Chapter 11: ForcesChapter 11: ForcesAlready Covered: Newton’s 1Already Covered: Newton’s 1stst and 2 and 2ndnd laws of laws of

motion:motion:1.1. An An object at rest object at rest remainsremains at rest at rest, and an , and an

object in motion object in motion remainsremains in motion with the in motion with the same velocitysame velocity, unless acted upon by an , unless acted upon by an unbalancedunbalanced force force

2.2. The The accelerationacceleration of an object of an object increases with increases with increased forceincreased force and and decreases with increased decreases with increased massmass

F = m x aF = m x a

2.3 Forces act in pairs2.3 Forces act in pairsNetwon’s 3Netwon’s 3rdrd Law: Law: 3. Every time one object exerts a 3. Every time one object exerts a forceforce on another on another

object, the second object exerts a object, the second object exerts a force that is force that is equal in size and opposite in directionequal in size and opposite in direction back on back on the first objectthe first object

Newton’s third law relates Newton’s third law relates actionaction and and reactionreaction forces forces

Forces Forces always always act in pairsact in pairs Jellyfish movement: squeeze water out of the umbrella-Jellyfish movement: squeeze water out of the umbrella-

like body: applies a like body: applies a force (of the water) in one direction force (of the water) in one direction (downward),(downward), and and moves in the opposite direction moves in the opposite direction (upward)(upward)

““Every time one object exerts a Every time one object exerts a forceforce on another object, on another object, the second object exerts a the second object exerts a force that is force that is equal in size and equal in size and opposite in directionopposite in direction back on the first object” back on the first object”

((For every action there is an equal and opposite For every action there is an equal and opposite reaction.)reaction.)

ActionAction and and ReactionReaction Pairs Pairs

Force exerted on an object/force the object Force exerted on an object/force the object exerts back = action/reaction force pairexerts back = action/reaction force pair

Jellyfish?Jellyfish? Book’s ex:Book’s ex:

Space shuttle launchSpace shuttle launch Stub your toeStub your toe Press down on a Press down on a

table (no motion)table (no motion)

Action and Reaction Forces Versus Action and Reaction Forces Versus Balanced ForcesBalanced Forces

Equal AND opposite, similar to balanced Equal AND opposite, similar to balanced forcesforces Balanced forces act on a Balanced forces act on a single objectsingle object: two friends : two friends

pulling on a back pack, which doesn’t movepulling on a back pack, which doesn’t move Action and Reaction Forces act on Action and Reaction Forces act on different different

objectsobjects: drag the back pack across the floor: drag the back pack across the floor Examples?Examples?

The baseball forces the bat to the left; the bat forces the ball to the right. The baseball forces the bat to the left; the bat forces the ball to the right. Together, these two forces exerted upon two different objects form the action-Together, these two forces exerted upon two different objects form the action-reaction force pair. Note that in the description of the two forces, the nouns in the reaction force pair. Note that in the description of the two forces, the nouns in the sentence describing the forces simply switch places.sentence describing the forces simply switch places.

Action: Baseball pushes glove leftwards.Reaction: _________________________Action: Baseball pushes glove leftwards.Reaction: _________________________

Action: Bowling ball pushes pin leftwards.Reaction: _________________________Action: Bowling ball pushes pin leftwards.Reaction: _________________________

Action: Enclosed air particles push balloon wall outwards.Reaction: _________________________

Action: Enclosed air particles push balloon wall outwards.Reaction: _________________________

Newton’s three laws describe and Newton’s three laws describe and predict motionpredict motion

Can explain the motion of almost any object Can explain the motion of almost any object (animals included)(animals included)

Work together (not independent of one another)Work together (not independent of one another) Can use the laws to make predictions about Can use the laws to make predictions about

motionmotion Spacecraft: can predict where Mars will be at the time Spacecraft: can predict where Mars will be at the time

a spacecraft reaches it, and can control the force on a spacecraft reaches it, and can control the force on the spacecraft to arrive at the right place/timethe spacecraft to arrive at the right place/time

Newton’s three laws of motionNewton’s three laws of motion

Acceleration is Independent of Mass

Until Galileo Galilei, "commonsense" told people that heavierobjects fall faster. Galileo showed the power ofexperiment over logic.

Inertia Trick

                              

  

Inertia is the tendency of an objectto remain at rest if it's already atrest, or to keep moving if it'salready moving.----------------------------------------------Tablecloth trick: Too little force, too little time to overcome "inertia" of tableware.

Inertia is Mass

Inertia is also called mass.

Mass is measured in kilograms.

One kilogram is the amount ofmass in a 2.2 pound weight

Three Examples of Inertia

Newton's Laws of Motion

                   

 Isaac Newton (1642-1727)

One newton is theapproximate weightof a cube of butter.

                           

     

Motion tends to continueunchanged.

The elephant at rest tendsto remain at rest.

Newton's First Law

Newton's Second Law:  Acceleration = Force / mass

 a = F / m                   

                    

          2 a = 2 F / m

                     

          a = 2 F / 2 m

Adding and Subtracting Forces

                                                          

    

Equilibrium Means "Zero Acceleration"

                                

   Forces in balance:  Equilibrium

            

     Equilibrium

                            

    

Forces are in balance, so block is in equilibrium, moving at constant speed

(forces acting on the same object)

Acceleration Due to Gravity

                         

  

Gravitational force (weight)is proportional to mass. Double the mass and thegravitational force will bedoubled also. Ratio of weight to mass isalways the same:  g

Gravitational force isproportional to mass.

Ratio F / m is alwaysthe same:  g

Weight

The weight of an object is theforce of the earth's pull and isgiven by the equation

Weight = m g

Example: A cube of butter has a mass ofabout 0.1 kg.

weight = mg            = 0.1 kg x 10 m/s2

           = 1 N

Terminal Speed

                      

     

Air resistance increases asthe speed increases. Eventually, the force R ofair resistance becomes equal to the force exertedby the earth, and theobject reaches equilibrium.

Newton's Third Law of Motion:  Action-Reaction 

                      

         

Whenever one object exertsa force on another object,the other object exerts an equal but opposite force Note:  each of the two forces in the pair acts on a different object.

Hammer pushes on stake.Stake pushes on hammer.

The hammer acts, thestake re-acts.

   

More Action-Reaction Pair Examples

                                                          

      

                                                          

      

 

If action-reaction forces areequal but opposite, why don't they cancel?