Transcript Chapter 1 Structure and Bonding

Chapter 4 Newtons 3rd Law
I.
Newtons 3rd Law of Motion
A.
Every force is opposed by an equal and opposite force
1) Action/Reaction Principle
2) Force = interaction of objects, must have equal and opposing partner
3) Only the force acting on the object affects its motion
a) F2 acts on you; -F1 acts on the chair
b) F2 affects your motion; -F1 affects the chair’s motion
B.
Analyzing Forces
1) Identify all interacting objects (draw a picture)
a) Weight
i. +W = earth pulling down on all objects
ii. -W = all objects pull up on earth
b) Normal Force = force perpendicular to
surface contacts
i. Table pushes up on the book -N
ii. Book pushes down on the table +N
2)
C.
What are the forces on the book? +W and –N
+W = -N
a) Ftotal = ma
a = 0, then Ftotal = 0 = W – N = 0
b) Earth has one force acting on it (-W)
Don’t affect book’s motion
c) Table has one force acting on it (+N)
d) +W and –N are not a 3rd Law pair, they just happen to be equal and
opposite (that’s how the book sits still)
e) If +W and –N are not equal, the book would be accelerating
How can we ever move anything?
1) All forces have opposite and equal partners
2) Only the forces acting on an object affect its motion
a) Cart: friction, weight,
normal, mule
b) Mule: friction, weight,
normal, cart
c) Mule will move if
sum of forces aren’t 0
d) Cart will move if sum
of forces aren’t 0
The only time objects don’t move is when all forces total zero!
II.
Applications of Newton’s Laws
A.
Acceleration of a car
Friction (f) of the tires on the road
Is opposed by –f, the road pushing the car
B.
Moving a Chair
W, N, P, f are the forces
Treat vertical and horizontal separately
C.
Skydiving
1) Why don’t we fall continually faster?
2) Air Resistance (R) is a special case of friction
3) Mass and Velocity determine R
4) Terminal Velocity = final speed due to
a) W = R
b) ~100-120 mph for a skydiver
D.
Thrown Ball
1) P, W, R to consider
2) Throw: P is major force
3) Flies: W, R only
E.
Connected Objects
1) Treat as one object: just like other examples
2) Treat as two objects: consider m1 and m2 and F1 and F2
FT = 36 N
m1 = 10 kg
m2 = 8 kg
a=?
F1 = ?
F2 = ?