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Newton's Three Laws of Motion by BUENO OLIVIER

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Isaac Newton ( ) Life & Character –Born at Woolsthorpe in Lincolnshire (England) –entered Cambridge University in 1661 –Professor of Mathematics in 1669 and Natural Philosopher –President of the Royal Society of London in 1703 until death.

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Scientific achievements –OPTICS –discovered measurable, mathematical patterns in the phenomenon of color, found white light as mixture of infinitely varied colored rays,…book: Opticks (1692). MATHEMATICS –discovered general methods of resolving problems of curvature, embraced in his "method of fluxions" and "inverse method of fluxions",..books: Principia I and II (1687)

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Scientific achievements GRAVITATION –calculated the relative masses of heavenly bodies from their gravitational forces, calculated the force needed to hold the Moon in its orbit book: Principia I and III (1687) MECHANICS –calculated the centripetal force needed to hold a stone in a sling, and the relation between the length of a pendulum and the time of its swing book: Principia I (1687)

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Newtons First law of motion Also known as law of inertia, States, –An object will remain at rest, or uniform motion in a straight line, with the same speed and in the same direction unless acted upon by an unbalanced force.

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Newtons First law of motion Comments –This means that if you leave a book on a bench over night, when you return in the morning, unless an outside force moved it, it will be in the same place No external forces applied-> the book remains at rest

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Newtons First law of motion Comments & Examples –But what is an unbalanced force? first consider a book at rest on a bench. There are two forces acting upon the book. - the Earth's gravitational force, and the push of the bench on the book (sometimes referred to as a F n ). Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium. Gravity pulls downward on the book The bench pushes upward on the book

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Newtons First law of motion Comments & Examples –Consider another example of a balanced force. There are two forces acting upon this person; The force of gravity and the force of the floor. these two forces are equal magnitude and in opposite directions, The person is at equilibrium. Gravity pulls downward on the person The floor pushes upward on the person

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Newtons First law of motion Involving Friction Comments & Examples Now consider a book sliding from right to left across a bench. Sometime in the prior history of the book, it may have been given a shove. The force of gravity and the force of the bench on the book balance each other. Yet there is no force present to balance the force of friction. As the book moves to the left, friction acts to the right to slow the book down. There is an unbalanced force. The book is not at equilibrium and subsequently accelerates The bench pushes upward on the book Gravity pulls downward on the book Force of friction between the bench/book

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Newtons First law of motion Involving Friction Lets exercise –Consider that the book weighs 0.2 kg. As it slides across the bench with a constant velocity, its coefficient of friction is What force must be exerted on the book, so that it maintains its constant velocity? (go to the next slide for the answer) FgFg F fr FnFn F ob = ?

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Newtons First law of motion Involving Friction Answer & explanations –We know that the magnitude of the force of gravity is mg. We recognize that the two object in contact are in relative motion (kinetic friction = F fr = μ k F n ). –Solving with the y-direction equation gives F n = mg, and solving for the x-direction, F = μ k mg) –The force that must be used on the book is F = μ k mg = (0.2)(0.15)(9.80 m/s) = N

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Newtons First law of motion Comments & Examples –Considering a soccer ball in the middle of a field with no external forces exerted (kicking, moving, high winds,…) on it. Normal force of the ground on the ball Force of gravity on the ball No external forces

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Newtons First Law of Motion Comments & Examples –If you kick the soccer ball, it will continue moving until it hits something.Newtons First Law of Motion Gravity pulls downward on the person The floor pushes upward on the person Fg Fg FnFn

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Newtons First Law of Motion Comments & Examples –Your foot can only interact with the ball through forces of contact (there is a gravitational force between your foot and the ball, but it is so tiny that it is completely negligible), so once the ball is not in contact with your foot, it no longer exerts any force on the ball. Gravity pulls downward on the person The floor pushes upward on the person Fg Fg FnFn Force of contact between the foot and the ball

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Newtons First law of motion involving Friction Comments & Examples –Once the ball is not in contact with the foot, the only object interacting with the ball is the ground. The ball will eventually stop even if it does not hit a wall (the friction between the ball and the ground, and between the ball and the air)Newtons First law of motion FgFg FnFn F fr Friction between the ball and the air Fg Fg FnFn

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Newtons First law of motion Comments & Examples –We feel the effects of Newton's First Law every day, but usually don't notice them because other forces interfere. If it was not for other forces we will be in constant motion.

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Newtons First law of motion Comments & Examples –On earth, the atmosphere will eventually slow down all moving objects, but in a vacuum (basically an empty space with no air or atmosphere), like space, it will be more obvious that objects obey Newton's Laws. FgFg Friction between the wind and the plane Direction of the force from the reactors Directio n of the force due to the reactors

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Newtons First law of motion Comments & Examples –In space, the First Law is much more obvious. Objects will follow their natural trajectories until they are stopped by an outside force.

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Newtons First law of motion Comments & Examples –One of the most common places people feel the First Law is in a fast moving vehicle, such as a car or a bus, that comes to a stop. An outside force stops the vehicle, but the passengers, who have been moving at a high speed, are not stopped and continue to move at the same speed

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Newtons First law of motion Comments & Examples –If the car hits a cement road divider it is stopped (outside force). The crash dummy, however is not so lucky. Since he is not wearing a seat belt, and is not connected to the car, he will continue to move at 60 mph, flying out through the front windshield.

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Newtons First Law of Motion Comments & Examples –The dummy will fly through the air until he hits the ground. This is because the earth's gravity stopped him from moving any further. If this collision had happened in zero-g, in a vacuum, the dummy would theoretically keep on hurtling away from the car at 60 mph.

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Newtons Second law of motion States, –The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. –The direction of the acceleration is in the direction of the net force acting on the object

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Newtons Second law of motion shortened => ΣF = ma –where f is a push or pull that gives energy to an object the motion of the object. a is the rate of change of velocity.

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Newtons Second law of motion Comment & Example –Newton's Second Law is more abstract than the first. The greater the mass, the greater the amount of force needed to accelerate the object. Heavy mass, needs more force Small mass, needs less force

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Newtons Second law of motion Example: –Betty is developing her muscles by pushing this car that weighs 1500 kg. She makes it go 0.02 m/s/s. Using Newton's Second Law, can you compute how much force I applied to the car? ( the answer in the next slide) Not really who you expect to push the car !!! F = mass car x g Force exerted by the ground on the car

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Newtons Second law of motion Comments & Examples Betty has not really move that much consider she has only exerted 30 Newton of force. ( F=MA, so you plug in the data and get F = 1500kg x.02 m/s/s. This comes out to 30 kg m/s/s, which is equal to 30 Newton. 30 Newton applied FnFn FgFg

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Newtons Second law of motion Example: –Here Betty is trying to do the impossible. She wants to push this 2500 kg van to a gas station. She computes 125 Newton on the car. How fast will she make it go? Shes trying hard !!! Force exerted by the ground on the car F = 2500 x g 125 N A = ?

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Newtons Second law of motion Answer & Explanations –It may seem impossible but Betty will make it go 0.5 m/s/s. Because using Newton's Second Law, we found that… ( F=MA, => A=F/M. So you plug in the data and get A = 125/2500kg. This comes out to 0.05 m/s/s. FgFg FnFn 125 N 0.05 m/s/s

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Newtons Third law of motion Comments: –Anytime an object exerts a force on another object, the second object exerts an equal and opposite force on the first.

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Newtons Third law of motion Comments –Newton's Third Law is probably the most famous of his laws. –The Third Law at first seems simple, but is a very important law. –Every time we interact with our surroundings we feel the Third Law.

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Newtons Third law of motion Comments & Examples –If use the convention that F means the force on object A from object B, then Newton's third law can be written F AB = - F BA Object A Object B

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Newtons Third law of motion Comments & Examples: –When you punch someone in his face your hand not only applies a force to the person's face, the person's face applies a force to your hand. Force exerted on the hand by his face Force exerted on his face by the punch

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Newtons Third law of motion Comments & Examples –The magnitude of the force on each body is identical and the forces on the on the two bodies are in the opposite directions to each other. F fp -F pf

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Newtons Third law of motion Comments & Examples: –The only reason why a rocket is able to launch, is that when its engine pushes out the gases, the gases exert an equal and opposite force back on the rocket, which accelerate. Force exerted on the rocket by the engine Force exerted on the engine by the rocket

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Newtons Third law of motion Comments & Examples: –One of the most unnoticeable Newtons third law, is when we walk. –We can walk forward because, when one foot pushes backward against the ground, the ground pushes forward on that foot. Force exerted on the floor by her foot Force exerted on her foot by the floor

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Newtons Third law of motion Comments & Examples: –Newton first law still applying in this case. –Her mass has also in influence on her walking. Force exerted on the floor by her foot Force exerted on her foot by the floor The floor pushes upward on the person Gravity pulls downward on the person

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Newtons Third law of motion Comments & Examples: –Even in the most unthinkable moment, we do exert Newtons third law. –We cannot be touched without being touched

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The End presented by BUENO OLIVIER

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