# Physics 140 Fall 2004 by liuqingyan

VIEWS: 6 PAGES: 16

• pg 1
```									Racquetball Striking a Wall                                   Copyright: Loren M. Winters

AP Physics C                           Prof. Tim McKay
University of Michigan

Andrew Davidhazy         Mt. Etna
Quick recap
• 1st Law: If motion          • 2nd Law: Quantifies
doesn‟t change, no            forces as
    
 F  ma
net force is acting
– Really a statement of
what has to be
explained                 • Mass is a measure of
• 1st Law: If motion            inertia: how hard it is
changes, it is                to change the motion
because of an                 of an object
unbalanced force          Pay attention to the forces
and you can predict motion
Another Newtonian insight: forces
always come from interaction
• Forces don‟t come            Flift
from nowhere
• In fact, all forces                  
emerge from                          v
interactions between
two objects
• In our suitcase:
msg
– Earth and suitcase (W)
– You and suitcase (Flift)
Newton‟s third law
• Forces always appear     • 3rd Law: “For every
in pairs, in               action there is an
interactions               equal and opposite
• When you pull up on        reaction”
the suitcase, it pulls   • If object A exerts a
down on you                force on object B
• When the Earth pulls       (FAB), then object B
down on the suitcase,      exerts and equal and
the suitcase pulls up      opposite force on
on the Earth               object A (FBA)
       
FAB   FBA
Third law thought experiments
• Horse pulls a stone        • Newton: two magnets
with a rope. The rope        with paper in between
stretches and will do
anything to be
A   B
shorter. So it pulls the
stone forward, and
the horse back.
These pulls are equal      • If FAB was bigger than
and opposite because         FBA, the whole thing
the rope doesn‟t know        would accelerate off
which way to pull…           to the left….
How can the 3rd law be true?
• Throwing a ball
       
FPB   FBP
• Why does the ball fly
off, and the person
remain still?
• Free-body diagrams                                FBP
FPB
help reveal this!
• Extra forces (friction)   NOT drawn after
act on the person…        the throw, but
during…
FFP
A locomotive pulls a series of wagons. Which
is the correct analysis of the situation?
1. The train moves forward because the locomotive pulls
forward slightly harder on the wagons than the wagons pull
backward on the locomotive.
2. Because action always equals reaction, the locomotive
cannot pull the wagons the wagons pull backward just as hard
as the locomotive pulls forward, so there is no motion.
3. The locomotive gets the wagons to move by giving them a
tug during which the force on the wagons is momentarily
greater than the force exerted by the wagons on the locomotive.
4. The locomotive‟s force on the wagons is as strong as the
force of the wagons on the locomotive, but the frictional force on
the locomotive is forward and large while the backward frictional
force on the wagons is small.
5. The locomotive can pull the wagons forward only if it weighs
more than the wagons.
In a tug-of-war two groups of students pull
as hard as they can on one another in
an effort to pull the other team across
the line. If the physics students team
wins over the history students team, it is
because:

1. The physics team pulled
harder on the history team
2. The history team pulled harder
on the physics team
3. Neither
More complicated free body diagram example

NT=FTable-on-Book
Book

Book                              WB=FEarth-on-Book
Table
FBook-on-Table

Table

Earth
WT=FEarth-on-Table

NF=FFloor-on-Table
FNTable-on-Floor

FgBook-on-Earth              FgTable-on-Earth
Earth
Some classifications for forces
• Two different              • Contact and non-
dialectics…                  contact forces
• Active and Passive           – Contact: surfaces
Forces                         actually touch (most
forces we encounter;
– Active have fixed
normal force, friction)
values (weight, a
push)                      – Non-contact: objects
are at a distance
(gravity, magnetic
a requirement (normal
forces)
force)
Weight: the attraction of the Earth
• Force of gravity
   
W  mg
• Sensation of weight?
off a chair. What do
you feel?
– What if you stand?
Sit? Stand on your
hands?
• Weightless: no
sensation of weight…
really freefall
An astronaut in the space shuttle orbits
the Earth, floating freely with respect
to the contents of the shuttle. Is the
astronaut weightless?

1. Yes
2. No
3. Depends on altitude
4. Depends on mass
Contact forces and the normal
force
• Useful to talk about         • Called “normal” force
two parts to contact           (for perpendicular, not
forces                         „usual‟)
• First prevents objects       • Classic passive force:
from moving through            as big as it has to
one another                    be…
• Acts  to interface          • How do inert objects
between objects                exert forces?

By bending: pillow, chair seat, plastic chair, a
hard floor, all push back by distorting….
Normal force examples
Push down
Simple
Pull up

N                         N                    N           Fpull

Book on Table                                      Book on Table

W                     Book on Table
W
W       Fpush
Fy = N – W = 0
N=W                                          Fy = N – W + Fpull = 0
N = W - Fpull
Fy = N – W – Fpush = 0
To find the normal force,        N = W + Fpush
see how large it has to
be: a passive force…
Consider a car at rest. We can conclude that the
downward gravitational pull of Earth on the car
and the upward contact force of road on it are
equal and opposite because:

1: The two forces form an 3rd law pair
2: The net force on the car is zero
3: Neither of the above

The normal force of the road pushing up on the
car and the gravitational pull of the Earth down
on the car can’t be 3rd law partners: they both
act on the same object!
I hold a book which has a weight of -20N y ˆ
against the ceiling by pushing up on it with a
ˆ
force of +30Ny. What is the normal force
exerted by the ceiling on the book?

ˆ
+30N y

1.   +20 N yˆ
2.         ˆ
-30 N y
3.   +50 N yˆ
4.         ˆ
-50 N y
5.         ˆ
-10 N y

```
To top