1. 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 front One 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

2. Newton’s 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!
NT,C
NC,T
subscripts are the key!!!
truck on car
car on truck

3. Mike is stuck in the middle of a frozen lake holding a large bag of gold. The surface is perfectly frictionless so you can’t walk. How does Mike get off the ice?
gold
Mike
Throw the gold in some horizontal direction!
NG,M
NM,G
Equal and opposite, TWO SEPARATE FBD’s!!!, and switched subscripts
3rd Law

4. What THING is pushing on the rocket?
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

5. Example: Elevator moving down and slowing down.
The magnitude of the elevator’s 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
NF,B
N - W = ma
5kg El
N - 50 = 5(2) = 10
WE,B
N = 60N
a =
= mg = 5(10) = 50N
Fnet =

6. Example:
The magnitude of the elevator’s 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 El
5kg
WE,B
a =
= mg = 5(10) = 50N
Fnet =

7. 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 elevator’s acceleration is 2m/s2. Let g = 10N/kg.
Fnet,El =
mEla = 200N
T-N-W = 200
TR.El = ?
mEl = 100kg
T = 200
NEl,B = 60N
1260N
5kg El
WE,El = 1000N
NB,El = 60N B
a =
WE,B
= 50N
Fnet =

8. 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