Transcript P5.28 (p.138)

Exam 1: Chapters 1-4
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Chapter 5 The Laws of Motion
5-1a Force – a push or pull.
CT1: The force of the elevator on Norbert and Zot is
A. up.
B. down.
Assume the elevator is near the Earth’s surface
and that it is right-side up in the cartoon.
CT2: The force of the non-sticky elevator
surface on Norbert and Zot is
A. up.
B. down.
Assume the elevator is near the Earth’s surface
and that it is right-side up in the cartoon.
CT3: The acceleration of Norbert and Zot is
A. up.
B. down.
C. zero
Fig. 5.1, p.113
5-1 B. Mass
The mass of an object is the
amount of matter (stuff) in that
object. It is the same for a given
object everywhere in the
Universe. Mass is also called
inertia because it resists being
moved.
5-2 Newton’s First Law: A body
remains in uniform motion (or
at rest) unless acted upon by a
net external force.
 acceleration due to Earth’s rotation  0.03 m/s2
 acceleration due to Earth’s orbit  0.006 m/s2
 acceleration due to Sun’s orbit  2 x 10-10 m/s2
An inertia frame of reference is a coordinate
system (or frame) in which Newton’s Law’s
hold.
We will assume that the Earth’s surface is an
inertial frame and not make errors greater than
0.03/10 = 0.3%.
CT4
A.
B.
C.
D.
E.
CT5
A.
B.
C.
D.
E.
5-3 Newton’s Second Law: The
net external force is equal to the
mass times the acceleration.
F = ma
Force and acceleration are vectors
 Fx = max
 Fy = may
remember Galileo!
 Fz = maz
Normally we will do problems in a plane with
only x and y components.
Calibrating a
“Force Meter”
Experiments
show forces
add as vectors.
Acceleration is Proportional to Force
Acceleration is Inversely Proportional to Mass
Applications of Newton’s Laws - Method
 Draw picture of the problem.
 Choose body (bodies) to isolate.
 Draw Free Body Diagrams (FBDs) for isolated bodies.
 Choose and label coordinate axes.
 Apply Newton’s 2nd Law: Fx = max and Fy = may
 Solve for F, m or a.
 Work out kinematics.
 Check solution is reasonable.
P5.6 (p.136)
Concept Question 6
P5.6b: If the parachutist comes to rest
over a shorter distance, the force exerted
by the ground will be
A. greater.
B. less.
C. the same.
5-4 Newton’s Third Law: If
body A acts on body B, then
body B acts back on body A
with a force equal in
magnitude but opposite in
direction.
A
FBA
FAB
B
FAB and FBA are 3rd Law pairs.
CT7
A.
B.
C.
D.
E.
Histogram for PHYS151 Exam 1 Spring 2008
Average = 65 Low = 27 High = 100
Number of Grades in Bin
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Histogram for PHYS151 Total Spring 2008
Average = 68 Low = 22 High = 102
Number in Grade Bin
12
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P5.18
(p.137)
Concept Question 8
P5.18b: If the 5 N force is applied to the
heavy box instead, the contact force
between the boxes will be
A. greater.
B. less.
C. the same.
5-5 The Vector Nature of Forces
Two Astronauts Pushing a Satellite
with Forces that Differ in Magnitude and Direction
P5.22
(p.137)
P5.28 (p.138)
Orbit of Moon about the Sun
Exaggerated and incorrect
Actual
5-6 Gravitational Force or Weight
y
Fg = mg (weight)
g = -9.81 j (m/s2 )
x
Apparent Weight is the
force you actually feel
pushing on you – often
the floor.
P5.35 (p.138)
Concept Question 9
P5.35b: If the constant speed of descent
is greater than 1.1 m/s, the force of air
resistance is
A. greater.
B. less.
C. the same.
Concept Question 10
P5.33a: Your normal weight is 610 N and
the scale in the elevator reads 720 N. The
acceleration of the elevator
A. upward.
B. downward.
C. zero.
Concept Question 11
P5.33c: Your normal weight is 610 N and
the scale in the elevator reads 720 N. The
magnitude of the velocity of the elevator is
A. increasing.
B. decreasing.
C. zero.
D. impossible to determine without more
information.
5-7 Normal Forces
The Normal Force
May Equal the Weight
The Normal Force
May Differ from the Weight
An Object on an Inclined Surface
P5.42 (p.138)
Concept Question 12
P5.42b: If the angle of the incline is
decreased, the normal force will
A. increase.
B. decrease.
C. stay the same.
CT13
A.
B.