Download Ch 6 Newton`s 3rd Law Notes

Name__________________________________Block_____Date____________________
Ch 6 Newton’s 3rd Law Notes
Mrs. Peck
6.1 Forces and Interactions
interaction- a mutual action between objects where each object exerts an equal and
opposite force on the other
force- a push or pull
a force never occurs by itself...but is always part of another equal and opposite force
6.2 Newton’s Third Law
Newtons 3rd law- whenever one object exerts a force on second object, the second object
exerts an equal and opposite force on the first object.
3rd law is the law of action & reaction
action force - on of the pair of forces described in Newton’s third law
reaction force - the force that is equal in strength and opposite in direction to the action
force, which acts simultaneously on whatever is exerting the action force
does not matter which force is designated the action or reaction force...they are partners in a single
interaction
forces are equal in strength and opposite in direction
“for every action there is an equal and opposite reaction”
without an action force there cannot be a reaction force, and therefore .....no resulting forward motion
? A car accelerates down the road. What is the force that moves the car?
6.3 identifying Action and Reaction
identify the interaction pair: object A interacts with object B
Action: Object A exerts a force on object B (direction)
Reaction: Object B exerts a force (equal and opp. direction) on object A
Action and Reaction occur simultaneoulsy
? What is the interaction pr between the Earth and you?
2
6.4 Action and Reaction on Different Masses
Acceleration = force
mass
Newton’s 2nd Law of Motion
acc. ~
1
mass
For an interaction pair:
The forces (both action and reaction) are equal and opposite in direction
The masses are different and inversely proportional to acceleration
A given force exerted on a small mass produces a greater
acceleration than the same force exerted on a large mass.
if one of the masses were extremely large compared to the other, its
acceleration would be imperceptible.
? Apply Newton’s third law to a tug-of-war. If the action is you pullling on the rope, is the reaction
force the ground pushing back on you or your opponents pulling back on the rope?
6.5 do Action and Reaction Forces Cancel? (not necessarily)
If action and reaction forces are internal to a system, they cancel each other and produce no
acceleration of the system.
Action and reaction forces do not cancel each other when either is external to the system being
considered.
Fig 6.10 consider the orange as a separate system and the apple is external to that system.
1. The apple pulls on the orange
this force affects the orange and causes it to accelerate toward the apple
2. the orange pulls back on the apple w/ = & opp force (which is external to the system)
this force affects the apple but not the orange