Collision

[Primarygun]

In collision, law of conservation of momentum is applied only if no external force is added. Is weight considered as an external force? I think so. Anyone tell me ?

[swansont]

W=mg It's a force. But it's an internal force if you are looking at both the object and the earth, and the momentum of the system is conserved.

 

Even if you aren't including the earth, you might still be able to solve the problem. If the collision happens quickly, and the amount of the momentum change caused by the force is small, it can be ignored. Conservation of momentum can be applied during the collision, and conservation of energy applied before and after.

[5614]

simple expansion:

 

w= mg

 

where:

w = weight

m = mass

g = gravitational force (on earth this is 10)

[Guest Mr.Therefore]

In collision, law of conservation of momentum is applied only if no external force is added. Is weight considered as an external force? I think so. Anyone tell me ?

 

Consider that Earths gravity decreases proportionate to the distance away from the Earth. The gravity force on the atoms of the objects colliding undergo greater acceleration that are nearer to the Earth than the ones further away which undergo less acceleration. Athough this could be considered neglible for a convenient explanation.

 

When considered negligeable, the force of gravity is equally constant on all atoms before, during and after the collision. Because the force of gravity is exact on each atom of the objects, gravity only changes where the collision will occur along the vertical axis, considering we could imagine the force of gravity not affecting the trajectory of the the colliding objects.

 

Abstractly:

 

Without gravity, each atom collides with no vertical displacement due to gravity.

 

With gravity, each atom in the collision is perfectly displaced at 9.8 seconds per second vertically.

 

With this in mind, I answer your question this way: Gravity is an external force occuring during the collsion, but since it has a perfectly even effect upon each atom in the same direction, you can deduce it's affect and quantify the conservation of momentum without it disturbing the amount of momentum before and after your objects collision.

[swansont]

g = gravitational force (on earth this is 10)

 

g is an acceleration.

 

In what units is it equal to 10?

[jordan]

Hey, that reminds me swansont. I think I meantioned this in another thread, but my physics teacher recently pointed out that a (in particular g) is better described by N/kg rather than the typical m/sec². Is one more correct than the other? Is there any advantage to one or the other?

[5614]

g is an acceleration.

 

In what units is it equal to 10?

 

g is the unit of acceleration caused by gravity... it is precisely 9.806 65 m/s^2 but is often referred to as 10m/s^2

[Martin]

Hey, that reminds me swansont. I think I meantioned this in another thread, but my physics teacher recently pointed out that a (in particular g) is better described by N/kg rather than the typical m/sec^{2[/sup']. Is one more correct than the other? Is there any advantage to one or the other?}

 

^{looks like swansont is offline so I will put in twocentsworth here}

^{the unit force newton is defined as 1 kg m/sec2}

 

^{so if you divide a newton by a kg you just get 1 m/sec2}

 

^{so the two definitions are algebraically equivalent}

 

^{N/kg = 1 (kg m/sec2)/kg = 1 m/sec2}

 

^{this just means there are two equivalent ways of describing THE METRIC SYSTEM'S UNIT OF ACCELERATION. So far this does not say anything about normal earth surface "gee" being 9.8 m/sec2, or equivalently}

^{9.8 newton per kilogram.}

 

^{I am not sure what you mean by "more correct" or "advantage".}

^{Indeed it is a good thing to remember that a kilogram normally weighs 9.8 newtons! The force of its weight is 9.8 N.}

 

^{but I would say that this definition (unit accel = 1 meter/sec2)}

^{is more PRIMITIVE AND BASIC because it deals with the most primitive measurements: distance and time.}

^{force is more complicated to standardize, and generally depends on measuring distance and time.}

^{So I would argue that the acceleration unit should primarily be thought of as 1 m/sec2, because more basic, and only secondarily the way your teacher says, just because it is sometimes convenient.}

[jordan]

It only seemed like N/kg was more correct because g is a measure of how many Newtons pull on a mass for a given number of kg (at the Earth's surface). I'm not sure exactly how m/sec² translates into a realistic description.

[Primarygun]

If an apple falls from a tree and hit the earth. Is the weight considered as external force?

Also, when they are in contact and starts to collide, the apple or the earth is back, in other words, the distance after impact is larger than that when it is at the moment of impact.

However, the earth would have a force that pulling the apple coming towards it again after impact, and the velocity then is affected from the law of conservation of momentum and thus I consider weight is an external force,isn't it?

Any advice is treasure

[swansont]

If an apple falls from a tree and hit the earth. Is the weight considered as external force?

Also' date=' when they are in contact and starts to collide, the apple or the earth is back, in other words, the distance after impact is larger than that when it is at the moment of impact.

However, the earth would have a force that pulling the apple coming towards it again after impact, and the velocity then is affected from the law of conservation of momentum and thus I consider weight is an external force,isn't it?

Any advice is treasure[/quote']

 

If you look only at the apple, then the weight is an external force and its momentum is not conserved. However, if you look at the apple-earth system, then it's an internal force and momentum is conserved for the system. In the center-of-momentum frame, where the two objects start at rest, they will have momenta of equal magnitude and opposite direction.

[Primarygun]

Thank you.