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Newton’s 3rd Law. For every ACTION there is an EQUAL and OPPOSITE REACTION. But what does that mean?. “Every action” Any force that Object A exerts on Object B “Equal and Opposite” same amount in the reverse direction “Reaction” The force that Object B exerts on Object A.

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Newton’s 3rd Law


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newton s 3rd law
Newton’s 3rd Law
  • For every ACTION there is an EQUAL and OPPOSITEREACTION
but what does that mean
But what does that mean?
  • “Every action”
    • Any force that Object A exerts on Object B
  • “Equal and Opposite”
    • same amount in the reverse direction
  • “Reaction”
    • The force that Object B exerts on Object A
so let s restate the law
So let’s restate the law:
  • EVERY TIME an object (A) exerts a force on another object (B), object (B) exerts an equal force in the opposite direction on object (A) AT THE SAME TIME
  • Note- it does NOT matter which object is A or B (or which you call action or reaction)
  • Is this ALWAYS TRUE?
    • YES, ALWAYS
exceptions to this law
EXCEPTIONS TO THIS LAW:
  • Um, um, um, um…
  • Let’s see…
  • Let me think…
did you see those exceptions
Did you see those exceptions?
  • That’s all of them!
painful example
Painful Example
  • You slam your fist into the wall.
    • Your hand exerts a LARGE force on the wall

LARGE FORCE THAT YOUR HAND PUSHES THE WALL

painful example continued
Painful Example (continued)
  • You slam your fist into the wall.
    • The wall exerts a LARGE force on your hand

LARGE FORCE THAT THE WALL PUSHES YOUR HAND

painful example continued11

LARGE FORCE THAT YOUR HAND PUSHES THE WALL

LARGE FORCE THAT THE WALL PUSHES YOUR HAND

Painful Example (continued)
  • See Action/Reaction PAIRS AT THE SAME TIME
so what do we know from experience
So what do we know from experience?
  • The harder you hit the wall, the harder the wall hits you
  • The gentler you hit the wall, the gentler the wall hits you
  • Is this ALWAYS TRUE?
    • YES, YES, YES!
how can we tell what the action reaction pair is
How can we tell what the action-reaction pair is?
  • Let’s take a closer look!
  • The action:
    • YOUR HAND pushes THE WALL to the LEFT

(Replacing “hits” with “pushes”)

  • The REaction:
    • THE WALL pushes YOUR HAND to the RIGHT
slide14
So…
  • If you switch the words around, you have the action-reaction pair!
  • Make sure to change the direction, too.
does it matter which is the action or reaction
Does it matter which is the action or reaction?
  • If the action is:
    • YOUR HAND pushes THE WALL to the LEFT
  • Then the reaction is:
    • THE WALL pushes YOUR HAND to the RIGHT

Or

  • If the action is:
    • THE WALL pushes YOUR HAND to the RIGHT
  • Then the reaction is:
    • YOUR HAND pushes THE WALL to the LEFT
faq s
FAQ’s:
  • Which happens first?
    • BOTH HAPPEN SIMULTANEOUSLY!
  • Do these forces “cancel out” each other (or lead to balanced forces)?
    • NO, NO, NO, But let’s see…
a look at canceling out
A Look at Canceling Out
  • The apple is pulling the orange (on the cart) to the right
  • We now know that (use 3rd Law here):
  • The orange (on the cart) is pulling the apple to the left.

You can’t balance a force on the orange with a force on the apple. Only one of the forces in any action-reaction pair (pear?) acts on each object.

a closer look
A closer look
  • This is the only force on the apple from the previous action-reaction pair.
  • This is the only force on the orange from the previous action-reaction pair

Force on apple by orange

Force on orange by apple

a closer look continued
A closer look (continued)
  • Another horizontal force on the apple is:
  • It is the NET FORCE on the apple which will determine its motion.
  • Just as it is the NET FORCE on the orange which will determine its motion.

Force on apple by orange

Force of Friction on the apple

Force on orange by apple

NO Force of Friction HERE DUE to FRICTIONLESS WHEELS

a closer look continued20
A closer look (continued)
  • SINCE the NET FORCE on the apple is ZERO the apple will NOT ACCELERATE!
  • SINCE NET FORCE on the orange is NOT ZERO the orange will ACCELERATE!

Force on apple by orange

Force of Friction on the apple

Force on orange by apple

NO Force of Friction HERE DUE to FRICTIONLESS WHEELS

do you really get it
Do you really get it?
  • What OTHER ACTION-REACTION PAIRS exist other than the orange/apple pair?
  • The force of friction is really the floor pushing the apple to the right, then the reaction is:
    • The apple pushing the floor to the left!

Force on apple by orange

Force of Friction on the apple by the floor

do you really get it22
Do you really get it?
  • EVERY FORCE IS PART OF A ACTION/REACTION PAIR
  • ONLY ONE FORCE FROM A PAIR CAN ACT on a SINGLE OBJECT!
  • The NET FORCE on an object is found by the FORCES THAT ACT ON THAT OBJECT ONLY.