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Chap. 4: Newton’s Law
of Motion
Force; Newton’s 3 Laws; Mass and Weight
Free-body Diagram (1D)
Free-body Diagram (1D, 2 Bodies)
Free-body Diagram (2D)
Equilibrium
Frictional Force
Circular Motion / Rotation
1
And Chap.5 Applying Newton’s Laws (more examples)
F
Who Wanted “Force”?
2
What is Force?
an interaction between two objects
a push or a pull on an object; either a contact force
or a long-range force
Push up on an apple by hand; contact force
Pull down on an apple by earth; long-range force
a vector
it has both a magnitude and a direction
causes an acceleration on an object
2
2
m/s 8.9
; ˆ)m/s 8.9(ˆ)0(
gg
jig
jmgiF ˆ)(ˆ)0(g
y
g
g
v
v
4
Newton’s Laws of Motion
Newton’s 3 Laws
Newton’s 1st Law
Law of inertia
“Mass” (kg) is a measure of the inertia of a body.
Newton’s 2nd Law
Dynamical analysis using Free-Body Diagram (FBD)
Newton’s 3rd Law
Action-reaction pair
m
Fa
5
Newton’s Laws of Motion
Kinematics
(r, v, a)
Structure of Newtonian Mechanics
Inertial Reference Frame
(Newton’s 1st Law)
Action-Reaction
(Newton’s 3rd Law)
Mass
(m)
The Nature of Force The Nature of Object The Nature of Motion
F = m a
(Newton’s 2nd Law)
Kinematics
(r, v, a)
Kinematics
(r, v, a)
Kinematics
(r, v, a)
6
Newton’s Laws of Motion
WHAT HAPPEN?
9
Newton’s Laws of Motion
The force on a hokey puck
causes the acceleration
If the net force on a hokey
puck is zero (equilibrium),
the acceleration is zero.
0
0
a
F
11
Force: Acceleration/Equilibrium Acceleration
Kinetic Equations
(see Chap. 2 &3)
Newton’s Laws of Motion
12
Diagram to Understand Forces
The force of the starting
block on the runner has a
vertical component that
counteracts her weight and a
large horizontal component
that accelerate her.
Vector Nature
(see Chap. 1)
Newton’s Laws of Motion
The two cases are identical as far as the acceleration of
the box is concerned. This demonstrates the VECTOR
NATURE of the force. FFFFF
321R
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Force: Vector
Newton’s Laws of Motion
1300 N
1300 N
90o
FFFF
21R
14
Similar to Fig. P4.37
Vector direction x-y coordinates
Find the magnitude of F2 and its direction relative
to F1.
Newton’s Laws of Motion
1300 N
1300 N
90o
FFFF
21R
15
Similar to Fig. P4.37 x
y
yyy
xxx
FFR
FFR
21
21
?
?
01300
13000
Find the magnitude of F2 and its direction relative
to F1.
Newton’s Laws of Motion
Find the tensions in each of three chains when the
weight of a car engine is W.
0321O)Point R(at
TTTF
17
Fig. 5.3
Also see Fig. E4.2
Newton’s Laws of Motion
What is 2nd Law?
m
Fa
m
Fa
m
Fa
m
Fa
z
z
y
y
xx
18
Newton’s Laws of Motion
y
x Impulsive Force “F ”
(for a short time
interval)
Top View
vk
(1) A hockey puck is initially
at rest on a flat ice surface.
(2) Then, it receives
a horizontal kick in
a direction of the
red arrow.
F = m a Change in velocity for non-zero F
[Quick Quiz] The motion of the puck
right after the kick is:
(a) Motion with constant velocity
(b) Motion with constant acceleration
(c) Motion with constant deceleration
(d) None of above 19
Newton’s Laws of Motion
Which of the path 1-5 below would the puck most
closely follow after receiving the force?
y
x
Top View
A hockey puck is sliding
at constant velocity
on a flat ice surface
5 4 3
2
1
v0
Impulsive Force “F ”
(for a short time
interval)
21
Newton’s Laws of Motion
vx2 = v0x
2 + 2 ax (x – x0)
x – x0 = 55 m
v0x = 28 m/s
Was: What is the acceleration?
25
Newton’s Laws of Motion
Fxnet = m ax
x – x0 = 55 m
m = 1500 kg
Fxnet?
v0x = 28 m/s
Was: What is the acceleration?
Now: What is the net force?
26
vx2 = v0x
2 + 2 ax (x – x0)
Newton’s Laws of Motion
vx2 = v0x
2 + 2 ax (x – x0)
Fxnet = m ax
x – x0 = 55 m
m = 1500 kg
Fxnet?
v0x = 28 m/s
Was: What is the acceleration?
Now: What is the net force?
vx2 = v0x
2 + 2 ax (x – x0) ax = 7.1 m/s2
SFx = m ax (1500 kg) x (7.1 m/s2)
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• If you exert a force on a body, the body always exerts a force (the “reaction”) back upon you.
• Figure 4.25 shows “an action-reaction pair.”
• A force and its reaction force have the same magnitude but opposite directions. These forces act on different bodies. [Follow Conceptual Example 4.8]
What is 3rd Law?
Newton’s Laws of Motion
Which force is greater?
From Giancoli 3rd ed.
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Newton’s Laws of Motion
What is 3rd Law? (II)
Push forward
on a rocket
by gas
Push backward
on gas
by a rocket
Action and reaction forces have the same magnitude
but are opposite in direction.
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Newton’s Laws of Motion
[Quick Quiz] A massive truck collides head-on
with a small sports car.
1) Which vehicle experiences the greater force of
impact?
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2) Which vehicle experiences the greater
acceleration?
Newton’s Laws of Motion
A stonemason drags a marble block across
a floor by pulling on a rope attached to the
block. Why does the block move while the
stonemason remains stationary?
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Newton’s Laws of Motion
FP-G
Force on
Rope exerted
By Person
FR-P =10 N
Force on
Box exerted
By Rope
Assume the rope is massless.
At rest
x
What is the tension here?
0? 10? 20? Or else?
Quick Quiz
FB-R =10 N
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Newton’s Laws of Motion
Free-Body Diagram
jmgiF ˆ)(ˆ)0(g
y
g
g
v
v
Draw only force(s) on the apple
41
Newton’s Laws of Motion
A hockey puck is sliding at a
constant velocity across a flat
horizontal ice surface. Which is
the correct free-body diagram?
[A]
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Quick Quiz
Newton’s Laws of Motion
Free-Body Diagram (1D)
46
Newton’s Laws of Motion
Free-Body Diagram (1D) M
oti
on
y
m = 10.0 kg
47
Find ay
Newton’s Laws of Motion
ay = 0
Weight in Elevator
y
ay = 4.9 m/s2
49
m = 60 kg
Fg = mg = 588 N
SFy= m ay FN = ?
Example 5.9
Newton’s Laws of Motion
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Weight on Planet
Newton’s Laws of Motion
FBD (1B/1D 2B/1D) M
oti
on
y
54
Find ay
Motion
Find ax
Newton’s Laws of Motion
Free-Body Diagram (x-axis)
Motion
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P4.43, P4.54
e.g., 2 boxes sliding on desk (no friction)
Find acceleration and tension.
Newton’s Laws of Motion
Find FP and FT
ax = 2.50 m/s2
FT FP
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P4.43
Exercise
1) 1B 2B
2) Now a is given. Then you are asked to find forces.
Newton’s Laws of Motion
2) Apply Newton’s 2nd law:
e.g., 2 boxes sliding on desk (no friction)
Find acceleration and tension.
Free-Body Diagram (x-axis) The rope has a mass …
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1) Draw F.B.D.
identify all forces exerted on box1, rope and box2
separately.
FP = 40.0 N
Newton’s Laws of Motion
Free-Body Diagram (y-axis)
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Newton’s Laws of Motion
Example 1: One paint bucket (mass m1) is hanging by
a massless cord from another paint
bucket (mass m2), also hanging by a
massless cord. The two buckets are pulled
upward with an acceleration by the
upper cord. Draw the free-body
diagram for each bucket. Determine the
tension on each code if a = 3.00 m/s2.
a
a
y
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P4.57
Newton’s Laws of Motion
Example 1: One paint bucket (mass m1) is hanging by
a massless cord from another paint
bucket (mass m2), also hanging by a
massless cord. The two buckets are pulled
upward with an acceleration by the
upper cord. Draw the free-body
diagram for each bucket. Determine the
tension on each code if a = 3.00 m/s2.
m1
m2 a
a
Fg1
Fg2
FT2
FT1
FT1
y
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Newton’s Laws of Motion
Acceleration?
Tension at the midpoint of the rope?
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P4.54
Newton’s Laws of Motion
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Newton’s Laws of Motion
2)
e.g., Box sliding on desk (no friction)
Free-Body Diagram (2D)
1)
Motion
x
y
73
m = 10.0 kg
Newton’s Laws of Motion
2) Apply Newton’s 2nd law:
m ax =
m ay (=0) =
e.g., Box sliding on desk (no friction)
1) Draw F.B.D -identify all forces exerted on the box.
FN
FG
FP Motion
x
y
FP cos(30.0o)
FP sin(30.0o) + FN m g
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Free-Body Diagram (2D)
Newton’s Laws of Motion
2) Apply Newton’s 2nd law:
10.0 ax =
10.0 (0) =
e.g., Box sliding on desk (no friction)
1) Draw F.B.D -identify all forces exerted on the box.
FN
FG
FP Motion
x
y
40.0 cos(30.0o)
40.0 sin(30.0o) + FN (10.0)(9.80)
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Free-Body Diagram (2D)
Newton’s Laws of Motion
e.g., Box sliding on desk (no friction)
FN
FG
FP Motion
x
y
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Free-Body Diagram (2D)
Try E4.4
Circular Motion
Free-Body Diagrams?