1. Newton’s First Law

2. Galileo’s 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

3. Galileo’s View of Force & Motion  Galileo questioned this view  He performed real experiments with a ball rolling down and up sloped ramps

4. Galileo’s 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

5. Galileo’s View of Force & Motion  And if the ramp is made less steep, the ball still rises to the same height

6. Galileo’s 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

7. Galileo’s 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

8. 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

9. 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 passenger’s inertia) and collides with the car’s windshield

10. 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

11. 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 car’s inertia makes it difficult to move when stopped, and difficult to stop, when moving

12. Newton’s 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 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” copy

13. 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

14. 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

15. 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

16. 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 object’s velocity to change, the vector sum of the applied forces on the object must be different than zero

17. Implications of Newton’s 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

18. 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 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

19.  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

21. 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

22. 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

23. 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

24. 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

25. 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 car’s hood deploys, softening the pedestrian’s landing

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