Newton’s Laws of Motion

HistoryAristotle

First to study motion

Galileo Dropped big and small rock

Fell at same rate If no interference – motion will go

on forever

History

Newton formalized ideas At 23 (1665) he overthrew

Aristotle’s 2000 year old ideas

LAWS OF MOTION

Newton’s First Law of Motion Law of Inertia

An object continues in its state of rest, or of uniform motion in a straight line, unless it is acted upon by an unbalanced force

“Object” – any body

“Continues” – keeps rest or moving

“Unbalanced force” – net force, not in equilibrium

Law of Inertia

Why Law of Inertia? Tendency of a body to resist

change of motion More matter = more inertia Ex. If chased by an elephant,

how should you run away?

Zig-zag!!

Mass and Inertia

Wood LeadCardboard

• Same volumes take up the same amount of space

• Which is harder to move?

- More inertia

- More mass (mass is a measure of inertia)

V V VI I IM M M

Mass and Weight

Does mass = weight? Mass – quantity of matter in a body Weight – measure of gravitational force which

acts on a body Depends on where the object is Ex. Earth, Moon, Space

g = 9.8 m/s2 g = 1.6 m/s2 g = ?

Mass and Weight

Gravity on the Moon http://www.youtube.com/watch?v=PE81zGhnb0w

Gravity around the Solar System http://www.aerospaceweb.org/question/astronomy

/q0227.shtml

Mass and Weight

How much would a 75 kg (165 lb) person weigh on the Moon? On Mars? gmoon= 1.62 m/s2

gmars= 3.77 m/s2

2.2 pound = 1 kilograms 1 pound = 4.45 Newtons

Moon: 121.5 N or 27.3 lbs Mars: 282.8 N or 63.5 lbs

Examples of Inertia

On bus (rest rest) Standing and bus moves forward You seem to fall backward Actually feet move forward with bus and body

remains at rest Bus stops?

Examples of Inertia

Dishes on Table (rest rest) Snap a cloth from beneath Friction between dishes and cloth not enough to

move dishes

Examples of Inertia

In stopping car (motion motion) Fly forward Keep moving while car stops

Newton’s Second Law of Motion Deals with acceleration Acceleration = a = v / t

Ex. Car died tow

5 N

50 N

Which provides the greater acceleration?

Which can get car from 0 mi/hr to 20 mi/hr faster?

GREATER FORCE

Newton’s Second Law of Motion

With same force, which can go from 0 mi/hr to 50 mi/hr faster?

Mass = 1000 kg

Mass = 5000 kg

SMALLER MASS

SMALLER INERTIA

Second Law

Force = mass x acceleration (F = ma)

The more force on an object, the more it accelerates. The more massive an object, the more it resists acceleration.

Second Law “The acceleration of a body is directly proportional

to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the body”

* “in the same direction as the net force” a in the same direction of body’s motion speed up a in opposite direction of body’s motion slow down a at right angles to direction of body’s motion deflect

circular Any other change in speed and direction

Units

mks cgs English

Length

Mass

Time

Force

Weight =

W = g =

Use ______________ as force units

A force is a __________ or a __________

Now also say: Force is anything that can ____________ a body.

History Revisited

Galileo gave no reason why bodies fall with an acceleration.

Newton did. Newton – a F pulls on a body and pulls it toward

the Earth. F = ma

Heavier body attracted with more F. Heavier body has more m and…

More I More resistance to change

Ex. If 2 x F and 2 x m same net acceleration

Newton’s Third Law of Motion Force: Causes acceleration A push or a pull Always involves two objects

interaction Ex. Hammer and Nail

Hammer exerts force on nail In process, nail slows down hammer Therefore, nail exerts force on

hammer Ex. Ice skater with rifle or with mitt

Newton’s Third Law of Motion To every action there is an equal and

opposite reaction One force is action force, other is reaction Neither can exist without the other

Action

Weightlifter lifts on barbell

Feet down on Earth

You sit on chair

Tires sit on road

You push on wall

Helical blades push down on particles

Rifle pushes bullet

Reaction

Barbell pulls on weightlifter

Earth pushes up on feet

Chair pushes up on you

Road pushes up on tires

Wall pushes back on you

Particles push up on blades

Bullet pushes back on rifle (KICK)

SAME FORCE: F/mb = a F/mr = a

Newton’s Third Law of Motion Sometimes not so obvious.

Reactions in free-fall Earth pulls on body = action Body pulls on Earth = reaction Therefore, can say boulder falls to Earth or Earth falls to

boulder F is equal but acceleration is very small

When step off curb – street comes up to meet you Bug against windshield – deceleration of car small,

deceleration of bug large

When objects are relatively the same mass, both forces are more easily noticed since both accelerations are about equal.