CHAPTER 4 The Laws of Motion

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CHAPTER 4The Laws of Motion
Newtons First Law An object at rest remains
at rest and an object in motion continues in
motion with constant velocity (constant speed
in straight line) unless acted on by a net
external force.
in motion or at rest with respect
to the chosen frame of reference net force
vector sum of all the external forces acting on
the object FNet,x and FNet,y
calculated separately Forces Contact
Forces Applied Forces (push or pull) Normal
Force (supporting force) Frictional Force
(opposes motion) Field Forces Gravitational
Magnetic Electrostatic The typical four
forces analyzed in our study of classical
mechanics
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Newtons Second Law The acceleration of an
object is directly proportional to the net
force acting on it FNet ma

• Mass The measurement of inertia (inertial
  mass)
• Inertia The tendency of an object to resist any
  attempt to change its motion
• Book Example
• Strike golf ball w/golf club
• Strike bowling ball w/golf club
• Which has greatest inertia?
• Which has greatest mass?

Dimensional Analysis F ma kg x m/s2
newton N
1 newton 1 kg m/s2
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Weight and the Gravitational Force
Mass an amount of matter (gravitational
mass) Your mass on the Moon equals your mass
on Earth. Weight the magnitude of the force of
gravity acting on an amount of matter
F ma Fg mg w mg
NOTE Your text treats weight (w) as a scalar
rather than as a vector.
Example Your mass is 80kg. What is your
weight? w 80kg 9.8m/s2 w 780
kgm/s2 w 780 N
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Newtons Third Law If two objects interact, the
force exerted on object 1 by object 2 is
equal in magnitude but opposite in
direction to the force exerted on object 2
by object 1
Book pushes down on table with force of
9.8.N Table pushes up on book with force of
9.8.N Net Force on book 9.8N 9.8N 0N Hence,
book does not accelerate up or down.
Earth pulls on Moon equal to the force the Moon
pulls on Earth.
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Problem Solving Strategy

• Remember We are working now with only 4 forces.
• Applied Force Fa
• Normal Force FN
• Frictional Force Ff
• Gravitational Force Fg

Draw a Sketch
Determine the Magnitude of Forces in x and in
y Direction FN often equals Fg (object does
not accelerate up off surface or accelerate
downward through surface) FNet,y FN Fg 0
N FNet,x Fa Ff ma Ff lt Fa
Label forces on Sketch Solve Problem
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Example 1 Sliding Box Problem (Horizontal
Fa) Box hockey puck shopping
cart tire dead cat etc. A 55 kg
shopping cart is pulled horizontally with a force
of 25N. The frictional force opposing the motion
is 15N. How fast does the cart accelerate?
Fa 25N Ff 15N FNet,x 25N 15N ma
10.N 55kga
Fg mg 55kg 9.8m/s2 540N FN Fg 540N
a .18m/s2
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Friction
Friction opposes motion. Kinetic Friction opposes
motion of a moving object. Static Friction
opposes motion of a stationary object. Ff ?FN
?static coefficient of static
friction ?kinetic coefficient of kinetic
friction
?s gt ?k
Why? Static condition peaks and valleys of the
two surfaces overlap each other. Kinetic
Condition surfaces slide over each other
touching only at their peaks
?s gt ?k ?Ff,s gt Ff,k Applied Physics
Example Anti-lock Brakes