From: niklas.holsti@tidorum.invalid   
      
   On 2020-10-25 1:48, JF Mezei wrote:   
   > On 2020-10-24 03:02, Niklas Holsti wrote:   
   >   
   >> If the orbiting object is in a circular orbit, the force acts   
   >> perpendicularly to the motion and so does no work.   
   >   
   >   
   > Changing direction of travel is "no work" ?   
   >   
   > If I am on a hockey rink, and a puck is coming at me and I wish to   
   > change its course so it ends up in the goal, I need to apply force to   
   > change its direction or travel, even if afterwards, it travels at the   
   > same speed as before.   
      
   Consider a puck bouncing off the wall around the rink. Suppose, first,   
   that it comes in perpendicular to the wall. If both the wall and the   
   puck are perfectly elastic, the collision will deform them until the   
   entire kinetic energy is stored as potential energy in the deformation.   
   This transformation was effected by the impact force working to move   
   (compress) the material along the direction of the force, which also   
   decelerates the puck to a stand-still. Next, the pressure force acts in   
   the opposite direction to decompress the wall and the puck and   
   accelerate (bounce) the puck back, with the same speed it had before the   
   impact, but in the opposite direction. This converts the potential   
   (compression) energy back into the kinetic energy of the puck. Total   
   energy remains constant.   
      
   If the puck and wall are imperfectly elastic, some energy is lost into   
   vibration, heating, and permanent (plastic) deformation.   
      
   If the puck hits the wall at a slant, the above applies to the component   
   of the puck's velocity that is perpendicular towards the wall, which is   
   changed to have the opposite direction, away from the wall. The velocity   
   component parallel to the wall is not changed, in the ideal case of   
   perfect elasticity and no friction. In reality, of course, the wall and   
   puck are not perfectly elastic, and the friction between the wall and   
   the puck converts some of the along-the-wall velocity into heat and into   
   puck spin.   
      
   Going back to your example of you changing the course of the puck, if   
   you don't need to change the speed of the puck, you could just put the   
   blade of your stick into the path of the puck at such an angle that the   
   puck bounces off the blade into the goal, assuming you can keep the   
   blade perfectly steady (like a wall). That may take muscular effort, but   
   only because humans are built that way (imperfectly), not because of   
   basic physics.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)