# Kinetic Energy and Potential Energy

It’s professor dave, I want to tell you about kinetic and potential energy. We now understand a more rigorous scientific definition of the word energy as being the capacity to do work, but as we said, there are a few different kinds of energy, so we must talk about all of these varieties. First we will discuss kinetic energy and potential energy. Kinetic energy is the energy an object possesses by virtue of its motion. If we recall Newton’s second law, we know that the application of some net force will move an object with some mass at some constant acceleration. We also know that work is equal to force times distance, so if we account for the distance travelled by this object we get that W equals Fd equals mad or simply W equals mad. We also know of this kinematic equation for an object experiencing constant acceleration. If we solve for ad we get this expression which can be plugged into the above equation for work. After some simplification, we arrive here, thus deriving the work-energy theorem. This term, one-half mass times velocity squared, represents the kinetic energy of a system. So now we can express the work-energy theorem by saying that when an external force does work on a system the amount of work will be equal to the change in kinetic energy of the system. If w is positive, the system increases its kinetic energy, and the change will be equal to the work done on the system. If w is negative, the system decreases its kinetic energy, and this change will represent the work that is done by the system. Note the presence of mass in the kinetic energy expression. This means that if two objects with different masses are moving at the same velocity, the more massive object will necessarily have more kinetic energy. Now let’s discuss potential energy, which is forever intertwined with kinetic energy. Potential energy is the energy an object possesses by virtue of its position in a field, whether that is a gravitational field, an electromagnetic field, or any other type of field. If you lift a ball up into the air, you’re increasing its gravitational potential energy as it moves farther away from the ground. If you release the ball, that potential energy is converted into the kinetic energy of motion as it falls. The closer an object is to the center of gravity of the earth, the less potential energy it has, and as you pull it up and away from the ground it gains potential energy because it has an increasing potential to fall some distance to the ground. It may seem strange to bestow an object with more energy just by lifting it into the air, but remember that we have to eliminate our prior conceptions of the word energy in favor of more rigorous definitions. The potential energy of a compressed spring or an arrow ready to be released from a bow is where the kinetic energy of motion comes from. It is like a kind of stored energy. In this course we will frequently be discussing gravitational potential energy for objects on earth, and for any object this will be equal to the mass of the object in kilograms, times the freefall acceleration due to gravity for the earth, times the height of the object in meters. The greater the height, the greater the potential energy, as this represents the work that can be done by gravity to produce the motion of a falling object. There is also elastic potential energy, such as with the spring or the bow and arrow, and we will discuss this later as well. For now, we just need to understand kinetic energy as the energy of motion and potential energy as the energy of location. Thanks for watching guys, subscribe to my channel for more tutorials, support me on patreon so I can keep making content, and as always feel free to email me:

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## 56 thoughts on “Kinetic Energy and Potential Energy”

1. Ali Hussain says:

you're too late , I already done my phisicks test about dinamics and got 14/20
But that's ok , I know it's impossible, time never stops…

2. MathMagic Mathamagic says:

it was really good sir

3. The Baphomet says:

I know this isn't the subject your doing right now but Iv just got done watching your biochemistry videos I'm going to uni and want to become a drugs developer you know making and producing new research drugs and I know what it sounds like but where I live all I see is teenagers an young adults addicted to opioid based prescribed drugs and Iv always wanted to do something for mankind can anyone point me In the right direction otherwise I'm going to end up studying about 3 subjects I don't need

4. SailorJenova says:

I wish I knew about you when I had to take Intro to Physics like 2 years ago. I would've gotten an A NOT a B! 🙁 Oh, well ….understanding it is more important than a grade.

5. Budget Reefer says:

I have a question, could a machine use kinetic energy by throwing weight in a certain direction say through electromagnetism. Through Throwing mass in a certain direction. Could it generate lift?

6. Bleepingbobby says:

I told people, hey , energy is everywhere but specificaly more in oceans and seas. Time to see all people living of their research otherwise we will be returning to being cave men!

7. Catplant ASMR says:

lol thanks for helping me 💕

8. Rob X says:

Is it correct to say there is no potential energy. For example if you spin a stone which is tied with a string, a circle start to appear, but in reality it is not a circle. In other words instead of your eyes seeing colors reflecting out of the stone from a single point of your eye sight, they capture the color of the stone from different places. Now, try to apply it to an object that is not moving. The particles the object is made up of reflect light from relatively small places, but it doesn't mean that they are not
moving.

I think you can apply it to texture as well. As a matter of fact, I don't think objects have shapes. For example if a blind man touches a small square he can tell it is a square, but what if he were to touch a relatively very large square (think of a kilometers wide one.). Now, how on earth will it be possible for him to say it is a square, without experiencing the whole square(why don't you apply it to a ball). Why experiencing the whole object matters? Grab a small square and close your eyes…! When you grab that small square tight, what your hand can experience is different pressure values which come from different part of that object. What your hand does is get that values relatively and compare them to the values that you got when you first touched a square (Remember what your mother told you what you should call it when you first experience those relative pressures?). So, they is no shape, but there is pressure

How about the shapes you see? Trust me it is even simpler. All you need are a chalk and blackboard. When you have not drawn anything on the board, do you see any shape? NO? Now, draw a circle using the chalk. So, how shapes come to existence without more than two colors and different color patterns. Again, there is no shapes, but there are colors (I know it is weird, and I would like to explain it more, if you agree above things)

9. Gemma Leigh says:

Our school played this for Science! 🙂

10. Ali MERMER says:

Abi gerçekten fevkalade videolar yapıyorsun, Türkiye'de İstanbul Teknik Üniversitesinde okuyorum. Ama Hocalarımızdan anlamadıklarımı senin aracılığınla anlıyorum. İyi ki varsın çok teşekkür ederim. Arkadaşlarıma da tavsiye ediyorum. Bilerek İngilizce yazmadım, çevirirsin diye. Bir de abi sen çok Türklere benziyorsun bir DNA testi yaptırsana; Sende Türklük olması çok muhtemel. İyi çalışmalar.

11. Jean Jean Tamon Tagle says:

plsssss give me a poem of kenetic and potential energy

12. arshil zia says:

I did it by other method
First I found k.E
1/2×50×11.2×11.2=3136J
As it transfers 1539J to another body so 3136-1539=1597J
So new kE is 1597J
1/2mv^2=1597
1/2×50×v^2=1597 ( as mass is given 50)
25v^2=1597
V^2=1597/25
V^2=63.88
V=7.99

13. Era Smith says:

Very nice explanation

14. Shyameet says:

professor dave i should tell u this but u should had explain that kinectic energy as u did with potential energy . You explained the potential well but i ma bit confused in kinetic.

15. Paul Hughes says:

I understand this except for the math at the end. I get why those values plug into the equation the way they do, and I know that joules is kg m^2 / s^2 in SI base units. After that I'm stuck – I can't see how the equation has been simplified and rearranged. Help please Prof. Dave! Loving your videos btw

16. Gowtham malar says:

not clear :/

17. Tom says:

does this mean that an object with a greater mass has more PE? if the object is at rest why must we still consider the mass of an object?

18. KRIPA SHANKAR SHUKLA says:

Sir what is the difference between -GMm/R and mgh

19. KRIPA SHANKAR SHUKLA says:

Sir why GPE is always negative?

20. young saxena says:

you are great sir . thanks a lot.

21. S. G Karle says:

fluently explained

22. EpicSelenium34 says:

I watched a video that proposed that potential energy is not stored within an object, but instead is directly associated with the system that the object inhabits. Since energy is not really a "thing" in the physical sense of the word (it is an intangible capacity), both perspectives seem equally valid. Which one is right?

23. Zoha Tariq says:

If work is done on the system then sign should be negative?

24. CAMRYN VEAL says:

wow

25. Educator Sharmin says:

Im searching your channel about topic renewable and non renewable energy.Do you make or u havr?

26. Bilas says:

27. MLEBINGE JACQUES says:

Thank you very much now I understand the different between potential energy and kinetic energy

28. Savannah Greaves says:

catch me outside how bout dat

29. Savannah Greaves says:

you say your smart but when i search that up the only thing it shows is me

30. Vanity Nutrition says:

I'm a bit lost as to how you got rid of the kg value from Joules. Don't the +50.0 kg and -50.0 kg cancel each other out, leaving no kg to cancel the one from Joules?

31. Ed Rozario says:

Why is the work in the comprehension in negative value?

32. Eguiarte Morett Luis Andrés says:

it would be so cool if you made versions of the videos with all the vector calculus involved

33. theunraveler says:

Hmm I got 6.83 m/s

I took -1539 = 1/2(50)(v squared) – 1/2(50)(11.2 squared) which gives -4675 and I solve for the unknown v squared.

What did I do wrong?

34. Jeremy Celis says:

Why was the other mass canceled out??

35. SuperShana97 says:

Your video was short and helpful compare to other videos. I kinda got the material gradually. When I do the problem, I almost got it except the negative signs.
I wonder why is negative ?

36. Michelle Khoshaba says:

You are a god sent, thank you!

37. Peter Wolf says:

Am I the only one that get's another solution for the task? stepping from line 3 to 4 -1,5…[j]/25[kg] doesn't get me to -61 but -0,061… A little note might help here! 🙂

38. GgEasy 84 says:

PE = mgh… isn't that mg = Fg, gravitational Force…times a distance… that's still work.., potentially speaking :)) lol nice video

So what we do is, using different values to express same things. We use work for both of the PE and KE, but we express it as different things. Einstein's theory E = mc^2 is the same thing. Work, or KE, is expressed using mass and speed..lol…6th grade Physics are the begging of the new era :))) bai

why many times we refer to the potential energy of an object and not of the system ?

40. Patricia Anders says:

This was a great helpful video when discussing with my middle school student! Thank you

41. Prooven Reality TV says:

Hi David what's up you are always good thank you.I have a question is the potential energy always positif ?or it can be negatif ? In which case.I have an exam can you answer me quickly

42. Octavio Octavius says:

Thanks for reading our lesson Dave lol!

43. Jessica Gaffney says:

44. Dann Kebede says:

Hey sexy

45. Hey There says:

Wow !
You defined potential energy quite perfect .

46. TheAlex0903 says:

How do you know the second object is 50kg in mass also?

47. Oppong Edmond says:

Does an object on the ground possess potential energy??

48. Scott Krumwiede says:

I enjoy Professor Dave's videos, helps me learn subject better!!

49. HowDareThou says:

Science jesus

50. Islam Mandour says:

Hi this video is so good thank for you

51. Raymond Flannagan says:

thanks

52. slifer god says:

Who's in 2020 in the future

53. purple stacks says:

I use to hate ur intro and the way you explain things. Lately I've been realizing that you are proficient when it comes to concepts especially when it comes to physics. Keep this subscription forever and I hope you continue uploading videos. Thank you for helping an electronics engineering student who is so terrible at physics.

54. Hafsa Imtiaz says:

In exmple y do uh assume works = -1539 kg

55. Cat life says:

Nice

56. Mitesh Hingnekar says:

What is energy? What is origin of energy?