Chapter 4 Newtons 3rd Law

I. Newtons 3rd Law of MotionA. Every force is opposed by an equal and opposite force

1) Action/Reaction Principle2) Force = interaction of objects, must have equal and opposing partner3) 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 Forces1) Identify all interacting objects (draw a picture)

a) Weighti. +W = earth pulling down on all objectsii. -W = all objects pull up on earth

b) Normal Force = force perpendicular to surface contacts

i. Table pushes up on the book -Nii. Book pushes down on the table +N

2) 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)

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

C. 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 0The only time objects don’t move is when all forces total zero!

Don’t affect book’s motion

II. Applications of Newton’s LawsA. 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 a = ?m1 = 10 kg F1 = ?m2 = 8 kg F2 = ?