ImpulseNonzero ForceForce of CollisionChange in MomentumImpulsive ForceAverage ForceBrief TimeImpulseImpulseNonzero ForceNonzero ForceWhen momentum changes in a system, an external When momentum changes in a system, an external force has acted.force has acted.•Constant force like gravityConstant force like gravity•Position-dependent force like the elastic forcePosition-dependent force like the elastic force•Time-dependent force during brief contactTime-dependent force during brief contactbefore afterpipfF(t)Force of CollisionForce of CollisionCollisions seem Collisions seem instantaneous.instantaneous.They actually take place They actually take place during a short time span.during a short time span.The force (an elastic force) The force (an elastic force) greatly increases during the greatly increases during the contact, then decreases.contact, then decreases.Change in MomentumChange in MomentumA force acting during a time A force acting during a time changes momentum.changes momentum.The change in momentum is The change in momentum is called the impulse (called the impulse (JJ).).•Impulse = change in Impulse = change in momentummomentum•Impulse = area under force Impulse = area under force vs. time curvevs. time curvepppdtFJdtpdFifttfiImpulsive ForceImpulsive ForceA karate strike to a board A karate strike to a board can break it.can break it.•Mass of the hand: 0.70 kgMass of the hand: 0.70 kg•Weight of the hand: 6.9 NWeight of the hand: 6.9 N•Velocity of the hand: 5.0 m/sVelocity of the hand: 5.0 m/s•Stopping distance: 6.0 mmStopping distance: 6.0 mmpi = (0.7 kg)(5.0 m/s) = 3.5 kg m/spf = 0J = pf pi = 3.5 kg m/sdirected upwards on the handAverage ForceAverage ForceAn impulse is associated An impulse is associated with a force.with a force.The average force is the The average force is the constant force that gives the constant force that gives the same impulse in the same same impulse in the same time.time.tJFavBrief TimeBrief TimeBreaking an object benefits Breaking an object benefits from a short time interval.from a short time interval.The average force is The average force is hundreds of times larger hundreds of times larger than the weight.than the weight.nextt = y / vav = y / (v/2)= (0.006 m) / (2.5 m/s)= 2.4 msFav = J / t = 3.5 kg m/s)/(0.0024 s)= 1.5
View Full Document
Unlocking...