Chapter 2Chapter 2

Newton’s First Newton’s First Law of MotionLaw of Motion

Aristotle on Motion Aristotle on Motion (350 BC)(350 BC)

Aristotle attempted to understand motion by classifying motion as either

• (a) natural motion• forces acting at a distance

• (b) or violent motion• contact forces

Large object tend to 'strive harder'.

“The Earth remains at rest.”

Geocentric Model - Earth Centered Universe

Copernicus (1500's)Copernicus (1500's)

"The Earth and planets orbit the Sun.”

He reasoned this from his astronomical observations.

Galileo (1600's)Galileo (1600's)Scientist who supported

Copernicus

Dropped objects with different weights from the Leaning Tower of Pisa

Found that all objects fall at the same rate if you can account for air resistance

Friction - a force that resists motion

• e.g. air resistance and sliding on rough surfaces

Inertia - the resistance of an object to change in its state of motion

Demo: Ball and incline plane

Galileo’s Incline PlanesGalileo’s Incline Planes

Isaac Newton (1642-1727)Isaac Newton (1642-1727)

His three laws of motion first appeared in his book called Principia.

Newton’s First LawNewton’s First Lawa.k.a “Law of Inertia”

A body remains at rest or moves in a straight line at a constant speed unless acted upon by a force.

Newton’s First Law ExamplesWeight and string

Card, cup, and coin

Fixing a Hammer

Demo - Coins on elbowDemo - Lead Brick and HammerDemo - Table setting

Figure 2.4

Mass Mass the quantity of matter in an object

the measurement of the inertia

measured in kilograms (kg)

WeightWeightthe force upon an object due to

gravity

Weight = Mass Acceleration of gravity

W = mg

measured in Newtons (N) in the metric system or pounds (lb) in the British system

The weight of a 10 kg brick is...• A) 98 N • B) 10 kg • C) 9.8 kg• D) 10 N • E) 98 kg

Mass and Weight should not be confused with...

Volume• the quantity of space an object occupies

Density • the quantity mass per unit volume

Mass and Weight

On the Moon the gravitational force is only 1/6 as strong as on the Earth.

In space you are “weightless” but not “massless”.

Your mass does not depend on where your are.

• (e.g. Earth, Moon, or space).

WeightLocation Mass

Earth

Moon

Space

18.4 kg

18.4 kg

18.4 kg

180 N

30 N

0 N

Chapter 3Chapter 3

Linear MotionLinear MotionSee Homework 5See Homework 5

Chapter 4Chapter 4

Newton’s Newton’s Second Law of Second Law of

MotionMotion

NEWTON'S 2nd LAW OF MOTION

Fa

or amF

F am

F am

m

F a

m

m

m

F a

F a

F aM

m

a1

M

M

Newtons’ Second LawNewtons’ Second LawF = m a

The acceleration of an object is directly proportional to the net force acting on the object…

…and inversely proportional to the mass of the object.

Example QuestionsExample Questions

How much acceleration does a 747 jumbo jet of mass 30,000kg experience in takeoff when the thrust of all of the engines is 120,000N?

Example QuestionsExample Questions

The same net force is applied to two blocks.

If the blue one has a smaller mass than the yellow one, which one will have the larger acceleration?

F F

If the net force is parallel to the velocity, then the speed of the object increases.

If the net force is anti-parallel to the velocity, then the speed of the object decreases.

If the net force is perpendicular to the velocity, the direction of the velocity changes.

Force and acceleration are vector quantities.

If v is parallel to F, speed increases.If v is antiparallel to F, speed

decreases.If v perpendicular to F, direction of

v changes.

When Acceleration Is Zero - Equilibrium

Static EquilibriumVelocity is zero

Examples:

Hanging from a tree

Weighing yourself on a set of scales

Computer setting on a table

Car parked on an incline

Normal up

Weight down

Weight down

Weight downWeight down

Scales pushing up

Tree pulling up

NormalFriction

When Acceleration Is Zero...…we say the object is in Mechanical

Equilibrium.

…the net force is zero.

For Static Equilibrium the velocity is zero.

For Dynamic Equilibrium the velocity is constant.

Dynamic EquilibriumVelocity is nonzero and constantExamples:Driving at constant velocity

Force from road

Weight down

Friction

Normal up

Weight downAir resistance

Terminal velocity in parachuting

When the Acceleration is g...…the object is in Free Fall.Consider a 1kg rock and a 1gram

feather.– Which object weighs more?

• Answer: The rock

– On which is the gravitation force stronger?• Answer: The rock

– Which has a greater acceleration when dropped from rest?

• Answer: Both have the same acceleration, g.

When the Acceleration Is Less Than g...…the object is not in Free Fall.

In this case there is a force other than gravity.

That force is air resistance.

Air resistance depends on size and speed.

Example: A heavy parachutists will fall faster than a light one.

When the force of air resistance is equal to weight of the falling object, the object will reach a Terminal Velocity.

See Questions on page 66,67 and 69.

After jumping from an airplane a skydiver will fall until the air resistance equals her weight. At that point...– A) she will fall with constant speed– B) she will fall no farther– C) she will fall faster– D) she opens her parachute – E) she will hit the ground

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