امثلة واقعية لمواضيع الديناميك (1)

Momentum and Energy Chapter 3 Joseph F. Alward, PhD Department of Physics University of the Pacific

Momentum

Momentum = mass x velocity = m v If the boulder and the boy have the same momentum, will the boulder crush the boy? Hint: Which would have the larger speed?

Impulse

Example: Wall exerts a force of 10,000 N. The contact time is 0.01 s. ------------------------------ Impulse = F t = 100 N-s

Momentum Change

D(mv) = Change in momentum D(mv) = final - initial = 0 - mv = - mv Ignoring negative sign: D(mv) = mv

Momentum Change = Impulse

D(mv) = Ft mv = Ft

Impulses and Contact Time





Rolling with the Punches

Spreading impulse out over a longer time means that the force will be less; either way, the change in momentum of the boxing glove, fist, and arm will be the same.

Force = Impulse / Time

Contact time is reduced if arm's deceleration is kept as small as possible. This is done by using "follow-through", which means to continue to push during the contact period.

Increasing Impulse

Curved blades prolong contact with water: Impulse = F Dt Greater impulse means greater change in momentum of the wheel.

Conservation of Momentum

Momentum Before = 0 ------------- Momentum After = 0 ------------- After firing, the opposite momenta cancel.

Conservation of Momentum

Momenta are equal but opposite. M v = m V 4 v = 0.010 x 300 = 3 v = 3 / 4 = 0.75 m / s

Conservation of Momentum in Collisions






Elastic Collisions

In elastic collisions no permanent deformation occurs; objects elatically rebound from each other. --------------------------- In head-on elastic collisions between equal masses, velocities are exchanged.

Inelastic Collisions

Inelastic collisions--as between the arm and wooden plates-- are characterized by permanent deformation. The kinetic energy of the swinging arm is converted into heat and chemical energy (to break the bonds between atoms in the wood).