Impulse

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Impulse
http//www.youtube.com/watch?vYFzNKBZehSU

• Chapter 8.2

An impulse transfers momentum
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Impulse Changes Momentum!

• The change in momentum depends on the impulse,
  which is calculated by the impact force times the
  length of time it acts.

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Impulse

• Impulse is the product of force (F) times time
  (t)
• The impulse equation is I Ft
• I is the symbol for impulse
• F is the impact force
• T is the momentum transfer time
• Momentum is transferred through impulse
• A change in momentum equals impulse
• The time it takes for momentum to be transferred
  (or change) determines how much force per unit
  time will be experienced.

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Impulse Objectives (leave space between each
question)

1. What is impulse?
2. What is the impulse equation?
3. What occurs through impulse?
4. How are impulse and change in momentum related?
5. How does the time of momentum transfer affect the
   force of impact?

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Impulse

• 1. What is impulse?
• Impulse is the product of force (F) times time
  (t)
• an impulse transfers momentum
• 2. What is the impulse equation?
• The impulse equation is I Ft
• I is the symbol for impulse
• F is the impact force
• t is the momentum transfer time
• Unit for impulse is N x s (same as kg x m/s)
• 3. What occurs through impulse?
• Momentum transfer occurs through impulse

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Impulse

• 4. How are impulse and change in momentum
  related?
• Impulse is equal to a change in momentum
• Ft ?mv
• 5. How does the time of momentum transfer affect
  the force of impact?
• The time it takes for momentum to be transferred
  (or change) determines how much force per unit
  time will be experienced.

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8.2 Impulse Changes Momentum
impulse change in momentum Ft ?mv

• The impulse you give will be the impulse you get
  back The force of the impulse will vary
  inversely with time.

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Momentum and impulse

• Momentum is mass in motion (p mv)
• An impulse transfers momentum (I Ft).
• Impulse remains the same, but time of transfer
  and impact force will vary inversely.
• Impulse change in momentum
• Ft ?mv

Same momentum, same impulse
More time, less force
Less time, more force
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Impulse
The impulse and the change in momentum are the
same in each case.
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Impulse

• By increasing the time needed to change the
  momentum of an object, the amount of force
  applied per unit time is reduced.

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Impulse

• By decreasing the time needed to change the
  momentum of an object, the amount of force
  applied per unit time is increased.

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8.2 Impulse Changes Momentum
If the change in momentum occurs over a long
time, the force of impact is .
small.
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8.2 Impulse Changes Momentum
If the change in momentum occurs over a short
time, the force of impact is
large.
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Remember

• Extend the time of momentum transfer, reduce the
  force of impact.

Impulse with large time has small force compared
to same impulse with short time, which would have
a large force.
RM
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Combination Momentum and Impulse Question

• A car with a mass of 1500 kg moves at 10 m/s.
  What is the momentum of the car? If the car
  stops in 5 s, what is the braking force needed to
  stop the car?
• Step 1solve for momentum
• p mv p 1500 kg x 10 m/s
• p 15,000 kg x m/s
• Step 2solve for force using the momentumimpulse
  theorem
• mv Ft or p Ft
• F change in momentum/time
• F 15,000 kg x m/s
• 5 s
• F 3000 N (braking force required to stop the
  car)

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Impulse and Safety

• Safety in collisions can be achieved by extending
  the momentum transfer time in which the impact
  force acts.

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SLOW TRANSFER OF MOMENTUM

• Transferring momentum over a longer period of
  time requires a smaller force.
• Consider how materials

GIVE
The safety net reduces the stopping force on a
fallen acrobat by substantially increasing the
time interval of the contact
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DEVICES THAT INCREASE MOMENTUM TRANSFER TIME

• Traffic Crash Cushions

When you extend the time of transfer, you reduce
the force.
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DEVICES THAT INCREASE MOMENTUM TRANSFER TIME

• Air Bags

When you extend the time of transfer, you reduce
the force.
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DEVICES THAT INCREASE MOMENTUM TRANSFER TIME

• Bungee Cords

When you extend the time of transfer, you reduce
the force.
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QUICK TRANSFER OF MOMENTUM

• Transferring momentum in a short period of time
  greatly increases the impact force!

When you reduce the time of transfer, you
increase the force.
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8.2 Impulse Changes Momentum

• think!
• Consider a dish falling and landing on a carpet
  and on a hard floor. The momentum of the dish is
  transferred
• Compare impulse, impact force and time of
  momentum transfer for each surface.
• Why does the dish break when it hits the hard
  floor but does not break when it lands on the
  carpet?

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8.2 Impulse Changes Momentum

• think!
• When a dish falls, will the impulse be less if it
  lands on a carpet than if it lands on a hard
  floor?
• Answer
• The impulse would be the same for either surface
  because the same momentum change occurs for each.
  
• It is the force that is less for the impulse on
  the carpet because of the greater amount of
  momentum transfer time.
• The impact force is much greater on the dish when
  it lands on the hard floor because the momentum
  transfer time has been reduced. This impact
  force is enough to cause the dish to break.