1. Newton’s third law
Forces always come in pairs, one on each object in the pair. The magnitudes of the forces in a pair are always ________, and the direction is _____________.
(They do not cancel each other…because they are on different objects)
Demo: carts and ropes

2. Newton’s third law
Show all the third law force pairs and objects in this problem
Which two forces happen to be equal and opposite but are not a third law force pair? How do we know they are not?

3. Newton’s third law
P1. The car accelerates forward when the gas pedal is pushed.
The force that actually accelerates the car is the force of:
A. the car pushing backward on the road
B. the car pushing forward on the road
C. the road pushing forward on the car
D. the road pushing backward on the car
Hint: the acceleration of the car must be in the same direction as the total force on the car.

4. How airplanes fly: 3rd law
Thrust
Lift

5. How rockets work: 3rd law
No atmosphere to push on…

6. Coupled objects accelerating
Two blocks, 3 kg and 6 kg, accelerate at 2m/s2.
What is the pushing force, and the contact force between the blocks?

7. Coupled objects accelerating
Method 1:
FBD for each object
Newton’s 2nd law for each FBD
Solve for unknowns

8. Coupled objects accelerating
Method 2:
FBD for the group
Newton’s 2nd law for each the group
FBD and 2nd law for one object to forces internal to the group

9. Pulleys
What external forces on this group contribute to the acceleration?
P2. As the masses accelerate, the tension in the rope is ____ mBg
A. less than
B. the same as
C. more than
D. none: tension is zero

10. Pulleys
What external forces on this group contribute to the acceleration?
P3. Assuming g = 10 (not 9.8), the net exterior force that accelerates this 2- mass system is
A. 20 N
B. 70 N
C. 90 N
D. 160 N

11. P5. A horse pulls a wagon that is much heavier than it is, and they accelerate together. Which is true?
a) the horse pulls harder on the wagon than the wagon pulls on it.
b) the horse has a forward force on it from the ground that is greater than the wagon’s force on the horse.
c) the wagon pulls harder on the horse than the horse pulls on it

12. Newton's Laws Lyrics by Walter F
Newton's Laws Lyrics by Walter F. Smith & Marian McKenzie Tune: Funiculi, Funicula by Luigi Denza (1880) Midi file
“A thing at rest will stay at rest,” says Newton. That’s his first law. That’s his first law. And some point out this only is transmutin’ His second law, but that’s no flaw. ‘Cause Newton’s second law says F is m a If there’s no force, no a of course! Acceleration zero means we can say No delta-v, so there you see! Chorus tune: Velocity depends upon your frame. At rest or moving, they’re really just the same! A tree for me stands still you see But from a train it moves with v! Newton’s His laws are good for you and me!

13. Newton's Laws Lyrics by Walter F
Newton's Laws Lyrics by Walter F. Smith & Marian McKenzie Tune: Funiculi, Funicula by Luigi Denza (1880)
Each action has an opposite reaction” -- Law number three confuses me. Both opposite and equal, this abstraction What does it mean? It seems obscene! Well, here’s the key that gives you knowledge ample: To be exact, the forces act On different objects and here’s an example: If I touch you, you touch me too! Chorus tune: If a horse starts pulling on a cart, The cart pulls back on the horse in equal part. The horse, of course, has no remorse, Because it feels a forward force From the ground upon the horse, As Isaac Newton would endorse!

14. P6. If you pull on the right hand rope to accelerate the blocks, which rope is most likely to break?
d. all the same

15. Lecture 6, third law and coupled objects
Basic concepts:
3rd law force pairs
Love your neighbor as yourself
Tension in rope stays the same across a low-mass pulley
Basic problems, skills:
Free body diagrams with 3rd law pairs
Advanced concepts:
Internal forces are determined by the masses they accelerate
Advanced problems, skills:
Finding between objects in a group