Newton's Three Laws of Motion by BUENO OLIVIER Isaac Newton (1642-1727) Life & Character –Born at...

Newton's Three Lawsof Motion

by

BUENO OLIVIER

Isaac NewtonIsaac 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.

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)

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)

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.


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

No external forces applied-> the book remains at rest

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.

Gravity pulls downward on the book


The bench pushes upward on the book



– 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


Newton’s First law of motion Involving Friction

Comments & ExamplesNow 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





Force of friction between the bench/book

Force of friction between the bench/book

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)

Fg

Ffr

Fn Fob = ?

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


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

Newton’s First Law of Motion

Comments & Examples– If you kick the soccer

ball, it will continue moving until it hits something.Newton’s First Law of Motion





Fg

Fn

Newton’s 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.





Fg

Fn

Force of contact between the foot and the ball

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

Fg

Fn

Ffr

Friction between the ball and the air

Fg

Fn


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.

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.

Fg

Friction between the wind and the plane

Direction of the force from the reactors

Direction of the force due to

the reactors


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.


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


– 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.

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.

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


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.


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


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 !!!

Not really who you expect to push the

car !!!

F = mass car x gF = mass car x g

Force exerted by the ground on the car



Comments & ExamplesBetty 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 applied30 Newton applied

Fn

Fg


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 !!!She’s trying hard !!! Force exerted by the ground on the car


F = 2500 x gF = 2500 x g

125 N

A = ?


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.

Fg

Fn

125 N

0.05 m/s/s

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.


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.


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


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 the hand by his face

Force exerted on his face by the punch

Force exerted on his face by the punch


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


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 rocket by the engine

Force exerted on the engine by the rocket

Force exerted on the engine by the rocket

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 the floor by her foot


Force exerted on her foot by the floor



Comments & Examples:– Newton first law still

applying in this case.

– Her mass has also in influence on her walking.





The floor pushes upward on the

person

The floor pushes upward on the

person

Gravity pulls downward on

the person

Gravity pulls downward on

the person


Comments & Examples:– Even in the most

unthinkable moment, we do exert Newton’s third law.

– We cannot be touched without being touched

The End

presented

by

BUENO OLIVIER

Newton's Three Laws of Motion by BUENO OLIVIER Isaac Newton (1642-1727) Life & Character –Born at...

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