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Think!Why do objects ofvarious masses fall withequal accelerations?
Gravity• Gravity acts on all objectson the earth’s surface andcauses them to acceleratewhen released.• It is known as theacceleration due to gravity(g). It is the same for allobjects on earth and is9.8 m/s2
Circular Motion• Gravity also affectsthe planets.• Sun’s gravitationalpull keeps them inorbit.
Think!• If the object in uniformcircular motion isaccelerating towardsthe center of the circle,why does it maintain acircular path at aconstant radius andnever get closer to thecenter of the circle?
ForcesIdentify theforces presentwhen a personsits on a couch.NormalWeight
ForcesIdentify the forces present on acar moving at constant velocity.ForwardFriction
Question!Are the forces exertedon the same object?On which objects arethe forces exerted?
ForcesWhen a person sits on a couch…The couch exerts normal force on the person.The person’s weight exerts a force on the couch.
Forcesa car moving at constant velocity…The car exerts a force on the ground to move forward.Friction tries to keep the car’s tires from moving.
InteractionNewton’s 3rd Law
Newton’s 3rd LawWhen one object exerts aforce on a secondobject, the second exerts anequal force in the oppositedirection on the first.
Newton’s 3rd LawFor every action,there is an equal andopposite reaction.
Newton’s 3rd Law• The forces exerted by two objects oneach other are often called andaction-reaction force pair.• Either force can be considered theaction force or the reaction force.• Action and reaction force pairs don’tcancel because they act on differentobjects.
Action and Reaction ForcesThorCapt. America
Action and Reaction Forces
Action and Reaction Forces
For everyaction, there is anequal and oppositereaction.
Newton’s 3rd Law vs. Balanced ForcesWhat arethe forcesactingon the cat?Pull ofGravityNormalForce
Newton’s 3rd Law vs. Balanced ForcesAre theforcesbalanced?Pull ofGravityNormalForceYES!(The cat is at rest…or resting). Forces are acting ON THE SAME object.
Newton’s 3rd Law vs. Balanced ForcesWhat is theaction-reactionforce pair?Cat’s weight onthe groundGround’s pushon the cat
Newton’s 3rd Law vs. Balanced ForcesAre the forcesequal and inoppositedirections?Cat’s weight onthe groundGround’s pushon the catForces are acting ON DIFFERENT objects.YES!(The cat is at rest…or resting).
How can kitty jump?Gettingtired ofsitting..
Newton’s 3rd Law vs. Balanced ForcesA greater forceexerted on theground by the cat(action) wouldresult to a greaterforce exerted onthe cat by theground (reaction).Cat’s pushon groundGround’spush on cat
Newton’s 3rd Law vs. Balanced ForcesA greater force exerted on the air by the bird(action) would result to a greater force exertedon the bird by the air (reaction).
Newton’s 3rd Law vs. Balanced ForcesIdentify the action-reaction force pair on USS Enterprise.
Newton’s 3rd Law vs. Balanced ForcesAs the gasmolecules collidewith the enginewalls, the wallsexert a force thatpushes them out ofthe bottom of theengine.Engine’s pushon fuelFuel’s push onengine
Question!Suppose a friend who hears aboutNewton’s law says that you can’t movea ball by kicking it because thereaction force by the kicked ball wouldbe equal and opposite to your kickingforce. The net force would be zero, sono matter how hard you kick, the ballwon’t move! What do you say to yourfriend?
AnswerNFgForces actingon the ballare balanced.
AnswerNFgAction-reaction forces are equal (and opposite).
AnswerNFgThe ball will move!
Question!A bug is flying at 1 m/s andtruck is traveling in the oppositedirection at 40 m/s. When thetwo collide, on which is theforce of the collision greater?Neither! Both will experience the sameamount of force.
Question!When a 0.5kg bullet is fired from an8kg rifle, the action-reaction pair is therifle pushing the bullet forward andthe bullet pushing the gun backward.If the force from the blast causes therifle to accelerate to 125 m/s^2, whatis the acceleration of the bullet?