Newtons First Law Galileos View of Force & Motion Prior to the 1600s, early scientists held simplistic views of motion They noticed that moving objects would spontaneously slow down for no apparent reason (thought they were lazy..seriously) Since they did not know about friction, they concluded that a constant net force was needed to keep an object moving They thought based upon the teachings of Aristotle that larger net forces made things move faster and smaller net forces made things move slower Galileos View of Force & Motion Galileo questioned this view He performed real experiments with a ball rolling down and up sloped ramps Galileos View of Force & Motion He reasoned that a ball speeds up as it rolls down a slope, then moves with constant velocity along the horizontal surface, and finally rolls up the far side to the same level it started from Galileos View of Force & Motion And if the ramp is made less steep, the ball still rises to the same height Galileos View of Force & Motion He further reasoned, logically, that if the slope of the second plane was zero, the ball, once rolling, would continue forever with no loss of speed, in an attempt to reach its original height Galileos View of Force & Motion Of course, when he performed the real experiment he observed that the ball rolling along the horizontal surface eventually stopped However, he was able to assume correctly that the ball slowed down to zero velocity because of friction Inertia Galileo concluded that once an object starts moving, it will continue moving with a constant velocity if there is no friction present Galileo used the concept of inertia to help explain his conclusion Inertia is the property of matter that causes it to resist changes in motion the inertia of an object depends on the mass of the object Examples of Inertia When a car stops suddenly, a passenger in the front seat not wearing a seat belt continue to move forward (due to the passengers inertia) and collides with the cars windshield Examples of Inertia A magician pulls a smooth tablecloth quickly out from under a place-setting of expensive china Due to the inertia, the dishes remain at rest where they were, and are not broken Examples of Inertia It is more difficult to get a stalled car moving by pushing it than it is to keep it moving Also, it is more difficult to stop a stalled car that is coasting by pushing backwards on it The cars inertia makes it difficult to move when stopped, and difficult to stop, when moving Newtons First Law of Motion: The Law of Inertia Although Galileo discovered the notion of inertia, it was Newton who coined the term when he summarized Galileos work Newtons 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 copy Implications of Newtons 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 objects motion. Eg, a passenger pushing on a dashboard of the car does not cause the cars velocity to change Implications 4. The external force must be unbalanced; that is, two equal opposing forces acting on an object will not change its velocity. For the objects velocity to change, the vector sum of the applied forces on the object must be different than zero Implications of Newtons First Law 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 copy Practice (together) 1. Older cars did not have headrests, but all new cards do. How do headrests help prevent injuries during a rear- end collision? Use Newtons 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 So how does a seat belt work? A seat belt uses the sudden decrease in velocity of the car to activate a gear mechanism In this design, the seat belt strap is attached to a spool which in turn is attached to a gear Beneath this gear is a pendulum that is free to swing back and forth So, how does a seatbelt work? When the car comes to a sudden stop, the pendulum swings forward due to inertia This causes the pendulum to move a metal stop into the teeth of the gear, locking the seat belt in place Interaction of Science & Technology The invention and use of airbags provides a typical example of the interaction of science and technology in our society A problem in society, in this case injuries and deaths in traffic mishaps, leads to research by scientists, which in turn leads to technological development Interaction of Science & Technology But then, more problems are identified that are a result of the initial solution to the problem the use of airbags can cause injuries and deaths in certain instances Thus, the process of scientific research starts again, followed by more technological development Interaction of Science & Technology Now, manufacturers are testing airbags that reduce deaths and injuries of pedestrians struck by cars Similar to interior airbags, the exterior ones are computer- controlled As the car approaches a pedestrian, an infrared detector senses the heat radiating from the human body and if necessary a front-end airbag deploys If needed, a second airbag on the cars hood deploys, softening the pedestrians landing Textbook: read pages 123 128 Answer #2,3,4,6, 10, 11, 12, 13, 14