LECTURE 7 Ch5 F18 Forces

DepartmentofPhysicsandAppliedPhysicsPHYS.1410Lecture7A.Danylov

Lecture 7Chapter 5

Dynamics: Forces and Newton’s Laws of Motion

Physics I

Team work of force demonstration

Course website:https://sites.uml.edu/andriy-danylov/teaching/physics-i/


Today we are going to discuss:

Chapter 5:

Some leftovers from rotational motion Force, Mass: Section 5.1

Weight—the Force of Gravity; and the Normal Force/Tension:Section 5.2-5.4

Newton’s First Law of Motion: Section 5.6 Newton’s Second Law of Motion: Section 5.5 Free-Body Diagram: Section 5.7


tanv

Rotational Kinematic Quantities (Review)

Angular descriptionof rotation

Linear descriptionof rotation

Connecting equations

Let’s look at accelerations


Accelerations

tanv

r

aR

tana

totala

Tangential acceleration atan is always tangent to the circle.

Rantotal aaa t

22t Rantotal aaa

Finally, any object that is undergoing circular motionexperiences two accelerations: centripetal and tangential.Total

acceleration:

;tanv

atan results from a change in the magnitude of vtan

aR (centripetal acceleration)results from a change in the direction of vtan

Centripetal acceleration aRalways points toward the

center of the circle.

aR

aR

In uniform circular motion (=const), although the speed is constant, there is a centripetal acceleration because the direction of the velocity vector is always changing.

(two servants of a king)


A particle moves with uniform circular motion if its angular velocity (and vtan) is constant.

Let’s relate the period and

The period T is related to the speed vtan:

The time interval to complete one revolution is called the period, T.

∙

1 2 2

2 2or

ThatishowtheperiodTisrelatedtotheangularvelocity

Acaristravelingaroundacircularcurveatasteady45mph.Isthecaraccelerating?

A) YesB) No

ConcepTest Car on a curve

ThereisaCentripetalaccelerationpointingtowardthecenter

raatan=0 since vtan=45mph (const)

Acarisslowingdownasitdrivesoveracircularhill.Whichoftheseisthetotalaccelerationvectoratthehighestpoint?

ConcepTest Car on a curve

tanara

totala

Sincethetangentialvelocityisslowing,atan pointsintheoppositedirectionrelativetovtan

Accelerationofchangingdirection

tanv

tanvtanv


Rotational Kinematic Quantities (Review)

Angular descriptionof rotation

Linear descriptionof rotation

Connecting equations

Kinematics is over

tanv

aR


Lecture 7Chapter 5

Dynamics: Forces and Newton’s Laws of Motion

Physics I

Team work of force demonstration

Course website:https://sites.uml.edu/andriy-danylov/teaching/physics-i/


Transition from Kinematics to Dynamics

In Kinematics we studiedHOW things move

Motion Forces

Motion Forces

Now in Dynamics we will study WHY things move

It’s time to complicate life


Force

push pull

AforcewasintroducedinPhysicstodescribeinteractionsbetweentwoobjects

A force is a push or pull

Forcehasmagnitudeanddirection:VECTOR!Toquantifyapushorpull,weneedtospecifybothmagnitudeandadirection.


There are two types of forces

Contact forces:forces that act on an object by

touching it at a point of contact.

Non-contact forces (Long-range forces): forces that act on an object

without physical contact (gravity).

The bat must touch the ball to hit it.

Frictional ForceTension ForceNormal ForceSpring Force

GravityElectric force

Magnetic force

My foot must touchto share fun through

a contact force


The Major Forces


A spring can either push (when compressed) or pull (when stretched).

The pull of a planet on an object near the surface is called the gravitational force. It comes from the entire planet

Gravity acts on all objects, whether moving or at rest.The gravitational force vector always points vertically downward.

Spring ForceGravity


NormalForce

• Whenanobjectsitsonatable,thetablesurfaceexertsanupwardcontactforceontheobject.

• Thispushingforceisdirectedperpendicular tothesurface,andthusiscalledthenormalforce.

Alwaysperpendicular

Atableismadeofatomsjoinedtogetherbymolecularbondswhichcanbemodeledassprings.Normalforceisaresultofmanymolecularspringsbeingcompressedeversoslightly.


Static friction is the contact force that keeps an object “stuck” on a surface, and prevents relative motion.

The static friction force is directed tangent to the surface.

Static friction points opposite the direction in which the object would move if there were no static friction.

Friction Force

Whenanobjectslidesalongasurface,thesurfacecanexertacontactforcewhichopposesthemotion.This is called kinetic friction.

Thekineticfrictionforceisdirectedtangenttothesurface,andoppositetothevelocityoftheobjectrelativetothesurface.Kinetic friction tends to slow down the sliding motion of an object in contact with a surface.

Kinetic friction Static friction


Tension Force

• Whenastringorropeorwirepullsonanobject,itexertsacontactforcecalledthetensionforce.

• Thetensionforceactsalongtheropeandawayfromtheobjectinquestion.

Abobsledderpusheshersledacrosshorizontalsnowtogetitgoing,thenjumpsin.Aftershejumpsin,thesledgraduallyslowstoahalt.Whatforcesactonthesledjustaftershe’sjumpedin?

ConcepTest Forces A)Gravityandkineticfriction

B)Gravityandanormalforce.

C)Gravityandtheforceofthepush.

D)Gravity,anormalforce,andkineticfriction.

E)Gravity,anormalforce,kineticfriction,andtheforceofthepush.

One of the key points:

the object has no memory of forces thatmay have been exerted at earlier times.It only responds to forces acting on it atthis instant. Once the woman stoppedpushing, her force disappeared from ourdiagram. An effect of that force is in adifferent velocity.

End of the class


Principle of superposition

F1F2

F3

+ +

If multiple forces are present, the net force on an object is given by the vector sum of all the forces

F1

F2

F3


Thank you

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LECTURE 7 Ch5 F18 Forces