Conservation of momentum when a ball hits a wall

[Amik]

Homework Statement

Can we use the conservation of momentum when the ball hit then wall?

Relevant Equations

(Px)f=(px)i

I think we can.Although the wall is not moving, it is just because the wall has a huge mass.As rhe law of the conservation of momentum states(suppose the ball hits the wall from the left), when the momentum decrease by J, the momentum of the wall increase by J, which means the momentum of the wall is 0+J=J,then J(which is the total momentum of the wall right now equals P) divided by huge m.The v is extremely small.Although it is small, it actually has(it is just we can not observe).
Am I right?

 

[PeroK]

The wall is usually attached to the Earth. The momentum of the ball/wall/Earth system is conserved. But, of course, the momentum of the ball is reversed.

 

[Amik]

Thank you.But Is my explanation right?

 

[PeroK]

Thank you.But Is my explanation right?

What you have written is unclear, though I believe you have the right idea. Note that momentum is a vector.

 

[Amik]

I just want to say when you push a object with a large mass, the object is actually moving(but we can not see it).
An I right?

 

[PeroK]

I just want to say when you push a object with a large mass, the object is actually moving(but we can not see it).
An I right?

Perhaps. If you push a wall, it doesn't move because your feet are pushing on the Earth in the opposite direction.

If you throw a ball against a wall, then the ball gets its momentum from the Earth initially and then it gets converted back when the ball eventually comes to rest.

 

[MartinCarr]

This is the same idea as when you start to walk, you have created momentum! In reality the Earth moves the opposite direction, but due to mass. Hartly at at all!

 

[Amik]

That helps so much!Thanks!

 

[ZapperZ]

I know I'm a bit late to the party, and it appears that the OP's query has been answered. But I want to post this because this type of question has appeared periodically on here.

The best way to see this is to actually look at a simulation of the collision of two objects. I have done this in my lesson on conservation of momentum with my students, and they seem to like it, so I'll repeat the gist of it here.

Open the PhET app on the Collision lab (you will need the capability of running Adobe Flash).

https://phet.colorado.edu/sims/collision-lab/collision-lab_en.html

First things first: click on "More Data" at the bottom of the screen so that you get to see the values of momentum and kinetic energy for each mass. You should have a screen that looks something like this:

In this exercise, the elasticity is at 100%.

Now, in my class, I asked the students to play around with the values of the masses such that they observe these cases (in all cases, m2 is stationary at the start):

1. m1 >> m2
2. m1 > m2
3. m1 = m2
4. m1 < m2
5. m1 << m2

I then ask them in which of those cases is the most accurate representation of these situations:

a) An 18-wheeler trailer colliding with a stationary ping pong ball
b) a pool ball colliding with another stationary pool ball
c) a bouncy ball bouncing off the floor

You will notice that as you increase m2 relative to m1, it will move less and less upon collision, until at some point (say m2 = 100 m1), m2 hardly will even move. Just think of how little m2 will move if m2 is the Earth and m1 is a bouncy ball. It moves so little that (i) m2 can still be considered as stationary, and (ii) m1 will have the same speed, but in the opposite direction, as the speed that it started with.

The conservation of momentum for the entire system (m1 + m2) is still valid. It is just that when one mass is significantly larger than the other, it may not be that obvious by itself until you perform an exercise such as this. This is what my students discover on their own when they did this exercise in class.

Addendum: We also did cases where the elasticity is 0% elastic, and for m1 << m2, this corresponds to dropping a silly putty onto the floor.

Zz.

 

[Amik]

Exactly.When one mass is huge, the velocity is too small to observe.