12.2 Force is the Rate of Change of Momentum

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12.2 Force is the Rate of Change of Momentum
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Chapter 12 Objectives

• Calculate the linear momentum of a moving object
  given the mass and velocity.
• Describe the relationship between linear momentum
  and force.
• Solve a one-dimensional elastic collision problem
  using momentum conservation.
• Describe the properties of angular momentum in a
  systemfor instance, a bicycle.
• Calculate the angular momentum of a rotating
  object with a simple shape.

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Chapter Vocabulary

• angular momentum
• collision
• law of conservation of
• momentum
• elastic collision
• gyroscope
• impulse
• inelastic collision
• linear momentum
• momentum

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12.2 Force is the Rate of Change of
Momentum

• Investigation Key Question
• How are force and momentum related?

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12.2 Force is the Rate of Change of
Momentum

• Momentum changes when a net force is applied.
• The inverse is also true
• If momentum changes, forces are created.
• If momentum changes quickly, large forces are
  involved.

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12.2 Force and Momentum Change
The relationship between force and motion follows
directly from Newton's second law.
Force (N)
Change in momentum (kg m/sec)
Change in time (sec)
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Calculating force
Starting at rest, an 1,800 kg rocket takes off,
ejecting 100 kg of fuel per second out of its
nozzle at a speed of 2,500 m/sec. Calculate the
force on the rocket from the change in momentum
of the fuel.

• You are asked for force exerted on rocket.
• You are given rate of fuel ejection and speed of
  rocket
• Use F ?? ?t
• Solve ?? (100 kg) (-25,000 kg m/s) (1s) -
  25,000 N
• The fuel exerts and equal and opposite force on
  rocket of 25,000 N.

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12.2 Impulse

• The product of a force and the time the force
  acts is called the impulse.
• Impulse is a way to measure a change in momentum
  because it is not always possible to calculate
  force and time individually since collisions
  happen so fast.

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12.2 Force and Momentum Change
To find the impulse, you rearrange the momentum
form of the second law.
Impulse (Nsec)
Change in momentum (kgm/sec)
Impulse can be expressed in kgm/sec (momentum
units) or in Nsec.