SPH3U UNIVERSITY PHYSICS

Transcription

1 SPH3U UNIVERSITY PHYSICS FORCES L (P ) Galileo s View of Force & Motion Prior to the 1600s, early scientists held a simplistic view of motion. They noticed that moving objects would spontaneously slow down for no apparent reason. 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. November 24, U2 - Newton's First Law of Motion 1 Galileo s View of Force & Motion Galileo questioned this view. He performed real experiments with a ball rolling down and up sloped ramps. He also performed virtual experiments to try to explain his ideas. November 24, U2 - Newton's First Law of Motion 2 1

2 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 slope to the same level it started from. November 24, U2 - Newton's First Law of Motion 3 Galileo s View of Force & Motion And if the ramp is made less steep, the ball still rises to the same height. November 24, U2 - Newton's First Law of Motion 4 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. November 24, U2 - Newton's First Law of Motion 5 2

3 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. November 24, U2 - Newton's First Law of Motion 6 Inertia Galileo concluded that once an object starts moving, it will continue moving at 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. INERTIA property of matter that causes it to resist changes in motion is directly proportional to the mass of the object November 24, U2 - Newton's First Law of Motion 7 Examples of Inertia NOTE! Since an object s mass is a measure of its inertia, and vice versa, there are many everyday examples of objects resisting a change in their motion, including the following: November 24, U2 - Newton's First Law of Motion 8 3

4 Examples of Inertia When a car stops suddenly, a passenger in the front seat not wearing a seat belt continues to move forward (due to the passenger s inertia) and collides with the car s windshield. November 24, U2 - Newton's First Law of Motion 9 Examples of Inertia A magician pulls a smooth tablecloth quickly out from under a placesetting of expensive china. Due to the inertia, the dishes remain at rest where they were, and are not broken. November 24, U2 - Newton's First Law of Motion 10 Examples of Inertia If a coin is balanced on a horizontal card on top of a glass, and the card is then flipped away, the coin, because of its inertia, will drop into the glass. November 24, U2 - Newton's First Law of Motion 11 4

5 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 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. November 24, U2 - Newton's First Law of Motion 12 : The Law of Inertia Although Galileo discovered the notion of inertia it was Newton who coined the term when he summarized Galileo s work in his book Principia Mathematica. Because it was included with Newton s other laws of motion,it is often referred to as Newton sfirstlawofmotion. NEWTON S FIRST LAW OF MOTION also known as law of inertia if the net external force on an object is zero, the object will remain at rest or continue to move at a constant velocity November 24, U2 - Newton's First Law of Motion 13 Implications This law has several significant implications, which must be clearly understood: 1. Objects at rest remain at rest unless acted upon by an external unbalanced force. For example, a ball on a horizontal floor will remain at rest forever, unless someone gives it a push. November 24, U2 - Newton's First Law of Motion 14 5

6 Implications This law has several significant implications, which must be clearly understood: 2. Moving objects continue to move in a straight line at a constant speed, unless acted upon by an external unbalanced force. For example, a car moving into a flat icy curve will tend to continue in a straight line, off the side of the road. November 24, U2 - Newton's First Law of Motion 15 Implications This law has several significant implications, which must be clearly understood: 3. An external force is required to change the velocity of an object. Internal forces have no effect on an object s motion. For example, a passenger pushing on the dashboard of the car does not cause the car s velocity to change. November 24, U2 - Newton's First Law of Motion 16 Implications This law has several significant implications, which must be clearly understood: 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. November 24, U2 - Newton's First Law of Motion 17 6

7 IMPLICATIONS OF Î objects at rest tend to remain at rest Ï objects in motion tend to remain in motion Ð if the velocity is constant, the net force acting on it must be zero Ñ if the velocity is changing (in direction and/or magnitude) the change mustbe caused by a net external force acting on the object November 24, U2 - Newton's First Law of Motion Older cars did not have headrests, but all new cars do. How do headrests help prevent injuries during a rear-end collision? Use Newton s first law to explain your answer. During a rear-end collision, the car will suddenly accelerate forward and so will your body because the seat exerts a force directed forward on your torso. In a vintage car with no headrest, there is no force applied to the head. November 24, U2 - Newton's First Law of Motion 19 As a result your head will continue to remain at rest (law of inertia). Your head will initially appear to snap backwards relative to your body as your body accelerates forward, possibly resulting in a neck injury known as whiplash. November 24, U2 - Newton's First Law of Motion 20 7

8 The headrest in a modern car helps push the head forward with the rest of the body. This helps to prevent whiplash since your neck does not bend backwards as far during a rearend collision. November 24, U2 - Newton's First Law of Motion 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. November 24, U2 - Newton's First Law of Motion How does a seat belt 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. November 24, U2 - Newton's First Law of Motion 23 8

9 APPLICATIONS OF... Newton s first law is observed and applied in many situations and technological applications including the restraint systems in automobiles(i.e. headrests, seatbelts, airbags,...). November 24, U2 - Newton's First Law of Motion 24 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. November 24, U2 - Newton's First Law of Motion 25 9

10 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. November 24, U2 - Newton's First Law of Motion 27 Interaction of Science & Technology INTERACTIONS OF SCIENCE& TECHNOLOGY problem identified (i.e. automobile deaths) research and a solution invented (i.e. air bags) solution causes other problems (i.e. air bag deaths) research begins again November 24, U2 - Newton's First Law of Motion 28 Newton s First Law 3. What is a virtual experiment? a thought experiment November 24, U2 - Newton's First Law of Motion 29 10

11 Newton s First Law 4. Suppose you are helping a friend move. The friend asks you stand in the back of a pickup truck to hold a piano because there is no rope available to tie it down. Explain why you should refuse this request. inertia object in motion wants to stay in motion November 24, U2 - Newton's First Law of Motion 30 Newton s First Law 5. Why does a child on a toboggan fall backward when the toboggan is given a sharp tug forward? inertia object at rest wants to stay at rest November 24, U2 - Newton's First Law of Motion 31 Newton s First Law 6. A friend carries their metal lunchbox on the ledge between the rear windshield and rear set of their car. Is this safe? no if the car stops suddenly the metal lunch box continues moving forward because of inertia November 24, U2 - Newton's First Law of Motion 32 11

12 Newton s First Law 7. Why must an object at rest have either no force or a minimum of two forces acting on it? a minimum of two forces are needed because one force is needed to balance the other one force would not be able to balance itself November 24, U2 - Newton's First Law of Motion 33 U Check Your Learning TEXTBOOK P.129 Q.4,6,10,12,13 November 24, U2 - Newton's First Law of Motion 34 12