Chapter 13Chapter 13

The Nature of ForcesThe Nature of ForcesForcesFrictionInertiaNewton’s LawsGravityAir Resistance

Section 1 - ForcesSection 1 - Forces Force

a push or pull that one bodyexerts on another that can makean object start or stop moving,OR change its speed or direction.

What forces are beingexerted on the football?

Fkick

Fgrav

Ffriction

Fgrav = Force of GravityFkick = Pushing force ofthe kicker’s footFfriction = molecules of air“rubbing” against thesurface of the football –air resistance!

Balanced Forces forces acting onan object that areopposite indirection andequal in size

no change invelocity

no movement the upward force

is called thenormal force

Section 1 - ForcesSection 1 - Forces

weight(force)

normalforce

Unbalanced Forces Forces that cause a change in

motion (these forces can be in thesame direction OR oppositedirections.

velocity changes (object accelerates)

Section 1 - ForcesSection 1 - Forces

Ffriction Fnet

Fpull

Balanced & UnbalancedForces

Draw the resultingforce arrow foreach of the 3 forcecombinationsshown to the left!!!

Section 1 - ForcesSection 1 - Forces

=

=

=

Section 2 - FrictionSection 2 - Friction

Friction force that opposes (restricts or

”fights”) motion between 2 surfaces The amount of friction depends on

the:• types of surfaces•force between thesurfaces

Friction is greater...between rough

surfaceswhen there’s a

greater forcebetween thesurfaces(e.g. more weight)

Section 2 - FrictionSection 2 - Friction

3 main types of friction:SlidingRollingFluid

What are some Pros andCons of friction?

Section 2 - FrictionSection 2 - Friction

DecreasingFrictionalForce

3 main types of friction:Sliding – when solid objects

slide past or over each other

Rolling – friction produced bysolid objects such as wheelsor ball bearings.

Fluid – friction produced whenan object moves through afluid such as air or water.

Section 2 - FrictionSection 2 - Friction

Section 2 - FrictionSection 2 - Friction

How do people decrease the forceof friction?We use lubricants.

•Def: any substance thatchanges sliding friction tofluid friction.

•Ex: motor oil, grease,

Section 3 - NewtonSection 3 - Newton’’s Firsts First

LawLaw

Newton’s First Law ofMotionAn object at rest will remainat rest and an object inmotion will remain in motionat a constant velocityunless acted upon by anoutside force.

Section 3 - NewtonSection 3 - Newton’’s Firsts First

LawLaw

Newton’s First Law of Motion “Law of Inertia”

Inertia tendency of an object to resist any

change in its motion (either restingor in motion)

increases as mass increases

Section 3 - NewtonSection 3 - Newton’’s Firsts First

LawLaw

Restated:An object will maintain its

speed and direction(velocity) or LACK ofvelocity (remainmotionless) unless anoutside force changes it.

Test Your UnderstandingTest Your Understanding

You are a passenger in a car and notwearing your seat belt.

Without increasing or decreasing itsspeed, the car makes a sharp left turn,and you find yourself colliding with theright-hand door.

Which is the correct analysis of thesituation?

Test Your UnderstandingTest Your Understanding

1. Before and after the turn, there is arightward force pushing you into thedoor.

2. Starting at the time of collision, thedoor exerts a leftward force on you.

3. Both of the above

4. Neither of the above

2. Starting at the time of the turn, thedoor exerts a leftward force on you.

Section 3 - Newton Section 3 - Newton’’s Seconds Second

LawLaw

Newton’s Second Law ofMotionNewton’s Second Law

shows how force, mass,and acceleration arerelated.

F = ma

Section 3 - Newton Section 3 - Newton’’s Seconds Second

LawLaw

F = maF: force (N)m: mass (kg)a: accel (m/s2)

1 N = 1 kg ·m/s2

m

F

a

Force CalculationsForce Calculations

What force would be required toaccelerate a 40 kg mass by 4 m/s2?

GIVEN:F = ?m = 40 kga = 4 m/s2

WORK:F = ma

F = (40 kg)(4 m/s2)

F = 160 N

mF

a

Force CalculationsForce Calculations

Mr. Miller weighs 557 N. What is hismass?

GIVEN:F(W) = 557 Nm = ?a(g) = 9.8 m/s2

SHOW YOUR WORK:m = F ÷ a

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg

mF

a

Force CalculationsForce Calculations

A 4.0 kg shotput is thrown with 30 N offorce. What is its acceleration?

GIVEN:m = 4.0 kgF = 30 Na = ?

SHOW YOUR WORK:a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2

mF

a

Section 3 - Newton Section 3 - Newton’’s Thirds Third

LawLaw

Newton’s Third Law of Motion When one object exerts a force

on a second object, the secondobject exerts an equal butopposite force on the first.

Section 3 - Newton Section 3 - Newton’’s Thirds Third

LawLaw

Simply put….•Every action hasan equal andoppositereaction.

Section 3 - Newton Section 3 - Newton’’s Thirds Third

LawLaw

How do the arrows indicatehow the forces are acting in thepicture?

Section 3 - Newton Section 3 - Newton’’s Thirds Third

LawLawExplanation: The forceof the water(blue arrows) iscounterclockwise. Theresulting force(red arrows) againstthe sprinkler head is inthe opposite direction –clockwise.

This is why thesprinkler spins!!

Section 3 - Newton Section 3 - Newton’’s Thirds Third

LawLaw

Action-Reaction Pairs The hammer exerts a

force on the nail to theright.

The nail exerts an equalbut opposite force onthe hammer to the left.

However, even thoughthe nail exerts as muchforce as it can, it is notas large as thehammer’s force and thenet result is the nailmoving toward the right.

Section 4 - GravitySection 4 - Gravity

Gravity force of attraction between any two

objects in the universe

increases as...• mass increases• distance decreases

G = m1 m2 d2

Section 4 - GravitySection 4 - Gravity Who

experiencesmore gravity -the astronautor thepolitician?

lessdistance

moremass

Whichexerts moregravity - theEarth or themoon?

Section 4 - GravitySection 4 - Gravity

Weight the force of gravity on an object

W = mgW: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)

WEIGHTdepends on gravity

(N or gxm/s2)

MASSalways the same

(kg or g)

Section 4 - GravitySection 4 - Gravity

Would you weigh more on Earthor Jupiter?

Therefore it has greater gravity

And you would have greater weight

Jupiter has a greater mass Jupiter because...

Test Your UnderstandingTest Your Understanding

Is the following statement true or false? An astronaut has less mass on the

moon since the moon exerts a weakergravitational force.

False! Mass does not depend ongravity, weight does. The astronaut hasless weight on the moon.

Section 4 - GravitySection 4 - Gravity

Acceleration due to gravity (g)

In the absence of airresistance, all falling objectshave the same acceleration!

On Earth: g = 9.8 m/s2

mWg =

elephant

m

Wg =

featherAnimation from “Multimedia Physics Studios.”

Section 4 - Air ResistanceSection 4 - Air Resistance

Air Resistance “fluid friction” or “drag” force that air exerts on a moving

object to oppose its motion depends on:

• speed of object• surface area of object• shape of object• density of fluid

Section 4 - Air ResistanceSection 4 - Air Resistance

Terminal Velocity maximum velocity

reached by a fallingobject

reached when… Fgrav = Fair

Fair

Fgrav

no net force⇒ no acceleration⇒ constant velocity

Section 4 - Terminal VelocitySection 4 - Terminal Velocity

Section 4 - Air ResistanceSection 4 - Air Resistance

Terminal Velocity

increasing speed ⇒ increasing airresistance until…

Fair = FgravAnimation from “Multimedia Physics Studios.”

Section 4 - Air ResistanceSection 4 - Air Resistance

Falling with air resistance

Fgrav = Fair

Animation from “Multimedia Physics Studios.”

heavier objects fall fasterbecause they accelerateto higher speeds beforereaching terminalvelocity

larger Fgrav⇒ need larger Fair⇒ need higher speed

JET CAR CHALLENGEJET CAR CHALLENGE

CHALLENGE:Construct a car that will travel as far aspossible (at least 3 meters) using onlythe following materials. scissors tape 4 plastic lids 2 skewers

2 straws 1 balloon 1 tray

How do each of Newton’s Laws apply?