Chapter 5: Newton's Second law of motion

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Chapter 5 Newton's Second law of motion
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Do Now 1/26/09

• 1.) What is the definition of acceleration?
• 2.) What is the equation used to find the
  acceleration of an object?
• 3.) What is the net force of an object?

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Acceleration

• Acceleration change in velocity
• change in time
• Acceleration describes how quickly motion
  changes.
• The cause for an objects acceleration is force.
  If no force is applied to an object, it doesnt
  accelerate it either stays at rest or continues
  at a constant motion.

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Net Force

• The net force is the combination of all the
  forces acting on an object.
• When an object has a net force that is not equal
  to zero, the object accelerates.
• The acceleration of an object is equal to the net
  force divided by the objects mass.
• To find the acceleration of an object, we divide
  the net force by the mass of the object
• a F
• m

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Acceleration Force / mass A F/m

• A 20 N net force is applied to an object that has
  a mass of 2 kg. What is the objects
  acceleration?
• A 30 N net force is applied to an object that has
  a mass of 6 kg. What is the objects
  acceleration?

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• 1.) A net force of 10 Newtons acts on a box which
  has a mass of 2 kg. What will be the acceleration
  of the box?
• 2.) Double the net force in problem 1. What
  will be the new acceleration of the box?
• 3.) Triple the net force in problem 1. What
  will be the new acceleration of the box?
• Acceleration and net force are directly
  proportional.

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• 1.) A net force of 10 N acts on a crate that has
  a mass of 10 kg. What will be the acceleration
  of the crate?
• 2.) Double the mass of the crate in problem 1.
  What is the new acceleration of the crate?
• 3.) Triple the mass of the crate in problem 1.
  What is the new acceleration of the crate?
• Acceleration and mass are inversely proportional.

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Newtons Second Law of Motion

• The acceleration produced by a net force on an
  object is directly proportional to the magnitude
  of the net force, is in the same direction as the
  net force, and is inversely proportional to the
  mass of the object.
• OR
• acceleration net force/ mass
• A F / m

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Extra Practice (Write out the equation that you
used to solve the problem)

• 1.) How much force must a 30, 000 kg jet plane
  develop to achieve an acceleration of 1.5m/s2
• 2.) How much force is needed for a 220 kg car to
  reach an acceleration of 30 m/s2?
• 3.) If a constant force is applied to a 20 kg
  mass, will the masss acceleration increase,
  decrease, or remain the same?
• 4.) A car can accelerate at 2m/s2, what
  acceleration can it attain if it tows another car
  of equal mass?

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Do Now 1/27/09

• 1.) A 2 kg football is kicked with a net force of
  30 N. What is the footballs acceleration?
• 2.) A 8 kg bowling ball is rolled with a net
  force of 48 N. What is the bowling balls
  accelerations?
• 3.) If a constant force is applied to a 20 kg
  mass, will the masss acceleration increase,
  decrease, or remain the same?
• 4.) A car can accelerate at 4 m/s2, what
  acceleration can it attain if it tows another car
  of equal mass? (Recall Newtons Second Law of
  Motion)

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Friction

• Friction is not just restricted to solids sliding
  or rubbing against each other.
• There is also fluid friction which occurs when
  an object pushes aside the fluid it is moving
  through.
• Air resistance is a type of fluid friction

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Net Force

• An object that moves at a constant velocity
  (constant speed in a constant direction) has no
  acceleration.
• Therefore, the object has a net force of zero.
• The net force is the combination of all the
  forces acting on an object.
• If I push a book across the table with a force of
  10 N, how can I get a net force of zero?
• Friction force occurs only when an object tends
  to slide or is sliding

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Zero Net Force No Acceleration Constant
Velocity

• An object will move with a constant velocity when
  the force pushing it balances the force of
  friction.
• Suppose a desk is dragged at a constant velocity
  with a force of 90 N. What is the acceleration
  of the desk? What is the force of friction that
  acts on the desk?
• Suppose a jet cruises with a constant velocity
  when the thrust from its engines is a constant
  10, 000 N. What is the acceleration of the jet?
  What is the force of air resistance acting on the
  jet? (Recall air resistance is a form of fluid
  friction!)

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Pressure

• Pressure occurs when some force is applied on a
  surface.
• Pressure is the amount of force divided by the
  area of application
• P F
• A
• Where F is the amount of force applied, and A is
  the Area of application
• Pressure is measured in Pascals (Pa)
• When the area of application is larger, the
  pressure is smaller.

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Do Now 1/28/09

• 1.) Define weight.
• 2.) Which exerts more pressure A textbook or a
  couch? Why?
• 3.) A shopping cart is pushed at a constant
  velocity with a force of 65 N. What is the
  shopping carts acceleration? What is the net
  force acting on the shopping cart? What is the
  friction force acting on the shopping cart?
• 4.) A 30 kg mass is pulled with a force of 120 N.
  Find the objects acceleration (Newtons Second
  Law of Motion!)

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A F/m

• Find the acceleration of an object in free fall
  using Newtons Second Law of Motion (A F / m)
• 1.) What is the acceleration of a 1 kg bag in
  free fall?
• 2.) What is the acceleration of a 30 kg boulder
  in free fall?

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On Your Own

• Come up with three different objects of different
  masses. Write down the masses on a piece of
  paper.
• Switch with the person next to you.
• Find the force of gravity on each object (the
  weight of each object) listed on your partners
  paper. Then find the acceleration of each object
  using Newtons Second Law of Motion.
• What do you notice?

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• The acceleration is always 9.8 m/s2
• That is why objects of different masses hit the
  ground at the same time when they are in free
  fall, when air resistance is negligible. The
  ratio of weight to mass is the same for all
  objects.
• All freely falling objects undergo the same
  acceleration.

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• Air resistance is a form of friction.
• What is the net force of an object when air
  resistance equals the force acting on it (when
  the friction force equals the applied force of an
  object)?
• Does the object accelerate?
• If a ball is falling through the air, and air
  resistance equals the force of gravity acting on
  the ball, what is the net force?

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Terminal Speed

• The net force of the ball is zero. Therefore, it
  falls through the air with a constant velocity
  (there is no acceleration)
• When an object reaches its terminal speed (or
  terminal velocity if we want to consider
  direction), acceleration terminates. No more
  acceleration occurs.

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Terminal Speed

• A skydiver will encounter air resistance. As
  soon as the skydivers weight (the force of
  gravity acting on the skydiver) equals the
  friction force (which is air resistance), the
  skydiver will reach his/her terminal speed
  he/she will no longer accelerate.

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Do Now 1/29/09

• 1.) A 20 N net force is applied to an object that
  has a mass of 2 kg. What is the objects
  acceleration?
• 2.) Double the mass in the previous problem.
  What is the new acceleration of the object?
• 3.) What happens to the acceleration when you
  double the mass of an object? Triple the mass?
• 4.) Double the force in problem 1 (mass remains
  at 2 kg). What is the new acceleration of the
  object?
• 5.) What happens to the acceleration when you
  double the force acting on an object? Triple the
  force?
• 6.) What is Newtons Second Law of Motion? Give
  the definition and then describe it in your own
  words

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• 1.) Apply a constant horizontal force to the cart
  (Hint Pull the scale so it reads "1 Newton". As
  you pull the cart, keep the scale reading 1
  Newton.) What happens?
• 2.) What happens if you apply an even larger
  force (like 4 or 5 Newtons)? Does the cart go
  faster or slower than when only 1 Newton was
  applied?
• Conclusion Applying a greater force will
  _________ the acceleration of an object.
• 3.) Add mass to the dynamics cart by placing some
  of your weights on it. Apply the same force to
  the cart from step 1. In which case is it harder
  to pull the cart In step 1 with little mass, or
  in step 3 with more mass? What does this tell
  you about the acceleration of the cart?
• Conclusion Applying more mass will _________
  the acceleration of an object.