Newton’s Three Laws of Motion and Forces

Newton’s Three Laws of Motion and Forces

Sections Covered – Honors Physics◦Chapter 6: pages 116- 143◦Chapter 7: Section 1: pages148 - 154Topics Covered◦Force◦Mass and Inertia◦Newton’s Three

Laws of Motion◦Equilibrium and Non-

equilibrium◦Free Body Diagrams

◦Force of Gravity and Weight

◦Normal Force◦Friction◦*Tension – Atwood

Machine◦*Weight in an

Elevator

Forces

◦Force - a push or a pull

*Mass

**Difference between pushing a bowling ball vs. softball?

What is mass? Good Question!*Little Kid Definition:

◦ How much stuff something has in it

*Chemistry Definition:◦ How much matter

something contains

Physics Definition:◦ Mass is a numerical measurement of inertia.

◦ Mass (m)◦ SI Units: kilograms (kg)

What is Inertia? Good Question!Main Theme of Nature

Inertia - object’s tendency to resist a change in motionObjects at rest want to stay at restObjects in motion want to stay in motion

Nature is lazy, doesn’t like change

Newton’s First Law: Law of Inertia

An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion will continue in motion with the same velocity unless acted upon by an unbalanced force.*So why is its nickname, the law of inertia

Newton’s Second Law

A force acting on a mass will cause the mass to…..

Formula for Force:

Accelerate!

F = maForc

e mass

acceleration

SI Units for Force:

Newton (N)

Force is a VECTOR quantity

Newton’s Second LawFor a single force: F = ma

Newton’s Second LawFor a single force: F = ma

For multiple forces:

Greek Letter – Capital Sigma

Means ‘Sum of…’

Newton’s Second LawTwo options with multiple

forces:1. Forces are balanced:

Equilibrium

2. Forces are not balanced: Non-

Equilibrium

Object is at restObject is moving at a constant

velocity (a = 0 m/s2)

The Sum of Forces – Working with Multiple Forces

Force is a VECTOR◦Can be broken down into

COMPONENTS

F

Fx

Fy

◦Sum of the forces in the x-direction

◦Sum of the forces in the y-direction

How Much is a Newton of Force?

So one Newton is actually pretty small.

One Newton is the force felt when you hold 100 grams in your hand.

Example 4: Calculations with Multiple Forces

Two men are trying to pull a car out of a ditch.

A. Write a statement for ΣFx and ΣFy

F1 is 40.0 N directed at

45.0o above the horizontal

F2 is 30.0 N directed at

37.0o above the horizontal

= 40.0 N

45.0o

= 30.0 N

37.0o

F1

F2

Example 5Calculate the sum of the forces in the x- and

y- direction for the object shown below.

F1 = 24.5 N

F2 = 33.8 N

θ = 68.0o

F3 = 21.0 N

What is the magnitude and angle of the resultant force?

Newton’s Third LawEvery action has an equal and

opposite reaction.

Important note!

Forces and accelerations go together like….

F

a

F

aAdditional note: when evaluating ΣFx and ΣFy – one direction may be in equilibrium while the other is in non-equilibrium and vice versa

Force of Gravity*Acceleration of gravity affects all

objects equally*But what causes

accelerations? *FORCES!

Ground

Force of Gravity

acceleration of gravity

F = ma

Force of Gravity uses the acceleration of gravity (g)F =

mg

g

Acceleration of gravity =

*ClarificationGrab a white board!

Write a definition in your own words for the three terms below. Are there any differences between these terms?◦ Mass◦ Weight◦ Force of Gravity

Weight and MassWhat is the difference between weight

and mass?Mass - is the numerical amount of inertia (how much an object resists a change in

motion)Weight - is the force of gravity on an object

Weight

Fg

Mass

# Inertia

SO WHAT DO BATHROOM SCALES ACTUALLY MEASURE?

What do bathroom scales measure?

On a piece of paper – write 1-2 sentences from your reading – what do they measure?Scales actually measure the

normal force they must exert to hold you up…

Fg

FN Equilibrium: FN = Fg Fg = FN = mg

Since g is a constant value, scales are calibrated to divide it out, leaving you your mass (lbs or kg)

Free-Body DiagramsFree-Body Diagrams (FBD) – Force diagram.

Steps to Drawing a FBD

◦ Step 1: Draw box

◦ Step 2: Draw forces as arrows, AWAY from the box.

F

*Let longer arrows represent greater magnitudes

◦ Step 3: Show any accelerations

a

What Keeps the Book from Falling?Normal Force (FN)A force exerted by surfacesAlways perpendicularly out of a

surfaceFN

FN

An attempt to maintain equilibrium

*Normal ForceThe normal force will only do as

much as is needed to maintain equilibrium.

Books on a desk

vs. Mrs. Whittaker on a deskThe very same desk has to exert a

greater normal force to hold up Mrs. Whittaker than the books.

Forces of Friction (Ff)Friction is a resistive force created by

contact between two surfaces.

◦Resistive Force – Force that resists or opposes motion

◦Forces of Friction are always PARALLEL to the surface. ◦Example

1Ff

m

θ

v

◦Example 2

Ff

FrictionThere are 2.5 kinds of

friction

Static Friction (Ffs): Friction between two object, not moving

= Coefficient of Static Friction

Static friction can change to maintain equilibrium

Maximum Static Friction (Ffsmax):

Break point - instant before the object moves

One specific point/value

Kinetic Friction (Ffk): Friction between two moving objects

= Coefficient of Kinetic Friction One specific

value

Coefficient of Static/Kinetic Friction

Variable: μs andμkGreek Letter: Mu

Unitless ratios between two surfaces

Both values are unique to every substance.

Mass on an InclineThere are several tricks to make this

genre of problem easier

Fg

FNFf

m

FN

Ff

FgFgy

Fgx

Review of ForcesForce – in general, a push or a pull

Force of Gravity (Fg = mg), also is weight

Normal Force (FN) – From surfacesForce of Friction (Ff) – Both static and

kinetic*Masses on an Incline

Force of Tension (FT) -

Force exerted by a rope, cord, string, etc..