A hockey puck sliding on a frictionless surface strikes a box at rest. After the

Question

A hockey puck sliding on a frictionless surface strikes a box at rest. After the collision, the two objects stick together and move at some final speed. Which of the following describes the change in momentum and energy of the puck during the collision? One cannot determine if the momentum or energy of the puck is conserved without knowing the final velocities and masses of the two objects. The puck conserves its original momentum, but loses all of its mechanical energy. The puck conserves its original momentum, but loses some, but not all, of its mechanical energy. The puck conserves its original momentum and mechanical energy. The puck loses some momentum in the collision, but conserves its mechanical energy. The puck loses some, but not all, of its original momentum and mechanical energy. Incorrect. Enough information exists to answer the questions regarding conservation of momentum and energy. Which of the following describe the change in momentum and energy of the combined puck and box during the collision? The system loses some, but not all. of its momentum and mechanical energy. One cannot determine if momentum or energy of the system is conserved without knowing the final velocities and masses of the two objects. The system conserves its original momentum and mechanical energy. The system loses some momentum in the collision, but conserves its mechanical energy. The system conserves its original momentum, but loses all of its mechanical energy. The system conserves its original momentum, but loses some, but not all, of its mechanical

Explanation / Answer

a) the puck loses some, but not all,of its original momentum and mechanical energy

before collision Pi = m Vi ,,,, after collision Pf = m Vf ,,,, then Pf - Pi = m ( Vf - Vi ) 0 ,,,,

before collision Ki = ( m Vi^{2} ) / 2 ,,,, after collision,,, Kf = ( m Vf^{2} ) / 2 ,,,

then,,,, Kf - Ki = ( m Vf^{2} ) / 2 - ( m Vi^{2} ) / 2 = [ m ( Vf^{2} - Vi^{2} ) ]/ 2 0

Since difference of linear momentum and kinetic energy are different to zero, those quantities are not conserved by the puck

since Vf < Vi then,,,, Pf - Pi < 0 (the puck lose linear momentum),,, and Kf - Ki < 0 ,,( the puck lose kinetic energy)

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