Today: Re-Teaching Newton

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Today Re-Teaching Newtons First and Second Laws
of Motion
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Balanced vs. Unbalanced Forces

• Balanced Forces

• Unbalanced Forces

• Cancel each other out
• NO net force acting on object (Fnet 0 N)

• Do not cancel out
• YES net force acting on object (Fnet gt 0 N)

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Check For Understanding

• Which statement below is true?
• When two unbalanced forces are applied to an
  object, the net force is 0 N.
• When two balanced forces are applied to an
  object, the net force is 0 N.
• Two unbalanced forces cancel each other out.
• When balanced forces act on an object, net force
  is negative.

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What is the net force acting on this box?
Fnet12 N 8 N 4 N to the right
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Practice Finding the Net Force
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Check For Understanding
You must have a magnitude and a direction for
Fnet.
Balanced or unbalanced? Fnet
Balanced
0 N
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Check For Understanding
Balanced or unbalanced? Fnet
Unbalanced
5 N to the left
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Check For Understanding
Balanced or unbalanced? Fnet
Balanced
0 N
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Check For Understanding
Balanced or unbalanced? Fnet
Unbalanced
15 N up
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Answer Balanced and Unbalanced Forces questions
on your sheet.5 minutes!
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Check For Understanding

• When balanced forces act on an object, what is
  the net force acting on the object?

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Check For Understanding

• If the net force on an object is greater than
  zero, what can you conclude about the forces
  acting on the object?
• It is being acted on by unbalanced forces.
• It is being acted on by balanced forces.
• It is being acted on by no forces at all.
• It is moving at a constant speed.

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Newtons First Law

• Objects at rest remain at rest unless acted upon
  by a net force.
• Objects in motion remain in motion at a constant
  speed unless acted upon by a net force.
• Law of Inertia

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FR
F
f
F FR f
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Newtons Second Law

• When a net force acts on an object, the object
  accelerates in the direction of the force.
• Acceleration is directly proportional to net
  force and inversely proportional to mass.

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Forces are either balanced, or unbalanced
When forces are equal, no acceleration occurs.
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Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
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Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
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Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
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Forces are either balanced, or unbalanced
But when they are unbalanced, thats when objects
accelerate
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Forces dont cause motion, forces cause
ACCELERATIONS!
Accelerations cause MOTION!
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FR
F
f
F FR f
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FR
f
F
The car will accelerate if F gt FR f
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Objects mass (kg)
Objects acceleration (m/s2)
Net force acting on object (N)
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F ma
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Check For Understanding

• A car is accelerating at 5 m/s2. The car has a
  mass of 1000 kg. What is the force that is
  pushing the car?

F
1000 kg
5 m/s2
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Check For Understanding

• Kristopher pushes with 150 N of force on a box
  with a mass of 50 kg. At what rate does the box
  acclerate?

150 N
a
50 kg
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Check For Understanding

• Tylesia pushes Mr. D with a force of 240 N. Mr. D
  accelerates forward at a rate of 3 m/s2. What is
  Mr. Ds mass?

240 N
m
3 m/s2
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Relationships in F ma

• ? net force (Fnet) ? ? acceleration (a)
• ? mass (m) ? ? acceleration (a)
• ? mass (m) ? ? net force (Fnet)

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? net force (Fnet) ? ? acceleration (a)
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How fast will this rock accelerate?
100 N
a
50 kg
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How fast will this rock accelerate?
200 N
a
50 kg
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? net force (Fnet) ? ? acceleration (a)
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? mass (m) ? ? acceleration (a)
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? mass (m) ? ? acceleration (a)
20 N
a
5 kg
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? mass (m) ? ? acceleration (a)
20 N
a
20 kg
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? mass (m) ? ? acceleration (a)
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? mass (m) ? ? acceleration (a)

• Kristopher pushes with 150 N of force on a box
  with a mass of 50 kg. At what rate does the box
  acclerate?

150 N
a
50 kg
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? mass (m) ? ? acceleration (a)

• Kristopher pushes with 150 N of force on a box
  with a mass of 100 kg. At what rate does the box
  acclerate?

150 N
a
100 kg
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Who will be harder to move?
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How much force is needed to accelerate this
wrestler at a rate of 2 m/s2?
F
200 kg
2 m/s2
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How much force is needed to accelerate this
wrestler at a rate of 2 m/s2?
F
20 kg
2 m/s2
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? mass (m) ? ? net force (Fnet)
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Check For Understanding

• A car is accelerating at 5 m/s2. The car has a
  mass of 2000 kg. What is the force that is
  pushing the car?

F
2000 kg
5 m/s2
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Check For Understanding

• A car is accelerating at 5 m/s2. The car has a
  mass of 5000 kg. What is the force that has to be
  pushing the car?

F
5000 kg
5 m/s2
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Balanced vs. Unbalanced Forces

• Balanced Forces

• Unbalanced Forces

• Cancel each other out
• NO net force acting on object (Fnet 0 N)
• NO acceleration (a 0 m/s2)
• Object keeps same speed

• Do not cancel out
• YES net force acting on object (Fnet gt 0 N)
• YES acceleration (a gt 0 m/s2 or a lt 0 m/s2)
• Object changes speed

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Balanced Forces Unbalanced Forces
Fnet is
Acceleration is
Object will
Described by which of Newtons Laws?
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Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is
Object will
Described by which of Newtons Laws?
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Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2
Object will
Described by which of Newtons Laws?
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Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2
Object will remain at rest or maintain constant speed accelerate (change speed)
Described by which of Newtons Laws?
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Balanced Forces Unbalanced Forces
Fnet is Fnet 0 N Fnet gt 0 N
Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2
Object will remain at rest or maintain constant speed accelerate (change speed)
Described by which of Newtons Laws? First Law Second Law
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Challenge Question

• A car moves along a road at a high speed. The
  driver pushes the gas pedal down farther,
  generating a forward force of 3000 N on the car.
  A strong gust of wind hits at the exact moment
  the driver pushes the gas pedal, exerting a
  backward force of 2500 N on the car. The road
  also applies a backward force of 500 N due to
  friction.
• What is the acceleration of the car at the exact
  moment the gust of wind hits the car?
• 0 m/s2
• 500 m/s2
• 2500 m/s2
• 3000 m/s2