physics question - momentum/ WEP

• lavastar

  30 Sep 07, 00:10

  A steel ball of mass 0.020kg is released from a height of 1.2m above a steel plate and rebounds vertically to a height of 1.1m. The ball is in contact with the surface for 0.9ms
  
  im stuck on one part of this question.that is the change of the momentum of the ball in its collision with its plate ( the answer is 0.19Ns )
  
  The answers for the 1st two parts are
  
  Velocity of ball as it arrive at the plate = 4.9m/s
  momentum of ball as it arrive at the plate = 0.097
  
  i tried alot of ways but cannot reach the answer? any help?
• eagle

  30 Sep 07, 00:16

  Originally posted by lavastar:

  A steel ball of mass 0.020kg is released from a height of 1.2m above a steel plate and rebounds vertically to a height of 1.1m. The ball is in contact with the surface for 0.9ms
  
  im stuck on one part of this question.that is the change of the momentum of the ball in its collision with its plate ( the answer is 0.19N )
  
  The answers for the 1st two parts are
  
  Velocity of ball as it arrive at the plate = 4.9m/s
  momentum of ball as it arrive at the plate = 0.097
  
  i tried alot of ways but cannot reach the answer? any help?

  We use back vi = -4.85 m/s
  
  vf = sqrt(2 * 9.81 * 1.1) = 4.65 m/s
  
  So change in momentum = (0.020) (4.65 - (-4.85)) = 0.19N
  
  Because we take momentum is a vector, velocity is a vector, and taking upwards as positive direction, hence vi = -4.85
  
  Regards,
  Eagle
  
• lavastar

  30 Sep 07, 00:21

  Originally posted by eagle:

  We use back vi = -4.85 m/s
  
  vf = sqrt(2 * 9.81 * 1.1) = 4.65 m/s
  
  So change in momentum = (0.020) (4.65 - (-4.85)) = 0.19N
  
  Because we take momentum is a vector, velocity is a vector, and taking upwards as positive direction, hence vi = -4.85
  
  Regards,
  Eagle
  

  omg thanks alot.. realised that i disregarded the minus sign even though momentum is a vector quantity.
• eagle

  30 Sep 07, 00:22

  Originally posted by lavastar:

  omg thanks alot.. realised that i disregarded the minus sign even though momentum is a vector quantity.

  students always make this same careless mistake
  
  Tip: everytime, when you answer questions that contain vectors, always declare which direction is positive. This way, it is easier on both you and the marker.
• ^tamago^

  30 Sep 07, 03:11

  for me i always declare the 2 axes for kinetics involving vectors, and they are usually along the axis of movement, or vertical-horizontal.
• eagle

  30 Sep 07, 15:16

  Originally posted by ^tamago^:

  for me i always declare the 2 axes for kinetics involving vectors, and they are usually along the axis of movement, or vertical-horizontal.

  Yep
  
  To add on another tip, every diagonal vector can be easily resolved into a vertical and a horizontal vector for overall easier calculations and understanding