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Impulse

Impulse. Storm Damage. A house takes damage from the force of a 60 mph gale-force wind. If the house is subject to a hurricane at 120 mph the force on the house is A) Equal. B) Two times stronger. C) Four times stronger. D) Eight times stronger. Linear Momentum.

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Impulse

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  1. Impulse

  2. Storm Damage • A house takes damage from the force of a 60 mph gale-force wind. If the house is subject to a hurricane at 120 mph the force on the house is • A) Equal. • B) Two times stronger. • C) Four times stronger. • D) Eight times stronger.

  3. Linear Momentum • Momentum is the measure of inertia in motion. • Momentum equals mass times velocity • Momentum is symbolized by p • Like velocity, momentum is a vector. • Momentum is parallel to velocity p m v

  4. Applying Force • Newton’s law uses mass and acceleration. • Acceleration is the time rate of change of velocity. • If the mass is constant the force depends on momentum.

  5. Change in Momentum • A force acting during a time changes momentum. • The change in momentum is called the impulse (J).

  6. At 60 mph the momentum of the air is due to the mass (m) and velocity (v). At 120 mph the speed is double. Half as much time to get the same amount of mass Storm Force p = mv p = m(2v)

  7. Nonzero Force • When momentum changes in a system, an external force has acted. • Constant force like gravity • Position-dependent force like the elastic force • Time-dependent force during brief contact pi pf F(t) before after

  8. Force of Collision • Collisions seem instantaneous. • They actually take place during a short time span. • The force (an elastic force) greatly increases during the contact, then decreases.

  9. Impulsive Force • A karate strike to a board can break it. • Mass of the hand: 0.70 kg • Weight of the hand: 6.9 N • Velocity of the hand: 5.0 m/s • Stopping distance: 6.0 mm pi = -(0.7 kg)(5.0 m/s) = -3.5 kg m/s pf = 0 J = pf-pi= 3.5 kg m/s directed upwards on the hand

  10. Average Force • The average force is the constant force that gives the same impulse in the same time. • Impulse = area under force vs. time curve

  11. Breaking an object benefits from a short time interval. The average force is hundreds of times larger than the weight. Brief Time Dt = Dy / vav = Dy / (v/2) = (0.006 m) / (2.5 m/s) = 2.4 ms Fav = J / Dt = (3.5 kg m/s)/(0.0024 s) = 1.5 kN next

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