For every_____________ , there is an equal and opposite____________. Newton’s ______ Law: In plain words: For every_____________ , there is an equal and opposite____________. 3rd action reaction In physics speak: If object A exerts a __________ (the action) on object B, then object B exerts a ___________ (the reaction) that is ________________________ but __________________________ on object A.” force F force –F equal in magnitude opposite in direction A F B -F reaction action

action: rocket pushes gases backward re-action: gases push rocket forward

action: runner pushes ground backward re-action: ground pushes runner forward

F -F Ex. Punching things: Action-reaction forces are ______________ in Case 1: punching paper force of paper on fist: force of fist on paper: -F F Which object exerts a greater force? neither Case 2: punching a cement block force of block on fist: force of fist on block: -F F Which object exerts a greater force? neither Action-reaction forces are ______________ in case 2, but both cases are opposite and _________ . greater equal

 Forces ALWAYS occur in __________ .  Either force could be called the ___________.  One force does not happen _____________  Both forces occur at the ___________________. pairs action “first.” same time A/ Both forces are the exact same ___________ . type Ex 1: F: Earth ________ you down with _________. -F: You _______ Earth up with _________. pulls gravity pull gravity Notice the "same types:" Fg Both forces are ____________ . Both forces are ____________ pulls Fg gravity

F: You ________ right on wall with a __________________. Ex 2: F: You ________ right on wall with a __________________. -F: Wall __________ left on you wall push normal force hand pushes normal force. FN FN Ex 3: F: Wire ________ up on picture with a _______________. -F: Picture _________ down on wire with a________________. pulls tension pulls T tension T

B/ Action:reaction pairs act on ______________ different B/ Action:reaction pairs act on ______________ objects in _____________ directions.  Action: A exerts a F on B.  Reaction: ___ exerts a -F on ____. opposite B A subject object direction Ex 1: F: Earth pulls you down with gravity. -F: ______ pull _______ ____ with gravity. You Earth up opposite ____________ direction subject object Notice how subject and object are _____________ , and the directions are ________________ . reversed opposite

Ex 2: F: You push right on wall with a normal force. -F: ______ pushes ______ on ______ with a normal force. left Wall you Ex 3: F: Wire pulls up on picture with a tension. -F: __________ pulls _________ on ________with a tension. Picture down wire In Sum: To find a reaction force, re-write with: 1. the same _________ of force (push/pull and gravity/normal/tension, friction, etc); but, 2. reverse _____________ and _____________ and the _______________ of the force. type subject object direction

in ___________ direction on ___ (object) with _____________force." B Action: Reaction: "A (subject) pulls/pushes in a certain direction on B (object) with a certain force." " ____ (subject) _________________ in ___________ direction on ___ (object) with _____________force." B pulls/pushes opposite A the same Which force, action or reaction, has a greater magnitude? neither

Ex: Write the action (A) and reaction (R) pairs. block table Case 1/ Between block and table: A: R: Block pushes down on table with normal force. Table pushes up on block with normal force. Case 2/ Between block and Earth: A: R: Earth pulls down on block with gravitational force. Block pulls up on Earth with gravitational force. Which of these are contact forces? Which are "at a distance" forces? case 1 case 2

Ex: Weight is the force of Earth’s ___________ pulling ______________ on an object: w = ____ = Earth pulls__________ on cat with a ________________ force. gravity downward w Fg downward w gravitational w What is the reaction force to a cat's weight, w? w The _______ _______ _______ on ________ with a __________________ force.” cat pulls up w Earth gravitational

m forces Ex. Don’t confuse ___________ (which are always equal and opposite) with ___________________ (which depend on _________ and usually are NOT equal and opposite). accelerations mass Ex: An apple and the Earth (not to scale) F gravitational F of Earth on ________ apple: a = 9.81 m s2 = apple m gravitational F of apple on _________ F Earth Earth: a = m ≈ m's Same _____ but different _____ b/c different _____ ! F's a's