Forces and the Laws of Motion Newton

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Forces and the Laws of Motion Newtons Second
Law

• PVHS Physics

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What we know

• How does force affect the motion of an object?
• How do free-body diagrams help in analyzing
  forces and motion?
• What are Newtons Laws of Motion?
• How can you determine forces required for
  equilibrium?
• How is mass related to motion?
• What are action-reaction pairs and why dont they
  produce equilibrium?
• What is the difference between mass and weight?
• What is a normal force?
• How can you determine the force due to friction?

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What we know

• Inertia
• Is an objects resistance to a change in motion
• it is the tendency of an object to maintain its
  state of motion
• Mass is a measure of inertia
• More mass more inertia
• Less mass less inertia

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What we know

• The Law of Inertia An object at rest tends to
  stay at rest and an object in motion tends to
  stay in motion with the same speed and in the
  same direction unless acted upon by an unbalanced
  force.
• Net Force
• The vector sum of all the external forces acting
  on an object

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What we know

• Forces are Vectors
• Equilibrium
• The state in which there is no change in an
  objects motion
• The sum of all forces equals zero

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Finding the Net Force
What happens when the Net Force is not zero?

• List givens and draw a free-body diagram
• Establish x-y axes (frame of reference)
• Resolve vector components
• Sum the vectors in each direction

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Forces and Motion

• Forces cause an objects velocity to change
• Since acceleration is the rate of change in
  velocity
• Force causes acceleration
• The acceleration is directly proportional to the
  force
• The bigger the force, the greater the
  acceleration
• The smaller the force, the smaller the
  acceleration

F
F
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Mass and Motion

• Inertia
• Is an objects resistance to a change in motion
• Inertia resists acceleration
• Mass is a measure of inertia
• Acceleration in inversely proportional to mass
• More mass, less acceleration
• Less mass, greater acceleration

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Force, Mass, and Motion

• Combining the effects of mass and force
• This is Newtons Second Law

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Newtons Second Law

• The acceleration of an object as produced by a
  net force is directly proportional to the
  magnitude of the force, in the same direction of
  the force, and inversely proportional to the mass
  of the object
• The 2nd Law is commonly written as
• SFma
• The sum of the forces on an object equals mass
  times acceleration
• Note it is the Net Force that causes the
  acceleration

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Application

• The net force on the propeller of a 3.5kg model
  airplane is 7.0N. What is its acceleration?
• A 2.0kg otter starts from rest at the top of an
  incline 85 cm long and slides to the bottom in
  0.50s. What is the net force on the otter?
• A soccer ball kicked with a force of 13.5N
  accelerates at 6.5 m/s2. What is the mass of the
  ball?
• The net force on a golf cart is 390N north. If
  the cart has a mass of 270 kg, what are the
  magnitude and direction of its acceleration?
• A car has a mass of 1.5 x 103kg. If the force on
  the car is 6.75x103N east, what is its
  acceleration?

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What we know

• How does force affect the motion of an object?
• How do free-body diagrams help in analyzing
  forces and motion?
• What are Newtons Laws of Motion?
• How can you determine forces required for
  equilibrium?
• How is mass related to motion?
• What are action-reaction pairs and why dont they
  produce equilibrium?
• What is the difference between mass and weight?
• What is a normal force?
• How can you determine the force due to friction?

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Questions?