3.3 Motion & Force3.3 Motion & Force

ObjectivesObjectives Explain how forces and motion are Explain how forces and motion are

related.related. Compare and contrast static friction Compare and contrast static friction

and sliding friction.and sliding friction. Describe the effects of air resistance Describe the effects of air resistance

on falling objects.on falling objects.

ForceForce Push or pull that one object exerts on Push or pull that one object exerts on

anotheranother Vector with size (strength of force) & Vector with size (strength of force) &

directiondirection SI Units = Newtons (N)SI Units = Newtons (N)

Can cause the motion of an object to Can cause the motion of an object to changechange Example: hitting a tennis ballExample: hitting a tennis ball

shooting pool/billiardsshooting pool/billiards

Balanced ForcesBalanced Forces Forces equal in Forces equal in

size but size but opposite in opposite in directiondirection doesndoesn’’t change t change

velocityvelocity

Net ForceNet Force When 2 or more forces act on an When 2 or more forces act on an

object at the same timeobject at the same time Sum of all forces acting on an objectSum of all forces acting on an object

Unbalanced ForcesUnbalanced Forces When forces When forces

combine to combine to produce a net produce a net force that is not force that is not zerozero Forces that arenForces that aren’’t t

equal equal The object The object movesmoves

Changes the Changes the velocity of an velocity of an objectobject

FrictionFriction The force between two objects, in The force between two objects, in

contact, that opposes the motion of contact, that opposes the motion of either objecteither object Unbalanced forceUnbalanced force

Why does a ball stop rolling?Why does a ball stop rolling? Why do we have to keep applying gas to the Why do we have to keep applying gas to the

engines in our cars to keep moving?engines in our cars to keep moving? Better yet, why do we put oil in a carBetter yet, why do we put oil in a car’’s s

engineengine Why do runners wear running shoes?Why do runners wear running shoes? Why do parachutes work?Why do parachutes work?

FrictionFriction Friction depends on the surfaces of Friction depends on the surfaces of

the objects in contactthe objects in contact

Smooth things tend to have less frictional Smooth things tend to have less frictional force force they are more slippery they are more slippery

Ice, oil on concrete, my old shoes, my road Ice, oil on concrete, my old shoes, my road bike tiresbike tires

Rough things tend to have more frictional Rough things tend to have more frictional force force less slippery less slippery

Asphalt, my new running shoes, my mountain Asphalt, my new running shoes, my mountain bike tiresbike tires

Frictional ForceFrictional Force Increases when the force pushing Increases when the force pushing

surfaces together increasessurfaces together increases

Static Friction = frictional force that Static Friction = frictional force that prevents 2 surfaces from sliding past prevents 2 surfaces from sliding past each othereach other

Sliding friction = force that acts in Sliding friction = force that acts in opposite direction to the motion of a opposite direction to the motion of a surface sliding past another surfacesurface sliding past another surface

Rolling friction = similar to sliding Rolling friction = similar to sliding friction friction why it is easier to move something on why it is easier to move something on

wheelswheels

FrictionFriction Air Resistance is a form of frictionAir Resistance is a form of friction

Caused by the interaction between an Caused by the interaction between an object and the air molecules it comes into object and the air molecules it comes into contact withcontact with

The bigger the object the more air The bigger the object the more air resistanceresistance

The faster the object the more air The faster the object the more air resistanceresistance

Air ResistanceAir Resistance Causes objects to fall at different Causes objects to fall at different

acceleration and speedacceleration and speed Acts in opposite direction to velocityActs in opposite direction to velocity

Air resistance not mass causes Air resistance not mass causes objects to fall at different ratesobjects to fall at different rates

v v

vv v

Terminal VelocityTerminal Velocity When an object falls with constant When an object falls with constant

velocityvelocity Upward air resistance becomes large Upward air resistance becomes large

enough to balance downward force of enough to balance downward force of gravitygravity

http://www.iop.org/activity/education/Teaching_Resources/Teaching%20Advanced%20Physics/Mechanics/Images%20200/img_mid_4140.gif

In-Class Assignment/HomeworkIn-Class Assignment/Homework Chapter Review WKTChapter Review WKT Later this week:Later this week:

Bill Nye Friction VideoBill Nye Friction Video Can a penny kill someone? Can a penny kill someone?

(Mythbusters episode)(Mythbusters episode)