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Impulse

Impulse. 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. p i. p f. F ( t ). before. after. Force of Collision.

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Impulse

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  1. Impulse

  2. 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

  3. 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.

  4. Change in Momentum • A force acting during a time changes momentum. • The change in momentum is called the impulse (J). • Impulse = change in momentum • Impulse = area under force vs. time curve

  5. 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

  6. Average Force • An impulse is associated with a force. • The average force is the constant force that gives the same impulse in the same time.

  7. 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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