Welcome back to Physics 211

1
Welcome back to Physics 211

• Todays agenda
• OH WF 3 4 Room 329
• Ch. 8 (please read)
• Ch. 9 sections 9.3 through 9.6
• Impulse and Momentum
• HW ch 8 due Friday 10/31
• Ch. 8 EP 10, 13, 20, 44

2
Car Turning

• Flat road exactly like 46, except f replaces T
• Banked road?? Do a FBD!

3
Projectile Motion and Circular Orbits
4
More on g later

• Cannot use g to calculate accelerations far
  earth surface
• Well show that g depends on distance between
  objects

5
Fictitious Forces

• Car turns, you push on door.
• Car stops, you are pushed into dashboard
• ? Arise from riding along in a non-inertial
  reference frame.
• Earth IS a non-inertial reference frame!
• Its spinning, i.e. accelerating.

6
Objects Moving in Circles

• Normal forces
• Tension forces
• Frictional forces

7
36 text

• D 5m
• 0.6 lt µ s lt 1.0
• 0.4 lt µ k lt 0.7
• mmin 30 kg
• Minimum angular speed in rpm?

8
42 text

• Tower 9 m high
• ? Chain 9 m long
• ? 1 rev/4s
• mmax 150 kg
• Good deal on chain Tmax 3000N

9
WHW11

• Ch 8 23, 25, 31
• Ch 9 7, 12

10
Impulse

• Force F2 on 1 acting on object 1 due to object 2
  for a time ?t yields an impulse
• J F2 on 1 ?t
• In general, for a time varying force need to use
  this for small ? t and add
• J ? F(t) ?t

11
Impulse for time varying forces
F(t)
area under curve equals impulse
t
t2
t1
12
Impulse ? change in momentum

• Consider first constant forces ...
• Constant acceleration equation
• vf vi at
• mvf - mvi mat
• If we call p mv momentum we see that
  ?p

13
Definitions of impulse and momentum
Impulse imparted to object 1 by object 2 J F2
on 1 ?t

• Momentum of an object
• p mv

14
Impulse-momentum theorem
J ?p

• The net impulse imparted to an object is equal to
  its change in its momentum.
• Note impulse and momentum are vectors.

15
Impulse demo

• Cart equipped with force probe collides with
  rubber tube
• Measure force vs. time and momentum vs. time
• Find that integral of force curve is precisely
  the change in p!