9/26 2nd and 3rd Law

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9/26 2nd and 3rd Law

• Text does not stress the 3rd Law but we will
• HW 9/26 Three Blocks due Monday 9/30
• Two Handouts on 2nd and 3rd Laws up frontOne is
  a worksheet we will do today
• Today we will practice using the 2nd and 3rd Laws
  (worksheet)
• Friday we will do a worksheet on friction

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Newtons Third Law

• All forces come in pairs. Here the car pushes on
  the truck and the truck pushes the opposite
  direction on the car, with the same size force!

subscripts are the key!!!
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Mike is stuck in the middle of a frozen lake
holding a large bag of gold. The surface is
perfectly frictionless so you cant walk. How
does Mike get off the ice?
gold
Mike
Throw the gold in some horizontal direction!
Equal and opposite, TWO SEPARATE FBDs!!!, and
switched subscripts
3rd Law
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Well, what is the rocket pushing on?

• What THING is pushing on the rocket?
• Is this a contact or non-contact force?

The rocket pushes the exhaust gasses down so...
The exhaust gasses push the rocket up.
Contact
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Example
Elevator moving down and slowing down.
The magnitude of the elevators acceleration is
2m/s2. Let g 10N/kg.
Find the normal force that the floor puts up on
the block
But first find the direction of acceleration
Draw FBD
Adjust the arrow length(s)
Apply 2nd law
N - W ma
5kg
N - 50 5(2) 10
El
N 60N
a
mg 5(10) 50N
Fnet
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Example
The magnitude of the elevators acceleration is
2m/s2. Let g 10N/kg.
Note the danger in calling g an acceleration!!!!
g tells us the Weight force on an object and is
not an acceleration.
But first find the direction of acceleration
NF,B 60N
5kg
a
mg 5(10) 50N
Fnet
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Example
Now find the Tension force that the rope puts up
on the elevator, using both the 2nd and 3rd Laws.
Elevator moving down and slowing down.
The magnitude of the elevators acceleration is
2m/s2. Let g 10N/kg.
Fnet,El
mEla 200N
T-N-W 200
mEl 100kg
T-60-1000 200
1260N
5kg
El
B
a
WE,B
50N
Fnet
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Example Try this
Moving up and slowing down
Draw FBD,s of both blocks consistent with the
2nd and 3rd Laws.
Then rank the magnitudes of all the forces.
2m
m