Physics Review

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Physics Review

• What Are Newton's Laws of Motion?

2
Forces

• Push or Pull that acts between two bodies
• Tension
• Gravitational force
• Frictional force
• Air resistance
• Electrostatic force
• Strong nuclear force
• Weak nuclear force

3
Newtons First Law

• Law of Inertia
• An object will continue in its state of motion
  unless compelled to change by a force impressed
  upon it.

• What net force is required to maintain a 5000 kg
  object moving at a constant velocity of magnitude
  7500 m/s?
• Net force of 0 ? constant motion

4
Newtons Third Law

• For every action, there is an equal, but
  opposite, reaction.

5
Weight

• The weight of an object is the gravitational
  force exerted on it by Earth (or whatever
  planetary mass the object is on).
• What is the mass of an object that weighs 500 N?

mg
F
w
N
F
500
w



kg
m
51
g
m
8
.
9
s
2
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Other Examples

• A book with a mass of 2 kg rests on a table.
  Find the magnitude of the force exerted by the
  table on the book.

F
mg
F
N
g
(
)
(
)



F
N
kg
F
20
8
.
9
2
m
N
g
2
s
Note This force is called the Normal force
because it acts perpendicular to the contact
surface of the object.
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Other Examples

• A can of paint with a mass of 6 kg hangs from a
  rope. If the can is to be pulled up to a rooftop
  with a constant velocity of 1 m/s, what must the
  tension in the rope be?

mg
F
F
g
T
)
(


m
8
.
9
N
kg
F
59
6
2
T
s
8
Friction

• Friction is a contact force that is parallel to
  the contact surface and perpendicular to the
  normal force.
• Static friction (Fs) occurs when a force tries
  unsuccessfully to set a body in motion.
• Kinetic (sliding) friction (Fk) occurs when a
  force acts on a body in motion.
• Generally Fs gtFk

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Friction Equations
m

F
F
N
s
s
(max)
m

F
F
N
k
k

• ? represents the coefficient of friction - a
  number that is related to the nature of the
  surfaces in contact with each other.
• Fs has a range of values dependent on the
  magnitude of the horizontal force being applied.

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Friction Problems

• A crate of mass 20 kg is sliding across a wooden
  floor. ?k between the crate and the floor is
  0.3. Determine the strength of the force acting
  on the crate.

m
m
mg
F
F
If the crate is being pulled by a force of 90 N
(parallel to the floor), find the acceleration of
the crate.
k
N
k
k
)
(
)
(
)
(

F
3
.
0
N
kg
m
59
8
.
9
20

k
s2
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Friction Problems

• A crate of mass 100 kg rests on the floor. ?s is
  0.4. If a force of 250 N (parallel to the floor)
  is applied to the crate, what is the magnitude of
  Fs on the crate?

m
m
mg
F
F
s
N
s
s, max
(
)
(
)
(
)

N
kg
m
390
8
.
9
100

F
4
.
0
s, max
s
2
NOTE Remember that Fs represents a range of
values. In this case, the applied force of 250N
is less than the maximum Fs so the actual
magnitude of Fs is 250N.
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Inclined Planes

• When a mass sits on an inclined plane, its weight
  has two components normal and parallel to the
  plane.

m
Normal force
Weight
Parallel force
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Tension
T1
T2
Fg 1
T2
Fg 2
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Pulleys

• Pulleys are devices that change the direction of
  the tension force in cords that slide over them.
• In problems, we generally ignore the mass and
  friction associated with pulleys.

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Pulleys
Step 2
Step 1

• Fnet 1 m1g - T
• Fnet 2 T m2g
• m1g - T m1a
• T - m2g m2a
• a m1-m2 g T m1m2 g
• m1 m2 m1 m2

m1
Step 3
m2
T
T
m2
m1
Step 4
m1g
m2g
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Uniform Circular Motion

• Describes an object that moves in a circle with a
  constant speed
• An object accelerates without changing its speed.
• The direction continuously changes as an object
  moves around a circle

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Circular Motion
Uniform circular motion is a particle moving at
constant speed in a circle.
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How do we describe the position of the particle?
The angle q is the angular position.
Again q is defined to be positive in the
counter-clock-wise direction.
Angles are usually measured in radians.
19
Circular Motion
Radians
For a full circle.
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Circular Motion
Angular velocity
We will worry about the direction later.
Like one dimensional motion - will do. Positive
angular velocity is counter-clockwise.
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Kinematical Equations
22
Centripetal acceleration
directed toward the center of curvature (center
of circle)
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An objects natural path is a straight line,
therefore to pull an object out of its natural
path a net force was applied
Centripetal Force
This force could be any of our everyday forces
(weight, normal, tension, applied, or friction)
or a combination of those five.
It is the force or forces that maintain(s) the
circle
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Centripetal force
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Centripetal Force
v
Fc
?
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