Physic 110 Lecture 15 from Chapter 6'''Section 1

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Physic 110 Lecture 15 from Chapter
6...Section 1

• Impulse and Momentum

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Homework assignment 15

• Assignment
• Conceptual Question
• Chapter 6, C.Q. 13 on page 181 Chapter 6,
  C.Q. 16 on page 181
• Problems
• Chapter 6, Problem 1 on page 181
• Chapter 6, Problem 10 on page 182 Chapter
  6, Problem 12 on page 182

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Momentum

• The linear momentum of an object of mass m
  moving with a velocity is defined as the
  product of the mass and the velocity
• SI Units are kgm/s
• Momentum is a Vector quantity
• Its direction is the same as the velocitys

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Impulse

• In order to change the momentum of an object, a
  force must be applied
• The time rate of change of momentum of an object
  is equal to the net force acting on it
• Gives an alternative statement of Newtons second
  law

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Impulse cont.

• When a single, constant force acts on the object,
  there is an impulse delivered to the object
• is defined as the impulse
• Units of impulse are N-s or kg-m/s
• Vector quantity, the direction is the same as the
  direction of the force

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Impulse-Momentum Theorem

• The theorem states that the impulse acting on the
  object is equal to the change in momentum of the
  object
• If the force is not constant, use the average
  force applied

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Average Force in Impulse

• The average force can be thought of as the
  constant force that would give the same impulse
  to the object in the time interval as the actual
  time-varying force gives in the interval

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Average Force cont.

• The impulse imparted by a force during the time
  interval ?t is equal to the area under the
  force-time graph from the beginning to the end of
  the time interval
• Or, the impulse is equal to the average force
  multiplied by the time interval,

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Impulse Applied to Auto Collisions

• The most important factor is the collision time
  or the time it takes the person to come to a rest
• This will reduce the chance of dying in a car
  crash
• Ways to increase the time
• Seat belts
• Air bags

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Air Bags

• The air bag increases the time of the collision
• It will also absorb some of the energy from the
  body
• It will spread out the area of contact
• decreases the pressure
• helps prevent penetration wounds

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Example 1

• A tennis ball is served with a speed of
• 65 m/s ( approx 146 mph).
• If the balls mass is 0.060 kg and is in contact
  with the racket for 0.030 s, what is the average
  force on the ball?
• Would this force be large enough to lift an
  average size person?

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Example 1

• vo0 m/s vf65 m/s m0.06 kg ?t0.03 s
• Impulse-Momentum
• ?

Assume average size person (of 60 kg has weight
of approx 600 N)
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Example 2

• Water leaves a hose at a rate of 5.0 kg/s with a
  speed of 50 m/s It strikes a wall which stops
  it. What is the force exerted by the water on
  the wall?

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Example 2

• Mass flow rate
• v0 50 m/s vf 0 m/s
• Impulse-Momentum

- 250 N (force of wall on water)
An equal but opposite force acts on the wall
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Example 3

• The force shown in the force vs. time diagram
  acts on a 1.5 kg object. Find
• a) The impulse of
• the force.
• b) The final velocity of
• the object if it starts
• with velocity -2.0m/s

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Example 3

• Principle Impulse-Momentum

Velocity changed from -2 m/s to 6 m/s
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Example 3

• Principle The area under the force-time graph
  represents the impulse or the change of momentum.

Impulse is 12 N-s or 12 kg-m/s