3rd Law lecture with lots of demos.
Inanimate objects can exert forces.
. Pull back fingers same result as leaning against a wall.
These forces are elastic, the bonds actually stretch.
. Demo with rubber band
. How does floor know how much force to exert to balance weight? Check to see if floor "stretches"
. Demo with laser pointer and mirror showing that wall actually does move when pushed.
Newton's 3rd Law Recipe
. Forces are interactions, they come in pairs
. Demo with finger in water
Newton's 3rd Law: If Object A exerts a force on Object B, then Object B exerts a force on Object A that is equal in magnitude and opposite in direction. There is NO time lag between these forces.
. Demo with force sensors. You can't pull harder on me than I pull on you
. Demo with tug-of-war. The person who wins the tug-of-war is not the person who pulls harder, it is the person who pushes harder against the floor.
. Drop apple, why does apple fall down and Earth not fall up - Newton's 2nd Law
Magic Tube Demo
. Tube pushes up on ball, ball pushes down on tube
Went over how to identify action reaction pairs of forces as opposed to forces that balance through Newton's 1st Law.
Action/Reaction forces will NEVER appear in the same free-body diagram and can NEVER cancel since they don't act on the same object.
Hand out CD 6-1 - due last time on Monday.
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