# I What slows an object during collision, force or loss of energy?

#### alkaspeltzar

I have an example from my machinery handbook which shows the kinetic energy of a pile driver and shows it will drive a pile some distance at a force until the energy is expended.

The formula is (Average force of blow times distance)= Weight of object (driver) times distance it falls)

Just curious, what slows down the driver as it hits the pile? Is it the loss of energy, or the reaction force from the pile, or both? It has to be both right?

As it does the work, it is expending the energy due to the reaction force trying stop it? Maybe someone can help me understand this situation.

Thank you

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#### HallsofIvy

Homework Helper
There has to be some reason to lose energy! It is the force that causes the loss of energy. With no external force we have "conservation of energy".

#### yoosnb

Kinetic Energy is converted to Heat Energy due to the nature of inelastic collisions

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#### alkaspeltzar

I guess what I am curious is about is how does the reaction force on the pile driver come into play with energy balance.

If the work done on the pile is the energy used by the pile driver, is it because it used that energy or the reaction force on the pile driver that slowed it to a stop.

Only thing that makes sense is that as the work is done on the pile, the driver feels an equal force which slows it down, giving up its energy to do the work on the pile. (or something like that)

#### yoosnb

Yes, but I don't see what that has to do with the original question. I notice there is another thread titled "Energy Loss during Inelastic collisions, Where does it go!". Did you intend to post under that?

Anyways, after reading the question then your reply, I have a question that may or may not be related to his.

Why do elastic collisions conserve kinetic energy? What makes them different from inelastic ones? Thank you for replying :)

#### Dale

Mentor
Just curious, what slows down the driver as it hits the pile? Is it the loss of energy, or the reaction force from the pile, or both? It has to be both right?
It is the loss of energy. A force is a change in momentum, that does not imply a change in speed or a change in energy.

Of course, you cannot change your speed without changing your momentum, but you can change your momentum without changing your speed. KE is proportional to the square of the objects speed.

#### alkaspeltzar

How does force not imply a change of speed when in this case the force is decelerating the pile driver. Less speed equals less ke

#### Dale

Mentor
How does force not imply a change of speed
Force can be perpendicular to the velocity.

when in this case the force is decelerating the pile driver
As I said above, you cannot change your speed without changing your momentum, but you can change your momentum without changing your speed.

In this case you are changing both. You cannot from that conclude which one is the cause. You have to investigate a different case where they do not both happen and see which possible cause is present when the effect is present and absent when the effect is absent.

If you investigate a different scenarios you can find one where a force exists and the speed and KE are unchanged. That tells you that it is the change in energy and not the force that is associated with the change in speed.

#### A.T.

how does the reaction force on the pile driver come into play with energy balance.

#### jbriggs444

Homework Helper
As it does the work, it is expending the energy due to the reaction force trying stop it? Maybe someone can help me understand this situation.
There are two distinct inelastic collisions that take place. You have the collision between the pile driver and the pile. And you have the collision between the pile driver plus pile combination and the ground.

The pile driver and pile are both made of steel and are quite rigid. The collision between the two is correspondingly brief. You know that the collision is inelastic because the driver does not bounce. Energy is lost in the collision. This energy will manifest as vibrations in the pile, which are mostly damped into the earth. The design of pile driver and pile is such that there is little or permanent plastic deformation of the pile and little associated localized heating. [Though I've never had the opportunity to examine the top of a driven pile for either heat or deformation].

The energy dissipated during this collision can be calculated based on conservation of momentum and the relative mass of driver and pile. If the pile is more massive than the driver, the bulk of the kinetic energy of the driver is lost in this collision.

Once the driver plus pile are moving downward as a unit, that bulk motion is resisted by the earth beneath. If you were to measure the distance the pile is driven per stroke and had calculated the initial energy of the pile plus driver you would be in a position to invoke $W=Fd$ to determine the "average" force with which the earth resists the driven pile.

Note that the "average" in this case would be a distance-weighted average.

#### alkaspeltzar

how does this explain the reaction force. It always looks at the force from the driver to the pile. But how does the reaction force play into the force of the pile on the driver? Wikipedia gives no explanation for that.

Bigger question is why is work never looked at from the reaction side of a force. We look at a single object and the forces/energy transfer to that object. But ignore the reaction force, why?

#### jbriggs444

Homework Helper
how does this explain the reaction force. It always looks at the force from the driver to the pile. But how does the reaction force play into the force of the pile on the driver? Wikipedia gives no explanation for that.

Bigger question is why is work never looked at from the reaction side of a force. We look at a single object and the forces/energy transfer to that object. But ignore the reaction force, why?
If you want to know how an object moves, you look at the forces on that object. You do not look at forces on other objects.

It really is just that simple.

#### alkaspeltzar

That is very true. Most physics problems looks at whats being moved. I was just curious as to the big picture but that is probably more confusing to me than I need to worry about.

"What slows an object during collision, force or loss of energy?"

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