Basic physics question.

frobisher

Ok, this might be very simple but it's been on my mind and I need to find out!

I'm basing this on remembering that energy is never created or destryed but is transfered. So if I pick up a ball and throw it at a wall am I right in presuming that I have transfered energy from my mass into the ball? And when the ball hits the wall where does the energy from the ball then go?

Tree

yeah, coz to get the ball to move you use your muscles adn the energy from them will be fueled by the breakdown of carbohydrates (or amino acids or fatty acids) which would be part of your mass


ball would make a lot of noise, and will still have some energy left when it bounces back off the wall.

prolly more ways to lose the energy but you've plenty there for a basic little model

Civilian_Target

frobisher wrote:

Ok, this might be very simple but it's been on my mind and I need to find out!

I'm basing this on remembering that energy is never created or destryed but is transfered. So if I pick up a ball and throw it at a wall am I right in presuming that I have transfered energy from my mass into the ball? And when the ball hits the wall where does the energy from the ball then go?

Yes - you transfer muscle and mass energy into the ball from your body by bracing yourself against the ground. Then, as the ball travels through the air, it enounters friction, which heats it up, until it hits the wall.

The wall takes some of the energy when it recoils (by a tiny amount, because its very heavy indeed), also releases sound energy, and some friction too (so heat energy) and the ball itself keeps some energy as it bounces back.

frobisher

I understand all that, what I'm wondering is where the energy from the ball finally "rests" ie what happens to the vibrations in the wall. They too are energy so if energy cannot be destroyed where has it gone? I'm guessing that in some way the energy might turn back into mass, is this true?

rugbug86

no system is 100% efficient
energy is lost in every cycle for everything. thats where the excess energy goes - its lost in the universe.

breadmonkey

Hey Frobisher, did you get you name from "Cloud Atlas"?

frobisher

breadmonkey wrote:

Hey Frobisher, did you get you name from "Cloud Atlas"?

Yes I did! Great book!

odarallo

The energy is not totaly lost on hitting the wall... The total lose of energey is when it bounces from the wall to the ground or to your hand.

For every action there is an opposite and equal reaction. Ball hits wall ball bounces gains some energy then full disperese on impact to the ground.

Easygainer

frobisher wrote:

I understand all that, what I'm wondering is where the energy from the ball finally "rests" ie what happens to the vibrations in the wall. They too are energy so if energy cannot be destroyed where has it gone? I'm guessing that in some way the energy might turn back into mass, is this true?

*correct me if I'm wrong*

Let's assume you start off with 100% energy ready to throw the ball. There is potential energy in how your muscle is ready to throw the ball. As you turn this to kinetic energy, you are throwing your arm through air, thus causing friction, so we lose some energy. The swish sound is also energy.

Now, the ball has left your arm. The kinetic energy has been transferred from your arm to the ball, BUT your arm follows through, you even overstep cos you've thrown hard. This means it was inefficient, and not all the enrgy from your arm went to the ball. More enrgy lost.

As the ball goes through the air it suffers the same friction, more energy loss. Then it hits the wall. It transfers energy to the wall (the wall recoils, imperceptibly), it makes a sound, it creates heat (friction), the ball itself shrinks a bit (meaning conversion to potential energy) and then this turns to kinetic as the ball moves back, through the air etc.

Basically, the whole process if very inefficient, and energy is dissipated in seemingly useless ways and in such small amounts that it might as well be gone. The ball would have more energy, and therefore bounce back further if you changed a few things - made the ball springy to increase the potential energy when it hits the wall (it "absorbs more of the force rather than trying to knock down the wall), doing it in a vacuum (no sound, air), pronated your wrist when throwing etc etc

ApeXaviour

frobisher wrote:

I understand all that, what I'm wondering is where the energy from the ball finally "rests" ie what happens to the vibrations in the wall. They too are energy so if energy cannot be destroyed where has it gone?

It causes vibrations, vibrations are dampened, thus dissipated as heat. Most examples like this will end up mostly as heat, some deformation (increased entropy) etc.

frobisher wrote:

I'm guessing that in some way the energy might turn back into mass, is this true?

No, your energy wasn't really mass in the first place but the energy taken from chemical bonds due to respiration in your body, which originally came from the sun btw (makes plants grow, feed animals, you eat plants/animals).
Mass energy exchanges dont happen that often in regular everyday occurences. For most examples of classical kinetics you can assume mass is constant.

frobisher

ApeXaviour wrote:

It causes vibrations, vibrations are dampened, thus dissipated as heat. Most examples like this will end up mostly as heat, some deformation (increased entropy) etc.


No, your energy wasn't really mass in the first place but the energy taken from chemical bonds due to respiration in your body, which originally came from the sun btw (makes plants grow, feed animals, you eat plants/animals).
Mass energy exchanges dont happen that often in regular everyday occurences. For most examples of classical kinetics you can assume mass is constant.

So is the answer that the energy disspates as heat caused by the kinetics surrounding the whole movement of the ball?

ApeXaviour

frobisher wrote:

So is the answer that the energy disspates as heat caused by the kinetics surrounding the whole movement of the ball?

Pretty much yeah.

The kinetic energy of the ball eventually dissipates as heat and some change in entropy.