“If I have seen farther than others, it is because I have stood on the shoulders of giants.”

Sir Isaac Newton (1642 – 1727) Physicist

Forces and the Laws of Motion

4-1 Changes in Motion Force – a push or pull applied to an object Unbalanced forces cause objects to accelerate.Cause objects to start moving, stop moving or change direction. SI Unit of Force is the Newton –

1 N = the amount of force acting on a 1 kg mass, produces an acceleration of 1m/s2

1 N = 1kg x m/s2

Forces can act through contact or without physical contact

Contact force – force through touch (contact)Field Force – electric and magnetic fields, gravity

FORCES COME IN PAIRS When a force is applied to an object, then the object

applies a counterforce. Force is a Vector – magnitude and direction. Force Diagrams used to show all forces on object. Isolate the object – only show forces acting on it.

FN – normal force

(always perpendicular to the surface the

object is touching)

Fg - force of gravity

(weight of object – always directed

straight down)

Fa – applied force

(forced applied to object by person or another

object)

Ff - force of friction

(usually opposes motion of object)

Free Body Diagrams – FBD’s

Review: Free Body Diagrams

4-2 Newton’s First Law

“An object at rest remains at rest and an object in motion remains in motion unless acted upon by an outside force”

Sometimes called “Law of Inertia” – the mass of an object (not it weight) causes it to resist changes in motion.Inertia – “laziness”

Newton’s 1st Law

1

Mass is a measure of inertia – easier to accelerate an object with less mass.

Objects are in equilibrium when they are either at rest or moving at constant velocity - The net external forces are zero.

• Mass and Weight are different! Mass never changes • Weight depends on the gravitational pull on 2 objects.

• Weight = mass of an object times the force of gravity (g)Formula: W = m x g

Weight is a force so the units of weight are in Newton's (N)

Fg (force due to gravity) = m x g

http://www.exploratorium.edu/ronh/weight/

Astronauts on the orbiting space station are “weightless” because...

a. there is no gravity in space and they do not weigh anything. b. space is a vacuum and there is no gravity in a vacuum.c. space is a vacuum and there is no air resistance in a vacuum.d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.

Astronauts on the orbiting space station are “weightless” because...

a. there is no gravity in space and they do not weigh anything. b. space is a vacuum and there is no gravity in a vacuum.c. space is a vacuum and there is no air resistance in a vacuum.

d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.

Find your mass in kilograms using the conversion: ___lbs = 1 kg

2.2 lbs

Then Determine how much you weigh in Newton's.

W = mass x gravity (m x g) = ______________ N

Newton’s 2nd Law

A net Force on an object will cause it to accelerate. Formula:

F = m x a

Units of Force: = kg x m/s2 = Newton (N)

Use equation for weight (force of gravity):

or  

Rearrange and get

mgFg maFg

m

Fa g

Falling and Air ResistanceA. Air resistance decreases the net forces

acting on a falling object1. When air resistance equals downward force on falling object (force of gravity– also called weight) then net force is zero and no further acceleration occurs.

terminal speed– when acceleration terminates

we call this maximum speed terminal velocity

Fg

Fdrag

mass

Fg = mg

FII

F ϴ

The Fg (force in Newton’s due to gravity) can be shown

as two useful vectors FII and F

F perpendicular = FN (normal)

Together these 2 forces replace the force of gravity

mass

Fg = mgFII = m x g sin

F = m x g cos

ϴ

ϴ

Using the law of similar triangles and your trigonometric functions

we can solve for FII and F Sin = opp / hyp = FII / m x g

Cos = adj / hyp = F / m x g

Hypotenuse = Fg = m x g

FII = m x g sin - parallel to the plane (x axis)F = m x g cos - the normal force that acts perpendicular to the plane (y axis)

The net force is the vector sum of all the forces.

• Static friction = friction between surfaces - Force required to get something moving

• Each surface has its own “coefficient of friction”• Coefficient of Static Friction ratio of the

static friction to the normal force (perpendicular force)

μs = Fs / F

2 Types of Friction: Static and Kinetic

Kinetic Friction (μk) = friction between moving surfaces.

Examples: Sliding, rolling, fluid friction (air or liquid) Coefficient of Kinetic Friction (μk) = ratio of kinetic friction

to the normal force (perpendicular force)

μk = Fk / F

Coefficient of Friction – it’s a Decimal – percent of the weight required to move an object

Example:How much force does it take to move a 100 N object that has a μk

= .33

Example:How much force does it take to move a 100 N object that has a μk

= .33

Given: Weight: 100 N (FN) = Fµk = .33

Answer: .33 x 100N = 33 N

Newton’s 3rd Law

For every action there is an equal but opposite reaction

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