Elastic collision against a wall

In summary, frictionless surface, or whatever, is a prerequisite for an elastic collision. Kinetic theory predicts that the momentum of the particles in the collision is unchanged, so there is no impulse.
  • #1
davidbenari
466
18
In an elastic collision against a wall, where the angle of incidence is equal to the angle of deflection, why is it assumed that friction effected no impulse, and only the normal force did? I can understand if this was stated in the problem by saying "frictionless surface, or whatever" but this seems to be the case if a shoot a tennis ball against the floor or a wall too. Why is it that friction is almost negligible here? Is this a property of very-quick collisions, like in Kinetic Theory?

Thanks.
 
Physics news on Phys.org
  • #2
Well, you did specify an elastic collision, which means perfect conservation of kinetic energy. If friction did anything at all, then there would be dissipation and no longer conservation of energy.

If you shoot a tennis ball against a floor at an angle, the interaction is vastly different from an elastic collision: friction plays a big role, as do deformation, angular momentum, etc. It's a very complicated interaction that makes for a pretty poor intro physics textbook problem. :wink:
 
  • #3
davidbenari said:
In an elastic collision against a wall, where the angle of incidence is equal to the angle of deflection, why is it assumed that friction effected no impulse, and only the normal force did? I can understand if this was stated in the problem by saying "frictionless surface, or whatever" but this seems to be the case if a shoot a tennis ball against the floor or a wall too. Why is it that friction is almost negligible here? Is this a property of very-quick collisions, like in Kinetic Theory?

The frictional force acts parallel to the direction of the wall. Does the momentum of the ball in that direction change? If not, there was no impulse from that force - and that's the case for the idealized elastic collision that you're describing.

It pretty much has to be that way because you've specified that the collision is elastic so kinetic energy is conserved and the speed of the ball on the way out is the same as the speed in, and that the angle of incidence is equal to the angle of deflection.
 
  • Like
Likes 1 person
  • #4
Oliver, Nugatory: Suppose you've never seen a tennis ball collide with a wall when thrown at an angle. What would make you predict that the outcome of this collision would be the way it's normally seen? Namely, angle of incidence = angle of deflection, collision is almost elastic.

Also, in the case of Kinetic Theory, is friction negligible when particles collide against the container? If this is the case, then why don't they slide along the container during the collision? Is this because the collision is almost instantaneous ?

Thanks.
 
  • #5
davidbenari said:
Oliver, Nugatory: Suppose you've never seen a tennis ball collide with a wall when thrown at an angle. What would make you predict that the outcome of this collision would be the way it's normally seen? Namely, angle of incidence = angle of deflection, collision is almost elastic.

I don't think I'd make a good prediction based on the theory of elastic collisions, because the physics of tennis ball bounces isn't much like an elastic collision (except superficially!).

That's what makes the physics of sports such an interesting topic. We have some idea from experience how certain kinds of balls, bats, etc. behave. What we observe usually doesn't fit very well with very simple (ideal physics) type models, and so we get to figure out why.

Also, in the case of Kinetic Theory, is friction negligible when particles collide against the container? If this is the case, then why don't they slide along the container during the collision? Is this because the collision is almost instantaneous ?
Well, I think in kinetic theory the point is all the molecules basically undergo elastic collision, and some molecules are more free to move than others. There is this notion, however, of a "no-slip boundary," which means the molecules don't just slip past the walls of the container uninhibited.
 
  • Like
Likes 1 person
  • #6
davidbenari said:
Oliver, Nugatory: Suppose you've never seen a tennis ball collide with a wall when thrown at an angle. What would make you predict that the outcome of this collision would be the way it's normally seen? Namely, angle of incidence = angle of deflection, collision is almost elastic.
I wouldn't make such a prediction until I had seen it - and in fact a tennis ball/wall collision isn't an especially good example of an elastic collision. A steel ball bearing bouncing off a polished steel plate might be easier to predict, as the modulus of elasticity of steel is so very high that we could safely ignore all the second-order effects that come from the deformation of the ball on impact.

But do remember that physics starts with observations, and the mathematical models follow from what has been observed and must be explained. We study the idealized perfectly elastic collision because it's a useful and computationally tractable model of the collisions that we've observed; when we see a collision in which the kinetic energy loss is insignificant and the incident and deflection angles are near as no never mind the same we use this model. Asking why the collision obeys the rules of the model is getting things backwards - we chose the model because it matched the observation.
 
Last edited:
  • Like
Likes 1 person

What is an elastic collision against a wall?

An elastic collision against a wall is a type of collision where an object collides with a wall and bounces off with no loss of kinetic energy.

What is the difference between an elastic collision and an inelastic collision against a wall?

In an elastic collision against a wall, the object bounces off the wall with no loss of kinetic energy. In an inelastic collision, some of the kinetic energy is lost and the object does not bounce off the wall.

What factors affect the magnitude of the rebound velocity in an elastic collision against a wall?

The magnitude of the rebound velocity in an elastic collision against a wall is affected by the mass and velocity of the object, as well as the properties of the wall such as its material and surface texture.

What is the conservation of energy principle and how does it relate to elastic collisions against a wall?

The conservation of energy principle states that energy cannot be created or destroyed, only transferred from one form to another. In the case of an elastic collision against a wall, the total kinetic energy of the object before and after the collision remains the same, demonstrating the conservation of energy.

What real-life examples can be explained using the concept of elastic collisions against a wall?

Real-life examples of elastic collisions against a wall include a game of billiards, where the cue ball bounces off the sides of the table, and a ball bouncing off a wall, such as in the sport of handball.

Similar threads

Replies
8
Views
1K
Replies
14
Views
1K
Replies
6
Views
972
Replies
4
Views
1K
Replies
8
Views
1K
Replies
2
Views
2K
Replies
1
Views
460
Replies
32
Views
4K
Replies
1
Views
2K
  • Electromagnetism
Replies
2
Views
657
Back
Top