# What happens to PE during elastic/inelastic collisions?

• southernson
In summary, collisions can be classified as elastic or inelastic based on the conservation of kinetic energy. In horizontal applications, potential energy is usually not involved. However, in the case of an inelastic collision, the potential energy may not be conserved as the object will not reach the same height as before. However, in most cases, the difference in potential energy before and after the collision is negligible. It is important to note that examination questions assume that collisions are instantaneous, meaning that the time taken for the collision is so short that the change in potential energy is negligible. Therefore, potential energy can be considered conserved in collisions. However, in real-world scenarios, there are ways for energy to be lost, such as through sound or heat
southernson
Elastic/Inelastic collisions are always defined in terms of KE, it being conserved in elastic and not in inelastic. But what happens to PE? In horizontal applications there's usually no PE involved, but consider a ball dropped onto a surface that experiences an inelastic collision. The ball won't reach the same height, so would one say that PE was not conserved? I've also had it explained to me that the PE immediately before the collision is the same as it is immediately after the collision, so PE is conserved.

collisions are "instantaneous"

southernson said:
I've also had it explained to me that the PE immediately before the collision is the same as it is immediately after the collision, so PE is conserved.

Yes, the difference in PE before and after is either zero or negligble.

Examination questions about collisions assume that a collision is "instantaneous".

In other words, the collision takes such a negligibly short time that you can regard the time as zero.

In zero time, the height doesn't change, so the PE doesn't change (or, if you prefer, in a negligibly short time the PE changes a negligible amount, so you can ignore it).

southernson said:
The ball won't reach the same height, so would one say that PE was not conserved?

Yup, there are ways to loose energy in our non-perfect reality. For example, energy can be lost via sound or heat.

## 1. What is the law of conservation of energy and how does it apply to elastic/inelastic collisions?

The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another. This law applies to elastic/inelastic collisions because the total amount of energy before and after the collision should remain the same.

## 2. How does the potential energy change during an elastic collision?

In an elastic collision, the potential energy is converted into kinetic energy as the objects collide. This means that the potential energy decreases while the kinetic energy increases.

## 3. What happens to the potential energy during an inelastic collision?

In an inelastic collision, some of the potential energy is converted into other forms of energy such as heat or sound. This means that the potential energy decreases even more compared to an elastic collision.

## 4. Does the potential energy of an object remain constant during a collision?

No, the potential energy of an object can change during a collision depending on the type of collision. In elastic collisions, the potential energy is converted into kinetic energy. In inelastic collisions, the potential energy is converted into other forms of energy.

## 5. How does the coefficient of restitution affect the potential energy during a collision?

The coefficient of restitution is a measure of how much kinetic energy is retained after a collision. A higher coefficient of restitution means that more kinetic energy is retained, which also means that less potential energy is converted into other forms of energy. Therefore, the potential energy will decrease less in a collision with a higher coefficient of restitution compared to a lower one.

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