Say we drop a ball into a bucket of honey. The falling ball will have a downwards momentum, but once falls into the honey, it will slow and then stop with no apparent opposite momentum. In such a case, how is momentum conserved?
$\begingroup$
$\endgroup$
4
-
$\begingroup$ Do you see how momentum is conserved if you drop a ball onto the floor and it bounces back? $\endgroup$– ToffomatCommented Dec 10, 2022 at 20:51
-
$\begingroup$ Definitely, as the ball bounces back. In this example, there is no upwards motion, and thus no upwards momentum. How is it conserved? $\endgroup$– TheoCommented Dec 10, 2022 at 22:48
-
$\begingroup$ Have you considered the particles the medium is made of? $\endgroup$– TriatticusCommented Dec 10, 2022 at 23:34
-
$\begingroup$ A body moving in a fluid, by the third principle of dynamics - action/reaction -, exerts an opposite force on the fluid. If a body slows down, it's receiving a force in the opposite direction of motion, and thus the fluid feels a force in the direction of motion of the body. The paricles of the fluid around the body are accelerated in that direction usually crearing a wake behind the body $\endgroup$– basicsCommented Dec 11, 2022 at 9:14
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
If the system is the ball by itself then momentum is not conserved because there are external forces acting on the ball, gravitational attraction of the Earth and viscous forces due to the honey.
Making the system, the ball, the bucket of honey and the Earth, will result in no external forces acting and thus in this case momentum is conserved.
Initial momentum of the system is zero with everything in the system at rest.
Release the ball and the ball falls down but also the Earth rises up (due to the attraction of the Earth by the ball) gaining the same amount of momentum in the opposite direction resulting in the net momentum of the system still being zero.
The ball hits the honey and slows down resulting in it having less momentum but the same thing happens to the Earth and the bucket with the change in momentum being in the opposite direction, ie the net momentum of the system is still zero.
The ball, bucket, honey and Earth stop moving, total momentum is still zero.
