’, it was Newton who coined the term when he summarized Galileo’s work Newton’s First Law of Motion Also known as the law of inertia (the property of matter that causes it to resist changes in motion; it is directly proportional to the mass of the object) “ If the net external force on an object is zero, the object will remain at rest or continue to move at a constant velocity”

Implications of Newton’s First law 1. Objects at rest remain at rest unless acted upon by an external unbalanced force. Eg. A ball on a horizontal floor will remain at rest forever, unless someone gives it a push

Implications 2. Moving objects continue to move in a straight line at a constant speed unless acted upon by an external unbalanced force. Eg. A car moving into a flat icy curve will tend to continue in a straight line, off the side of the road

Implications 3. An external force is required to change the velocity of an object. Internal forces have no effect on an object’s motion. Eg, a passenger pushing on a dashboard of the car does not cause the car’s velocity to change

Implications The external force must be unbalanced; that is, two equal opposing forces acting on an object will not change its velocity. For the object’s velocity to change, the vector sum of the applied forces on the object must be different than zero

Implications of Newton’s First Law copy Objects at rest tend to remain at rest Objects in motion tend to remain in motion If the velocity is constant, the net force acting on it must be zero If the velocity is changing (in direction and/or magnitude) the change must be caused by a net external force acting on the object

Practice (together) Older cars did not have headrests, but all new cards do. How do headrests help prevent injuries during a rear- end collision? Use Newton’s First law to explain your answer. During a read-end collision, the car will suddenly accelerate forward and so will your body because the seat exerts a force direction forward on your torso - In a vintage car with no headrest, there is no force applied to the head

As a result, your head will continue to remain at rest (law of inertia) Your head will eventually appear to snap backwards relative to your body as your body accelerates forward possibly resulting in a neck injury

Textbook: read pages 123 – 128 Answer #2,3,4,6, 10, 11, 12, 13, 14