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Newton's Three Laws of Motion. by BUENO OLIVIER. Isaac Newton (1642-1727). Life & Character Born at Woolsthorpe in Lincolnshire (England) entered Cambridge University in 1661 Professor of Mathematics in 1669 and Natural Philosopher

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Newton's Three Lawsof Motion

by

BUENO OLIVIER


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Isaac Newton(1642-1727)

  • 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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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)

Scientific achievements


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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)

Scientific achievements


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Newton’s 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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Newton’s 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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Newton’s 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 Fn). Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium.

The bench pushes upward on the book

Gravity pulls downward on the book


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Newton’s 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.

The floor pushes upward on the person

Gravity pulls downward on the person


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Newton’s 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

Force of friction between the bench/book

Gravity pulls downward on the book


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Newton’s First law of motion Involving Friction

  • Let’s exercise

    • Consider that the book weighs 0.2 kg. As it slides across the bench with a constant velocity, its coefficient of friction is 0.15. What force must be exerted on the book, so that it maintains its constant velocity?(go to the next slide for the answer)

Fn

Fob = ?

Ffr

Fg


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Newton’s 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 = Ffr = μkFn).

    • Solving with the y-direction equation gives Fn = mg, and solving for the x-direction, F = μkmg)

    • The force that must be used on the book is F = μkmg = (0.2)(0.15)(9.80 m/s) = 0.294 N


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Newton’s 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

No external forces

Force of gravity on theball


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Newton’s First Law of Motion

The floor pushes upward on the person

  • Comments & Examples

    • If you kick the soccer ball, it will continue moving until it hits something.Newton’s First Law of Motion

Fn

Fg

Gravity pulls downward on the person


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Newton’s First Law of Motion

The floor pushes upward on the person

  • 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.

Fn

Force of contact between the foot and the ball

Fg

Gravity pulls downward on the person


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Newton’s 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)Newton’s First law of motion

Fn

Fn

Fg

Friction between the ball and the air

Ffr

Fg


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Newton’s 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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Newton’s 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.

Direction of the force due to thereactors

Friction between the wind and the plane

Direction of the force from the reactors

Fg


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Newton’s 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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Newton’s 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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Newton’s 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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Newton’s 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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Newton’s 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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Newton’s 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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Newton’s Second law of motion

Heavymass,needs more force

  • 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.

Small mass, needs less force


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Newton’s 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 !!!

Force exerted by the ground on the car

F = mass car x g


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Newton’s Second law of motion

Fn

30 Newton applied

Fg

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.


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Newton’s 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?

She’s trying hard !!!

Force exerted by the ground on the car

125 N

A = ?

F = 2500 x g


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Newton’s Second law of motion

Fn

125 N

0.05 m/s/s

Fg

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.


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Newton’s 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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Newton’s 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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Newton’s 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

      FAB = - FBA

Object A

Object B


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Newton’s 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 his face by the punch

Force exerted on the hand by his face


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Newton’s 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.

Ffp

-Fpf


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Newton’s 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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Newton’s Third law of motion

  • Comments & Examples:

    • One of the most unnoticeable Newton’s 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 her foot by the floor

Force exerted on the floor by her foot


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Newton’s Third law of motion

The floor pushes upward on the person

  • Comments & Examples:

    • Newton first law still applying in this case.

    • Her mass has also in influence on her walking.

Force exerted on her foot by the floor

Force exerted on the floor by her foot

Gravity pulls downward on the person


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Newton’s Third law of motion

  • Comments & Examples:

    • Even in the most unthinkable moment, we do exert Newton’s third law.

    • We cannot be touched without being touched


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The End

presented

by

BUENO OLIVIER