How dampening works versus conservation of momentum

[sr241]

if linear and angular momentum are separately conserved how dampening works? like in a suspension of a car

 

[tiny-tim]

hi sr241!

same reason linear momentum isn't conserved when a ball bounces off the ground … there's an external force from the ground on the ball, or from the chassis and the axle on the spring

(the dampening forces are internal, and therefore don't change the total momentum … eg if you take the spring out of the car and lay it on the floor with zero momentum, then extend it and let it go so that it settles down fairly quickly, the total momentum remains zero )

 

[Blibbler]

Sorry to be pedantic, but this is one of my bugbears.

Dampening refers to making something damp. So a dampening field is a wet meadow.

To remove energy from an oscillating system is to damp the oscillation, not to dampen it.

I wince whenever a sci-fi character goes on about inertial dampening - what does it mean? Emptying a bucket of water over a stationary massive object? Standing perfectly still in the rain?

A critically damped oscillation is critically damped, not dampened.

You get the idea...

 

[sr241]

sorry for the typo ( dampened instead of damped)

if one side is made maximum elastic collision and other side is maximum damped. can we make reaction-less space engines.

how conservation of momentum fails in damped collision

 

[rcgldr]

if linear and angular momentum are separately conserved how dampening works? like in a suspension of a car

You need to include the Earth as part of this closed system. The total linear and angular momentum of the Earth and car are conserved. The suspension of a car is composed of two main components, the springs which allow the tires to move with respect to the car, and the shocks which damp that movement so it doesn't oscillate.

 

[sr241]

The total linear and angular momentum of the Earth and car are conserved.

Is it total of angular and linear momentum is conserved or is the linear and angular momentum is separately conserved.

how damping force affect linear momentum and its conservation?

 

[rcgldr]

Is it total of angular and linear momentum ...

In a closed system (no external forces) linear and angular momentum are separately conserved. Energy may be converted between potential (chemical, electrical, gravitational), angular, and linear energies, but the total energy is conserved (ignoring losses to heat that escape the closed system).

How damping force affect linear momentum and its conservation?

Damping doesn't affect the total momentum. The purpose of damping is to reduce or prevent oscillations. During this process, damping may convert mechanical energy into heat, reducing the overall energy due to heat escaping the system.

 

[K^2]

In other words, damping force transfers momentum between bodies to reduce relative velocity. It is never lost or gained in the process.

 

[tiny-tim]

Is it total of angular and linear momentum is conserved or is the linear and angular momentum is separately conserved.

linear momentum is a vector (ie it looks different in a mirror), but angular momentum (like any cross-product) is a pseudovector (ie it looks the same in a mirror, since -r x -v = r x v) …

you can't add a vector to a pseudovector, any more than you can add metres to square metres!

 

[vlado_skopsko]

The linear momentum of the car through the piston by friction is transversed to the linear momentum of the fluid (oil) molecules inside the dumpers, their speed increases (temperature goes up).
If there is temperature difference between the dumpers and the surrounding air, heat is transversed from the dumpers to the atmosphere (by the collisions between the molecules).

So in other words the momentum goes from macro to micro level but stays unchanged, if you watch the car dumpers and the atmosphere as a closed system.

Or maybe its better to say, the momentum goes from organized motion of particles to unorganized.

 

[Rap]

So in other words the momentum goes from macro to micro level but stays unchanged, if you watch the car dumpers and the atmosphere as a closed system.

Or maybe its better to say, the momentum goes from organized motion of particles to unorganized.

This is not right. Only the energy goes from macro organized to micro disorganized. Linear momentum and angular momentum are always conserved. When the car bounces up and down, the Earth bounces down and up so that up/down linear momentum is conserved, and that sums to zero. As the car bounces up, the Earth bounces down. As the energy is absorbed in the shock absorbers, the up and down motion of the car and Earth diminishes, but they still sum to zero. When the car finally stops bouncing, the Earth does too. Again the up/down momentum is zero, as it always has been. All the energy of the bouncing has been absorbed by the shock absorbers and has been turned into heat energy and so energy has been conserved as well.

 

[vlado_skopsko]

Yes, you are right. I was thinking of the energy but I thought that i can interchange those terms which it's not the case.

 

[sr241]

consider the scenario two balls of equal mass (say 1) is placed between two immovable walls. ball 'a' hits ball 'b' with a velocity = 10. trajectory of ball 'a' passes through center of mass of both balls and is normal to both walls. collision between two balls is damped (due to deformation) but collision between balls and both walls are perfectly elastic will the linear momentum will be conserved. what would be the position of center of mass of both balls( after both balls stop) relative to their initial position(I mean center of mass of both ball combined)

 

[Rap]

Hard to say. You know a few things - position and velocity of the center of mass won't change as the result of a collision between balls, but it will change when a ball hits the wall. The sum of the linear momentum of the balls will not be conserved. The sum of the kinetic energies of the balls will not be conserved. Also, it is possible that the final state is not that the two balls have stopped, but that they are stuck together as a unit, both bouncing between the walls. Actually, I think this will be the most likely situation unless the initial velocities and positions of the balls take very particular values.

 

[sr241]

Consider the scenario one ball is placed between two movable walls; movable walls are connected so that there is no relative movement between these walls. Ball hits wall with a velocity = 10. trajectory of ball passes through center of mass of both walls and is normal to both walls. collision between first wall is damped but collision between ball and other wall is perfectly elastic. Then, will the linear momentum will be conserved? What would be the position of center of mass of both ball and wall ( after both ball and wall stops or do they stop moving?) relative to their initial position(I mean center of mass of both ball and wall combined)

 

[Rap]

Consider the scenario one ball is placed between two movable walls; movable walls are connected so that there is no relative movement between these walls. Ball hits wall with a velocity = 10. trajectory of ball passes through center of mass of both walls and is normal to both walls. collision between first wall is damped but collision between ball and other wall is perfectly elastic. Then, will the linear momentum will be conserved? What would be the position of center of mass of both ball and wall ( after both ball and wall stops or do they stop moving?) relative to their initial position(I mean center of mass of both ball and wall combined)

The linear momentum of the walls and balls together will be conserved. Just the balls - no. The kinetic energy of the balls will not be conserved, but the kinetic energy and heat energy generated by the collisions will be conserved. I think generally, after a long time, when everything settles down, the balls will be stuck together, but moving, bouncing off the walls. In certain special cases, they will be stuck together, not moving with respect to the walls.

 

[tiny-tim]

hi sr241!

… two balls of equal mass (say 1) is placed between two immovable walls. ball 'a' hits ball 'b' with a velocity = 10. trajectory of ball 'a' passes through center of mass of both balls and is normal to both walls.

collision between two balls is damped (due to deformation) but collision between balls and both walls are perfectly elastic …

no, the two collisions are as elastic (or damped) as each other

elasticity is a measure of the energy retained after the collision (and has nothing to do with momentum)

will the linear momentum will be conserved. what would be the position of center of mass of both balls( after both balls stop) relative to their initial position(I mean center of mass of both ball combined)

in both cases, the total momentum (of the two balls) is zero, both before and after

internal forces do not alter the total momentum (including mutual forces between two bodies in the same system, such as the two balls)

 

[Rap]

hi sr241! no, the two collisions are as elastic (or damped) as each other.

I think the original thread was postulating that the two collisions were different. Nothing wrong with that. Maybe the balls are made of iron on one side, rubber on the other.

in both cases, the total momentum (of the two balls) is zero, both before and after

Not necessarily. Consider one ball bouncing between the two walls, elastically. At every collision, the momentum of the ball changes

internal forces do not alter the total momentum (including mutual forces between two bodies in the same system, such as the two balls)

Right, but in the collision with the walls, the total momentum of the balls does change.

 

[sr241]

in the video collision with red wall is damped and collision with all the green walls is perfectly elastic. total mass of front and back walls is equal to double the mass of ball.

you can see in the first case linear momentum is not conserved, that means damping can affect conservation of linear momentum, right?

 

[rcgldr]

In the video collision with red wall is damped and collision with all the green walls is perfectly elastic.

There's a problem with that video, noticable at the very start. After the first collisions with the green walls, the bottom U shaped object is moving faster than the top object. If you pause the video just before the ball hits the left wall on the bottom object you can see it's moved further ahead, which it shouldn't have if those objects were supposed to be identical except for the red wall on the upper object.

 

[sr241]

There's a problem with that video, noticable at the very start. After the first collisions with the green walls, the bottom U shaped object is moving faster than the top object. If you pause the video just before the ball hits the left wall on the bottom object you can see it's moved further ahead, which it shouldn't have if those objects were supposed to be identical except for the red wall on the upper object.

actually red wall is damped and green wall is perfectly elastic. so the linkage between these 2 walls are damped. I think that is why this happens

 

[Rap]

in the video collision with red wall is damped and collision with all the green walls is perfectly elastic. total mass of front and back walls is equal to double the mass of ball.

you can see in the first case linear momentum is not conserved, that means damping can affect conservation of linear momentum, right?

No, damping does not affect conservation of linear momentum. Taking a system that is subject to outside forces DOES affect conservation of momentum. If you have a system that is not subject to outside forces, momentum is conserved, otherwise its not. If you have two balls that are subject to the outside forces of two walls, then their momentum will not be conserved. If you consider the walls to be part of the system, and neither the walls nor the balls are subject to outside forces, then the total momentum of the walls and balls will be conserved. A car bouncing up and down on shock absorbers will not conserve momentum because it is subject to the outside force of the earth. If you take the car and the Earth together (and ignore the force of the sun and moon, etc.), then the total momentum of the car and the Earth is conserved.

 

[sr241]

can damping be assumed as the force that affect conservation of linear momentum

 

[tiny-tim]

can damping be assumed as the force that affect conservation of linear momentum

nothing affects conservation of linear momentum except external forces

(similarly, nothing affects conservation of angular momentum aout a point except external torques about that point)