Newton's Three Lawsof 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 - 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)

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.

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-gt the book remains at

rest

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 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

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.

The floor pushes upward on the person

Gravity pulls downward on the person

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

Force of friction between the bench/book

Gravity pulls downward on the book

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 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

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 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

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

No external forces

Force of gravity on the ball

Newtons 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.Newtons First Law

of Motion

Fn

Fg

Gravity pulls downward on the person

Newtons 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

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

Fn

Fn

Fg

Friction between the ball and the air

Ffr

Fg

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.

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.

Direction of the force due to the reactors

Friction between the wind and the plane

Direction of the force from the reactors

Fg

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.

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

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.

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.

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

Newtons Second law of motion

- shortened gt SF 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.

Newtons Second law of motion

Heavy mass, 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

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 !!!

Force exerted by the ground on the car

F mass car x g

Newtons 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. (FMA, 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.

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

125 N

A ?

F 2500 x g

Newtons 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 (FMA, gt AF/M. So you plug in the

data and get A 125/2500kg. This comes out to

0.05 m/s/s.

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.

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.

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 - FAB - FBA

Object A

Object B

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 his face by the punch

Force exerted on the hand by his face

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.

Ffp

-Fpf

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

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 her foot by the floor

Force exerted on the floor by her foot

Newtons 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

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

The End

- presented
- by
- BUENO OLIVIER