# What happens to kinetic energy of an object when I stop its motion? [duplicate]

Assume an object is moving and it is stopped at a point. Now kinetic energy will transform into some other energy. What can be those energies and on which cases these will happen respectively?

Furthermore, when that kinetic energy of an object (going upward against gravity) transforms into potential energy, the net potential energy of the object will be more than $mgh$ (where height is $h$ with respect to base). Is this fact correct? How can this unusual energy amount make sense?

## marked as duplicate by Jim, Kyle Kanos, bobie, Martin, Brandon EnrightJan 12 '15 at 16:42

• Samiur Rahman Mir. reply to your comment on my ans. when you stop a ball with your hand then a part of the kinetic energy of the ball is absorbed by your hand(or body) and a part converts into sound and a part into heat. – Paul Jan 13 '15 at 10:03
• So I stop object (on both cases - after throwing it with a velocity (a) or taking it upwards with a constant force b to $h'$ ) at $h'$ , the potential energy of the object will be same as $mgh'$ and KE will transform precisely into heat , sound and "potential energy" in my hand . correct me if I am wrong . Will there be any significant result to feel or to notice as potential energy will increase in the opposing body (here hand) ? – Md Samiur Rahman Mir Jan 13 '15 at 12:57
• I have edited.if you dont understand let me know. – Paul Jan 13 '15 at 15:20
• Well , let's assume There was brick (of mass $m'$) and it stopped the moving object at $h'$ . Now a part of kinetic energy will be absorbed by the brick . What does this absorbtion of energy mean ? Increase of potential energy ? Then the brick will have potential energy more than $m'gh'$ which is not possible . – Md Samiur Rahman Mir Jan 14 '15 at 12:22
• Okey..I havent used the word absorption....(again edited) – Paul Jan 14 '15 at 12:36

Assume an object is moving and it is stopped at a point . Now kinetic energy will transform into some other energy . what can be those energies and on which cases these will happen respectively ?

kinetic energy is converted into sound,heat,potential energy etc etc.

When a body go upwards against gravity its kinetic energy changes to potential energy and at maximum height the whole kinetic energy is converted into potential energy.

But if air is present the kinetic energy also converted into heat and sound so the potential energy at the maximum height is less than the initial kinetic energy of the object.

EDIT:Potential energy only depend on the final position.It doesnt matter how the object reach the height $h'$ ,potential energy is still $mgh'$. when you throw the object upward the speed of object keep decreasing that means the kinetic energy also keep decreasing. But potential energy of the object keep also increasing .

Now if you throw an object with kinetic energy $1/2 mv^2$(i will assume now that there is no air) If you dont stop the object with your hand(or anyhow) it will go to a height $h$,and at height $h$ its potential energy will be equal to $1/2 mv^2$.(That means the whole kinetic energy is converted into potential energy).

Now what happen when you stop the object at a height $h'$? In this case since at height $h'$ Potential energy is equal to $mgh'$(which is less than the kinetic energy you provided atfirst).

a part of the kinetic energy surely has gone somewhere?

ans is yes,when you stop the object with your hand ,a part is converted into sound and and a part to heat.

• a) let's assume I send an object upwards and it can go to $h$ height where its velocity gets zero . Now I stop the object at $h'$ height ($h'<h$) instantly . <br/> Now into which may the kinetic energy transform at $h'$ ? What will be the potential energy at this point , $mgh'$ or more than $mgh'$ where extra energy comes from kinetic energy ? b) Here the only force applied on the object is gravitational force . If I took the object with a constant force (greater than mg ) and stop it at some point , then what will be the next conversion of kinetic energy of the object ? – Md Samiur Rahman Mir Jan 13 '15 at 6:46
• @Md Samiur Rahman Mir and how will you stop the object at $h'$? – Paul Jan 13 '15 at 9:54
• An opposing force , brick , hand . – Md Samiur Rahman Mir Jan 13 '15 at 12:58
• In an inelastic collision where the center of mass of both objects comes to rest, basically all the energy becomes transformed into heat and soundwaves (and the soundwaves eventually disperse as heat). In statistical mechanics, the molecules that make up a substance are constantly bouncing around and carrying kinetic energy with them, in various "degrees of freedom", like the three components of their velocity vector and kinetic energy due to rotation and vibration (along with potential energy for molecules where the distance between bonded atoms can shrink and expand as the molecule vibrates) – Hypnosifl Jan 13 '15 at 15:40
• (continued) According to the equipartition theorem, the total temperature of an object is proportional to the kinetic and potential energy contained in these molecular degrees of freedom, so a small increase in the temperature of the surroundings through heat in a collision means an increase in the average kinetic and potential energy per molecule as it moves around, rotates, and vibrates. – Hypnosifl Jan 13 '15 at 15:42