Newton’s ______ Law: In plain words: For every_____________, there is an equal and opposite____________. 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.” A B F -F actionreaction action reaction 3rd force F force –F equal in magnitude opposite in direction

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

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

NP View of Earth from North Pole: walking east force of foot on ground Will Earth speed up or slow down? What if they walk west? force of foot on ground counter clockwise rotation

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

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

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

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

Ex 2: F: You push right on wall with a normal force. -F:______ pushes ______ on ______ with a normal force. Ex 3: F: Wire pulls up on picture with a tension. -F: __________ pulls _________ on ________with a tension. Wall left you Picturedown 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

"A (subject) pulls/pushes in a certain direction on B (object) with a certain force." Action:Reaction: " ____ (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: Case 2/ Between block and Earth: A: R: Which of these are contact forces? Which are "at a distance" forces? Block pushes down on table with normal force. Table pushes up on block with normal force. Block pulls up on Earth with gravitational force. Earth pulls down on block with gravitational force. case 1 case 2

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

Ex. Don’t confuse ___________ (which are always equal and opposite) with ___________________ (which depend on _________ and usually are NOT equal and opposite). Ex: An apple and the Earth (not to scale) gravitational F of Earth on ________ gravitational F of apple on _________ apple: a = 9.81 m s 2 m ≈ forces accelerations mass m F = Earth: a = F 0 Same _____ but different _____ b/c different _____ ! F's a 's m 's apple Earth

Open your 3-ring binder to the Worksheet Table of Contents. Record the title of the worksheet: Newton’s 3 rd Law WS

box Earth

box Earth

50 N 100 N

80 N50 N 100 N

When determining if an object accelerates, you do NOT add in the reaction forces.