Welcome back to Physics 211

1
Welcome back to Physics 211

• Todays agenda
• More on Newtons Laws
• Free Body Diagrams

2
Relationship between Mass, Weight and Gravity
Mass

• scalar quantity that describes an objects
  inertia
• describes the amount of matter in an object
• intrinsic property of an object
• tells us something about the object, regardless
  of where the object is, what its doing, or
  whatever forces may be acting on it

3
Gravity
Somewhat more loosely, gravity is a force that
acts on mass. When two objects with masses m1 and
m2 are separated by distance r, each object pulls
on the other with a force given by Newtons law
of gravity.
4
Gravity cont.
where G 6.67 x 10 -11 Nm2/kg2
near Earth
5
Example Problem 41

• Sam, whose mass is 75 kg, takes off across level
  snow on his jet-powered skis. The skis have a
  thrust of 200 N and a coefficient of kinetic
  friction on snow of 0.10. Unfortunately, the skis
  run out of fuel after only 10s.
• What is Sams top speed?
• How far has Sam traveled when he finally coasts
  to a stop?

6
Chapter 7. Newtons Third Law

• Topics
• Interacting Objects
• Analyzing Interacting Objects
• Newtons Third Law
• Ropes and Pulleys
• Examples of Interacting-Object Problems

7
Interacting Objects
If object A exerts a force on object B, then
object B exerts a force on object A. The pair of
forces, as shown, is called an action/reaction
pair.
8
Newtons 3rd Law
9
Physics Education Research
fact that a large majority can recite Newtons
third law when they enter the class. Halloun and
Hestenes (1985b) have characterized student
beliefs about interactions in terms of a
dominance principle the larger (or faster or
more active) object exerts a larger force than
the smaller (or slower or less active) object.
Students tend to view an interaction as a
conflict in which the stronger wins. Its not
hard to understand how this common-sense view
comes about. After all, the effect of the
collision on the compact car is much larger than
its effect on the truck.
10

• Some of the more specific difficulties students
  have with Newtons third law and with interacting
  objects are
• Students dont believe Newtons third law. Its
  too contrary to common sense.
• Students have difficulty identifying
  action/reaction force pairs
• They match two forces on the same object.
• They place forces on the wrong objects.
• They dont believe that long-range forces (e.g.,
  gravity) have reaction forces.
• Students confuse equal force with equal
  acceleration.

11
Students dont understand tension

• They think that tension is the sum of the forces
  exerted at the two ends of a string.
• They think that tension exerts a force only in
  the direction of motion.
• They think that tension can pass through an
  object to another string on the other side.
• Students often dont recognize that objects
  connected by an inextensible string must have
  accelerations of equal magnitude.

12
Demos

• Pasco cars
• Equal masses
• Unequal masses
• 1 student cart medicine ball
• 2 students cart rope

13
Ropes, pulleys, strings and springs

• Unless otherwise stated, all of these are
  massless and frictionless.
• These automatically accelerate with the
  connecting objects.
• These can be ordered from special imaginary
  physics equipment catalogs.

14
How does the tension in string (a) compare to
that in string (b)?

• Ta gt Tb
• Ta lt Tb
• Ta Tb
• Not enough information

15

• T1 gt T2
• T1 lt T2
• T1 T2
• Not enough information

16
Block A is held in place. How does the tension in
the section of the string tied to block A compare
to the section tied to block B?

• Ta gt Tb
• Ta lt Tb
• Ta Tb
• Not enough information

17
Block A is released. How does the magnitude of
the acceleration of block A compare to that of
block B?

• aa gt ab
• aa lt ab
• aa ab
• Not enough information

18
Block A is released. How does the tension in the
string now (T2) compare to the tension (T1) when
block A was held in place?

• T1 gt T2
• T1 lt T2
• T1 T2
• Not enough information

19

• Ch.7 32
• The 1 kg. block (m1) is tied to the wall with a
  rope. It sits on top of the 2 kg block. The lower
  block is pulled to the right with a tension force
  of 20 N. The coefficient of kinetic friction at
  both the lower and upper surfaces of the 2 kg
  block is µk 0.40.
• What is the tension in the rope holding m1?
• What is the acceleration of m2?

m1
m2
20
Demo Pulleys
2 pulleys 2T W F T W/2
T
F
N pulleys F W/N!
W
21
FHW7 Due F 10/16/09

• Ch.6 EP 1, 5, 18, 20, 21

WHW8 Due F 10/21/09

• Ch.6 EP 29, 37, 46 Ch. 7 CQ 12