Aristotle◦ First to study motion

Galileo◦ Dropped big and small rock Fell

at same rate◦ If no interference – motion will go

on forever

Newton formalized ideas◦ At 23 (1665) he overthrew

Aristotle’s 2000 year old ideas

◦ LAWS OF MOTION

Force◦ The cause of an acceleration (or the change in an

object’s motion) ◦ Force = mass x acceleration◦ Units:

Newton – SI Unit of Force (kg x m/s2) Dyne – CGS Unit of Force (g x cm/s2) Pound – English Unit of Force (slug x ft/s2)

Inertia ◦ The tendency of an object to maintain its state of

motion Equilibrium

◦ The state of a body in which there is no change in its motion

Weight◦ The magnitude of the force of gravity acting on an

object◦ Weight = mass x acceleration

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

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?

Zigzag!!

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

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

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

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

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

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

y/q0227.shtml

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

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?

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

move dishes

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

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

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

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.

“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

mks cgs English

Length

Mass

Time

Velocity

Acceleration

Force

m

kg

s

Newtons (N)(kg x m/s2)

cm

g

s

Dyne (g x cm/s2)

ft

slugs

s

Pounds (lbs)(slugs x ft/s2)

m/s ft/scm/s

m/s2 cm/s2 ft/s2

Weight =

W = g =

Use ______________ as force units

A force is a __________ or a __________

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

mass x gravity

N or lbs -9.8 m/s2 or -32 ft/s2

N or lbs

push pull

accelerate

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

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

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

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.