Transcript Physics 1422 - Introduction

Physics 203 – College Physics I
Department of Physics – The Citadel
Physics 203
College Physics I
Fall 2012
S. A. Yost
Exam, Chapter 4
Newton’s Laws – Part 2
Physics 203 – College Physics I
Department of Physics – The Citadel
Exam 1
Average: 51%,
High: 92%
(normalized to 85 points)
Exam 1
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Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 1
Indicate your current status
A
Freshman (4th class)
B
Sophomore (3rd class)
C
Junior (2nd class)
D
Senior (1st class)
E
Other
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 2
Select the highest high school physics courses you
have completed.
A One semester of HS Physics
B Two semesters of HS Physics
C One semester of AP Physics
D Two semesters of AP Physics
E No Physics
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 3 (one of two)
If the highest math course you completed is in this
list, select it. (There will be another list following.)
A MATH 104
B MATH 105
C MATH 106
D MATH 107
E None of these
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 4 (two of two)
If the highest Citadel Math course you have
completed is in this list, select it.
A MATH 119
B MATH 131
C MATH 132
D STAT 160
E None of these
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 5
Select Citadel Biology Courses you have completed.
A BIOL 101 only
B BIOL 101 and BIOL 102
C BIOL 130 only
D BIOL 130 and BIOL 140
E None of the above
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 6
Select Citadel Chemistry Courses you have
completed
A CHEM 103 only
B CHEM 103 and CHEM 104
C CHEM 151 only
D CHEM 151 and CHEM 152
E None of these
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 7
The figure shows a right triangle. Estimate the length
x indicated.
A 11.0 cm
x
B 6.5 cm
C 5.5 cm
D 8.1 cm
E 7.4 cm
4.0 cm
7.0 cm
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 8
An auto travels East from home at 65 mi/hr for
3.0 hours. Then it returns West at 50 mi/hr for
4.0 hours. How far is the auto from home?
A 105 miles East
B
5 miles East
C
14 miles East
D
20 miles West
E
5 miles West
Physics 203 – College Physics I
Department of Physics – The Citadel
Survey: Question 9
The quantity Z depends on X and Y according to the
equation
1
1
=
Z
1
+
X
Y
.
What is the value of Z if X = 4 and Y = 6?
A 0.1
B 6.4
D 1.0
C
E 10.0
2.4
Physics 203 – College Physics I
Department of Physics – The Citadel
Schedule
This week: Chapter 4 (continued). Finish reading
it by next time.
The topic is Newton’s Laws.
A problem set will be up later today and due a
week from today.
Physics 203 – College Physics I
Department of Physics – The Citadel
Newton’s First Law
If an object does not interact with any other object,
there exist reference frames in which the object
has zero acceleration.
Such a reference frame is called an inertial frame.
In an inertial frame, an isolated object moves with a
constant velocity.
Any frame moving at a constant velocity with
respect to an inertial frame is also inertial.
Physics 203 – College Physics I
Department of Physics – The Citadel
Newton’s Second Law
Forces act as vectors. If the vector sum of
forces on an object is zero, the object
experiences no acceleration.
If the
→ total force vector acting on an object is
F, then the acceleration of the object is
determined by the net force and the object’s
mass:
→
→
F = ma
Physics 203 – College Physics I
Department of Physics – The Citadel
Newton’s Third Law
All interactions occur in pairs. Forces always
exist between a pair of objects.
→
If object 1 acts on object 2 with a force F21 ,
then object 2 acts on object 1 with a force
→
F12 that is equal in magnitude and opposite
in direction:
→
→
F21 = – F12 .
Physics 203 – College Physics I
Department of Physics – The Citadel
Car on the Highway
A car is accelerating onto the highway.
What force is responsible for this?
Friction between the tires and road.
F
F
Physics 203 – College Physics I
Department of Physics – The Citadel
Car on the Highway
As you accelerate, coffee cup slides backwards
off the dashboard.
What force is responsible for this?
None: without friction, the cup would maintain a
constant velocity while the car accelerates
around it.
The coffee cup does accelerate backward
relative to the car, but the car is a
non-inertial reference frame, so no force is
responsible for this acceleration.
Physics 203 – College Physics I
Department of Physics – The Citadel
Push and Fall
Mike pushes Johnny
with a force of strength
F and Johnny falls.
Newton’s Third Law
→
→
FMJ = – FJM
Mike pushes
→
The force Johnny exerts
on Mike in the process
must be
A) less than F
B) greater than F
C) equal to F
FJM
→
FMJ
Johnny
falls
Physics 203 – College Physics I
Department of Physics – The Citadel
Units, Weight
Forces are measured in Newtons.
Weights are forces, and can be given in units of
Newtons. It is incorrect to measure weights in kg,
because this is a mass, not a force.
The weight of a mass m is W = mg.
1 kg weighs 9.8 N on Earth.
The units follow from Newton’s 2nd Law:
1 N = 1 kg m/s2
Physics 203 – College Physics I
Department of Physics – The Citadel
Units, Weight
Technically, pounds are a unit of weight, but are
commonly used to measure mass on Earth.
The correct English unit of mass is called the slug,
which weighs about 32 lb on Earth. The English
pound is 1 lb = 1 slug ∙ ft/s2.
1 kg weighs 2.20 lb on Earth, but this would be
different elsewhere. The weight of 1kg on Earth is
9.8 N, so 1 lb = 9.8 N / 2.20 = 4.45 N (anywhere).
Roughly, a Newton’s about a quarter pound.
Physics 203 – College Physics I
Department of Physics – The Citadel
Weight and Normal Force
If you are standing on a floor, the
floor pushes up on you with a
force opposite your weight to
prevent you from falling through
the floor.
The perpendicular component of
the contact force between two
objects is called the normal
force. (Here, “normal” means
“perpendicular”, not “ordinary”.)
N
W
N+W=0
Physics 203 – College Physics I
Department of Physics – The Citadel
Weight and Normal Force
Are the weight and normal force an
interaction pair, which must be
equal and opposite by the third
law?
No: The normal force is between
the person and the floor, while
gravity is between the person
and the Earth.
N
W
N+W=0