At 8:30 the gravitational potential energy is the work done to bring a mass m from infinity to a distance r from a mass M. That is obviously negative since PE reduces as you get closer to the mass M. But PE increases as you move away from the mass M.
You can choose your own convention for signs. The one I am using assumes that potential energy becomes zero at an infinite distance away. Since potential energy increases as you go further away that means it must be negative in order to increase to 0. But as long as your signs are consistent it doesn't really matter what convention you choose.
Force is a vector quantity. It has magnitude and direction. The negative sign is an indication that it is an attractive force (ie the force is in the opposite direction to increasing value of r). You don't have to include the minus sign as long as you remember which way the force operates.
Did you find any ? I'm struggling to find one myself. I did for the record find a channel called allery chemistry but I didn't really watch it's videos yet.
what surprised me was you took the time to explain mv2/r. it was taught a long time ago to me and certainly I forgot and didn't understand this when I was watching you. Every aspect of your video revolve around the student and that is why this is so great, clear, engaging and easy to understand!
A field doesn't do anything until you put something in. Like a temperature field that just sits there. Fields are capable of doing things, whether they do them or not. Put a block of ice in a temperature field and the ice melts. That's a brilliant start to the video! 🙌🏽🤯🔥🎊
Thank you for your videos. They are reviving my interest in Physics from 50 years ago when passing the exam was more important than understanding what you are doing I am a bit confused about what is the difference between Gravitational Potential Energy (about 8:05) and the more conventional mgh kg m²/s² as you mention in your Electric Field Video comparing the two (at about 11:34) Substituting values for the variables they certainly don't agree. Many Thanks.
Calculate the force on the 3rd due to the first and then the second using F = GMm/r^2. Since the forces will be in opposite directions the net force is one minus the other.
Yes. If you think of the earth and the sun, the gravitational force from the earth to the sun is directed towards the sun whereas the distance vector from the sun is measured away from the sun. The key thing is not the minus sign but rather that you are consistent in your signing convention.
It is the mass of the earth. The mass of the satellite cancels out at an earlier stage so does not feature in the equation here. Thanks for kind comment.
I take it you mean at 21:47 where I say r = 4.23 x 10**4 km. I haven't yet double checked it but could it be that you have calculated the value in m not km - in which case it would indeed be 4.23 x 10**7 m.
For escape velocity, assuming a conservative system where we end up with the kinetic and potential energy of the system as: K_f + Ep_f = K_i + Ep_i, where sub 'f' is final and sub 'i' is initial, should we not set up the equation where the left side is 0. The final kinetic energy and the potential at infinity would be zero. That leaves us with 0 = 1/2mV^2 -- GMm/r. This rids the (--) as shown in 26:31 when we rearrange to get 1/2mV^2 = GMm/r.
All I have done is to take the 2 pi r into the square root. In order to to do that I had to square it first so that when you take the square root you get back to where you started.
DrPhysicsA I instead substituted v = 2 pi r / T straight into the equation and then rearranged for T^2 after squaring both sides. Thanks for the great video by the way :)
Thank u v much for the content. Just one clarification though, If the total energy = KE + PE, when the object moves to infinite distance (r=infinity) both become KE=PE=0. However when r 0, Infinity and make it equals to (mv^2)/2, then I can get the same escape velocity. But still cannot understand what happened to all PE. If somebody can help me, really appreciated. Thanks.
I may not have followed the point you are making. But if you square both sides then you will indeed get an expression without the square root. But then the left hand side will be T^2. Or have I missed the point?
There are so many books that I cant organise according to them all. I try to organise the videos in some logical order and hope the titles explain what they cover. I'm not sure what your Unit 4 OCR covers. Is it nuclear and particle physics? If so, there are videos on this in the A Level playlist - and more advanced videos on both in other playlists of mine.
yo man got a B in physics, and i thought id thank you if i did cuz i swear to god i didnot know a word 3 days before the exam. i actually had to the buy the text book then cuz id lost mine, your videos saved me. thankyou very much
Drops lid whilst talking *oops* continues btw thanks so much for these videos, it has helped me understand mechanics a lot better than how my teacher explained it.
I assume you are referring to the section starting at about 11:23. You are right in all you say except "small v is the length". This is a vector diagram so v is velocity not length. See my video on adding and subtracting vectors for tail to tail subtraction of vectors. So v on both sides of the equation is velocity.
Gravity is the curvature of space time. Objects are not pulled towards earth they are are essentially pushed towards earth because the earths mass bends the fabric of space around it. Most people say the suns gravity causes the earth to orbit the sun. But in reality the sun warps the space around it, which pushes earth towards the sun.
kurdman12345678 The curvature of spacetime due to earth does not push objects towards the earth rather when a object moves near the earth it tries to move in a geodesic path (path between two points that has the shortest length) but due to the curvature of spacetime around earth those geodesic paths are curved which tend to converge towards the earth.
I had the same issue. If you square the whole expression, (2 x pi x r) / v, while substituting the root GM/r for the v, you get the expression you derived without the square root in the expression...
Question: In the video, you say velocity, v, is a vector, but then you say it's speed at 12:32. How do you convert a vector to a single variable like that?
Two weeks before midterm Statics & Dynamics exams my British professor said that we could skip the next week's classes to study for the exam, but suggested that we show up anyway because he would "revise everything we have learned thus far". I almost passed out... in American usage 'revise' means 'change what was previously looked at', so it sounded like he was tossing out everything we had previously studied and replacing it with something else.
Really? I struggle to see why. G is a constant. M and m are both scalars. F and r are the only vectors in the equation. And F is in the opposite direction to r. It may be they want you to say that Force is the differential of potential energy and PE is always negative (because it increases as you go further away from the earth and is 0 at infinity)
assuming im orbiting on the surface of the earth. how come that didn't get 24 hours (T) when I divide the circumference of the earth by v? I always get T=5068.7367s, or 1.4 hours.
Geostationary orbits - eg for telecomms satellites - are 22000 miles up. The earth's period of rotation does not have to equal the period of orbit, if you are orbiting just above the earth's surface.
technically gravity is just the result of the curvature of the fabric of space time and is not a pulling force. a large mass would curve or distort the space around it like rubber sheet being distorted by say a bowling ball. essentially other masses would "roll" towards the center of the distortion (like a marble rolling down the curve in the rubber sheet we talked about earlier made by the bowling ball). so gravity is not a "pull" but a "push. still like your calculations
in my book says ' the magnitude of minimal velocity of a rocket in order for the rocket to be able to escape from grabity is = square root of 2g× m over r. is that correct ?. there was one similar except it doesnt have a square root
The minus sign is there as gravitational force is attractive. I get the distance in the opposite direction of the force, however that will just confuse students (why cant they measure from bottom to top etc.)
Thank you, your videos helped a lot! I take IB HL physics but the topics covered are nearly the same ( from the videos that I've watched) so it helps a lot.
DrPhysicsA Thank you for the reply. An interesting video indeed. But in fact i was thinking of the two bodies problem in the planetary context. I have read something with a reduced mass to write the exact equation of the movement of a satellite which ends with this r = K²/(GMt( 1 + e cos θ )). I didn't understood how to arrive to this result. I was hoping you could shine some lights. (Sorry for poor english)
Thank you for videos, they do help me a lot. However, I am going through this video, and I seem to be confused that you have v^2 is negative for the escape velocity, also I thought that gpe+ke=0 when working out. Which might correct it?? Thanks M
You can choose any convention as long as you are consistent. In this case I am assuming that at an infinite distance the kinetic energy is zero and the potential energy is zero. This means that as an object approaches infinity its kinetic energy will reduce to 0 and its potential energy will increase to 0.
you have helped a lot over the past few weeks of revision for me. got my first A2 exam in just under an hour. I will let you know how it goes in a couple of months time. (predicted A, although will be happy with a C). Fingers crossed I guess!
Hello, I have an exam Q that says g is proportional to a body’s density and its radius but I thought it was /r squared. It says to multiply the density and radius to find the highest number that then equals the greatest g. Is this wrong? Help!!!!
If it was proportional to density, then a 1 kg ball of steel would have more gravity than Earth. It doesn't. You have to work damn hard to get a measurable amount of gravity between objects at the human scale. It is the mass to surface area ratio that matters, for the gravity at the surface of a spherically symmetric body. This is how Earth, with a density that is nearly that of cast iron can have approximately the same gravitational field as Saturn, which is approximately the density of cork.
thin you for this explanation,doctor,but i have a question,in this forumla ,you h'avent considered ,,,the earth mouves in orbit of the sun,this mouves infulences ,the satelite mouves ,why can you expain this in the formula,frendly
I am a little confused with the equation F = (-GM1M2)/r^2 Do the masses need to be in a certain order? As in is M1 the mass of the sun and M2 the mass of the planet for example? (As the sun is heavier than the planet). If that is correct then is that the same for the equation r1 = (m2*d)/(m1 + m2)?
Infernosion The order doesn't matter in the first equation. The formula tells you the gravitational force between any two objscts. You just have to remember which one is the m1 and which is m2.
Sir, your videos are awesome ! but there is something i awalys wanted to know... and i didnt found it in your channel... can you explain how the formula E= -GMm/r² come? was it determined mathematically or experimentally? Thanks !
Hi, erm, I'm a bit confused. Why does F= -GMm/r2 again? I mean, why is there a minus? Because the distance is measured in the opposite direction to the direction the force acts? Could you clarify that idea in the context of the forces being attractive? lol
"It is terrible to do so but I have blocked out the sun" haha I love your videos you're awesome thanks! :)
i heard he watered out the sun lol
This is far more intelligent than A-Level students! All the A-Level students I knew at school were thick!
My physics teacher is crap, so i have to teach it all to myself and this really helps, cheers man!
Same bro, how did your exam go?
At 8:30 the gravitational potential energy is the work done to bring a mass m from infinity to a distance r from a mass M. That is obviously negative since PE reduces as you get closer to the mass M. But PE increases as you move away from the mass M.
You can choose your own convention for signs. The one I am using assumes that potential energy becomes zero at an infinite distance away. Since potential energy increases as you go further away that means it must be negative in order to increase to 0. But as long as your signs are consistent it doesn't really matter what convention you choose.
thank you for making physics less intimidating, i have you to thank for if i score well in my upcoming A levels :)
Celyn Tan Thanks. All good wishes for the exams.
Sometimes its the tumble dryer.
Force is a vector quantity. It has magnitude and direction. The negative sign is an indication that it is an attractive force (ie the force is in the opposite direction to increasing value of r). You don't have to include the minus sign as long as you remember which way the force operates.
if only there was a drchemistryA level
Did you find any ? I'm struggling to find one myself. I did for the record find a channel called allery chemistry but I didn't really watch it's videos yet.
Niket try a guy called E Rintoul
Thanks
Allery chemistry, clearly the best!
E Rintoul
what surprised me was you took the time to explain mv2/r. it was taught a long time ago to me and certainly I forgot and didn't understand this when I was watching you. Every aspect of your video revolve around the student and that is why this is so great, clear, engaging and easy to understand!
A field doesn't do anything until you put something in. Like a temperature field that just sits there. Fields are capable of doing things, whether they do them or not. Put a block of ice in a temperature field and the ice melts.
That's a brilliant start to the video! 🙌🏽🤯🔥🎊
Sir you're the best! I wish there were more teachers like you.
Thank you for your videos. They are reviving my interest in Physics from 50 years ago when passing the exam was more important than understanding what you are doing
I am a bit confused about what is the difference between Gravitational Potential Energy (about 8:05) and the more conventional mgh kg m²/s² as you mention in your Electric Field Video comparing the two (at about 11:34) Substituting values for the variables they certainly don't agree. Many Thanks.
Calculate the force on the 3rd due to the first and then the second using F = GMm/r^2. Since the forces will be in opposite directions the net force is one minus the other.
Yes. If you think of the earth and the sun, the gravitational force from the earth to the sun is directed towards the sun whereas the distance vector from the sun is measured away from the sun. The key thing is not the minus sign but rather that you are consistent in your signing convention.
The arrow on the Gravitational field line points towards the larger mass. Always.
At 21:13- Period of geostationary, orbiting body: one sidereal day, 86,164 seconds.
Yes, from the centre of the earth to the centre of mass of the man. But I was ignoring the extra meter or so in 6400km.
Very well explained. Will check out your other vids. Thanks for uploading!
Sunny? In the UK?!
Damn he provides a whole new perspective. Thanks sir.
It is the mass of the earth. The mass of the satellite cancels out at an earlier stage so does not feature in the equation here. Thanks for kind comment.
Washing machine in the background
+John Fletcher or a work against gravity....
+John Fletcher i kept thinking it was mine
I thought it was a kettle
There was also a cow....
Abeel Goraya sounds like at around 6:00
I take it you mean at 21:47 where I say r = 4.23 x 10**4 km. I haven't yet double checked it but could it be that you have calculated the value in m not km - in which case it would indeed be 4.23 x 10**7 m.
For escape velocity, assuming a conservative system where
we end up with the kinetic and potential energy of the system as:
K_f + Ep_f = K_i + Ep_i,
where sub 'f' is final and sub 'i' is initial, should we not set up the equation where the left side is 0. The final kinetic energy and the potential at infinity would be zero. That leaves us with 0 = 1/2mV^2 -- GMm/r.
This rids the (--) as shown in 26:31 when we rearrange to get 1/2mV^2 = GMm/r.
6:42 - It sounds like there's a rocket preparing to launch.
Sir, i have a question about squaring both sides at @18:25.. how did u divide the square root of r/GM with 2 pi r ?
Pls. explain
All I have done is to take the 2 pi r into the square root. In order to to do that I had to square it first so that when you take the square root you get back to where you started.
DrPhysicsA I instead substituted v = 2 pi r / T straight into the equation and then rearranged for T^2 after squaring both sides.
Thanks for the great video by the way :)
Yep. Its a cuckoo clock with a cow instead of a cuckoo.
Thank u v much for the content. Just one clarification though, If the total energy = KE + PE, when the object moves to infinite distance (r=infinity) both become KE=PE=0. However when r 0, Infinity and make it equals to (mv^2)/2, then I can get the same escape velocity. But still cannot understand what happened to all PE. If somebody can help me, really appreciated. Thanks.
Good Luck on your IB physics exam tomorrow! I'm taking the same exam and I agree, these videos are indeed very helpful!
Thanks DrPhysicsA! :)
Excellent lessons. I am thrilled to have found these video lessons. Thank you. Thank you.
I am delighted to hear it. Well done.
I learned the whole topic and more in 30 min than my school teacher who teaches us for a week and i learn 0 from him. Please make more.
Wow ! The best physics teacher ever ❤️
I may not have followed the point you are making. But if you square both sides then you will indeed get an expression without the square root. But then the left hand side will be T^2. Or have I missed the point?
Sir, it would be helpful if you could tell me where I can find the video on electric fields. Its not there in your classical mechanics playlist.
ua-cam.com/video/GDFpTefpDME/v-deo.html
great video, very clear and engaging. Thank you sir.
as well as logical
There are so many books that I cant organise according to them all. I try to organise the videos in some logical order and hope the titles explain what they cover. I'm not sure what your Unit 4 OCR covers. Is it nuclear and particle physics? If so, there are videos on this in the A Level playlist - and more advanced videos on both in other playlists of mine.
yo man got a B in physics, and i thought id thank you if i did cuz i swear to god i didnot know a word 3 days before the exam. i actually had to the buy the text book then cuz id lost mine, your videos saved me. thankyou very much
What is the speed falling from space to earth with a changing gravity strength. Do you do a video about this?
this channel is too good
Drops lid whilst talking
*oops*
continues
btw thanks so much for these videos, it has helped me understand mechanics a lot better than how my teacher explained it.
Awesome stuff, such a huge help! I love all your videos.
I assume you are referring to the section starting at about 11:23. You are right in all you say except "small v is the length". This is a vector diagram so v is velocity not length. See my video on adding and subtracting vectors for tail to tail subtraction of vectors. So v on both sides of the equation is velocity.
i LOVE this, the explanation is good and easy understand.TQ so much ^^
This was posted 10 years ago but my God you are good....
Gravity is the curvature of space time. Objects are not pulled towards earth they are are essentially pushed towards earth because the earths mass bends the fabric of space around it. Most people say the suns gravity causes the earth to orbit the sun. But in reality the sun warps the space around it, which pushes earth towards the sun.
Mr genius every one knew it . but this is simple Physics IT DOES NOT INCLUDE RELATIVITY
kurdman12345678 The curvature of spacetime due to earth does not push objects towards the earth rather when a object moves near the earth it tries to move in a geodesic path (path between two points that has the shortest length) but due to the curvature of spacetime around earth those geodesic paths are curved which tend to converge towards the earth.
kurdman12345678 gravity is a force but doesn't bend space time because if it did you wouldn't age on the moon because it has less gravity
In the part going on at 15:42, is 'v' equal to the speed at both points (since magnitude is equal but direction is not)? And, if not, what is 'v'?
v is the magnitude of the orbiting (or tangential) velocity.
Hi sir woaaahh just wowwww i really love physics and you have deepen it
Thank you very much for this video. I honestly have learnt alot from this! Will be looking into more of your videos soon! :)
This video really helped MR thanx!!
I was in year 6 when this was filmed...
such a wonderful explanation, thnqw
the vedio made me understand more stuff then i understood in the class
so thank u sooo much
WE NEED YOU PLEASE COMEBACK, IT VERY USEFULL IN PANDEMIC (2022)
Seems to work for me. Is it OK now?
Thanks. All good wishes for the exams.
I had the same issue. If you square the whole expression, (2 x pi x r) / v, while substituting the root GM/r for the v, you get the expression you derived without the square root in the expression...
Hello sir. Very nice presentation.
you sir, deserve a medal!
cheers
I really like the proof of centripetal acceleration= vsquared over r
Ive never seen it because most teachers don't explain why that is
Do you have a video on the gravitational potential graphs etc. please?
Does something similar for chemistry exist?
like a DrchemistryA :/
E Rintoul is closest I've found :)
At 6:06, -GM/(r^2) = 9.81 . r^2 must be positive, and G is a positive constant so doesn't this imply that mass is negative?
No. Its just a question of which direction you count as the positive direction.
thank you very much i find your videos concise and extremely helpful !
Fantastic videos.
Question: In the video, you say velocity, v, is a vector, but then you say it's speed at 12:32. How do you convert a vector to a single variable like that?
yh sure my teacher is a no brainer, n wif vidz lyk thez lyfz made a lot easier......................anks a lot man, u da best
Two weeks before midterm Statics & Dynamics exams my British professor said that we could skip the next week's classes to study for the exam, but suggested that we show up anyway because he would "revise everything we have learned thus far". I almost passed out... in American usage 'revise' means 'change what was previously looked at', so it sounded like he was tossing out everything we had previously studied and replacing it with something else.
ur video is really a good tool for revision
thank u :)
Really? I struggle to see why. G is a constant. M and m are both scalars. F and r are the only vectors in the equation. And F is in the opposite direction to r. It may be they want you to say that Force is the differential of potential energy and PE is always negative (because it increases as you go further away from the earth and is 0 at infinity)
assuming im orbiting on the surface of the earth. how come that didn't get 24 hours (T) when I divide the circumference of the earth by v? I always get T=5068.7367s, or 1.4 hours.
Geostationary orbits - eg for telecomms satellites - are 22000 miles up. The earth's period of rotation does not have to equal the period of orbit, if you are orbiting just above the earth's surface.
thanks
15:26 Moooo!
technically gravity is just the result of the curvature of the fabric of space time and is not a pulling force. a large mass would curve or distort the space around it like rubber sheet being distorted by say a bowling ball. essentially other masses would "roll" towards the center of the distortion (like a marble rolling down the curve in the rubber sheet we talked about earlier made by the bowling ball). so gravity is not a "pull" but a "push.
still like your calculations
in my book says ' the magnitude of minimal velocity of a rocket in order for the rocket to be able to escape from grabity is = square root of 2g× m over r. is that correct ?. there was one similar except it doesnt have a square root
The minus sign is there as gravitational force is attractive. I get the distance in the opposite direction of the force, however that will just confuse students (why cant they measure from bottom to top etc.)
if you listen very closely you can hear your brain frying
You need glasses.
Thank you, your videos helped a lot! I take IB HL physics but the topics covered are nearly the same ( from the videos that I've watched) so it helps a lot.
What a great teacher you are. Thank you for your videos.
Could you make one describing the simple two bodies system and the equation that follows ?
There is a video in the A-level playlist which looks at momentum in two dimensions.
DrPhysicsA Thank you for the reply. An interesting video indeed. But in fact i was thinking of the two bodies problem in the planetary context. I have read something with a reduced mass to write the exact equation of the movement of a satellite which ends with this r = K²/(GMt( 1 + e cos θ )). I didn't understood how to arrive to this result. I was hoping you could shine some lights.
(Sorry for poor english)
At 21:00 is M the mass of the earth or of the satellite?
Thank you, very helpful
shouldn't the units of G be Nm-2Kg2
as the disitance between the centres (r2 )is the denominator so when brought up wouldn't it change sign
(at 3.55)
F = Gm1*m2/r^2 so G= F r^2/m1*m2
Thanks! Unfortunately though, CIE tends to ask why there is a negative sign, and they don't accept the direction argument.
can anyone please help me in which video does he discuss quantum physics basics(photons)?
102 Maneka try the wave particle duality video
Morgan Marshall yeah I saw that, still thanks
Thank you for videos, they do help me a lot. However, I am going through this video, and I seem to be confused that you have v^2 is negative for the escape velocity, also I thought that gpe+ke=0 when working out. Which might correct it??
Thanks
M
You can choose any convention as long as you are consistent. In this case I am assuming that at an infinite distance the kinetic energy is zero and the potential energy is zero. This means that as an object approaches infinity its kinetic energy will reduce to 0 and its potential energy will increase to 0.
you have helped a lot over the past few weeks of revision for me. got my first A2 exam in just under an hour. I will let you know how it goes in a couple of months time. (predicted A, although will be happy with a C).
Fingers crossed I guess!
Dan Hyde good luck. Hope it goes well.
I wonder if it went well.
Zarish Ahmed got a B so I was super happy. Completely forgot about that comment.
Daydreaming in Physics lesson to throw ball in space so it crashes on to your neighbor alien’s window
Hello, I have an exam Q that says g is proportional to a body’s density and its radius but I thought it was /r squared. It says to multiply the density and radius to find the highest number that then equals the greatest g. Is this wrong? Help!!!!
If it was proportional to density, then a 1 kg ball of steel would have more gravity than Earth. It doesn't. You have to work damn hard to get a measurable amount of gravity between objects at the human scale.
It is the mass to surface area ratio that matters, for the gravity at the surface of a spherically symmetric body. This is how Earth, with a density that is nearly that of cast iron can have approximately the same gravitational field as Saturn, which is approximately the density of cork.
Thank you so much! these are very useful!!
Thanks so much DrPhysicsA, your videos are so helpful! :)
thin you for this explanation,doctor,but i have a question,in this forumla ,you h'avent considered ,,,the earth mouves in orbit of the sun,this mouves infulences ,the satelite mouves ,why can you expain this in the formula,frendly
I am a little confused with the equation F = (-GM1M2)/r^2 Do the masses need to be in a certain order? As in is M1 the mass of the sun and M2 the mass of the planet for example? (As the sun is heavier than the planet). If that is correct then is that the same for the equation r1 = (m2*d)/(m1 + m2)?
Infernosion The order doesn't matter in the first equation. The formula tells you the gravitational force between any two objscts. You just have to remember which one is the m1 and which is m2.
What mass of the Earth did you use? at 21:43?
6 x 10^24 kg
There is only 1 mass of the Earth xD
What mass? How many masses does earth have? 2?
This really helps. THank you so much
shoudln't G = 6.67x10^-11 N kg2/m2 consdiering r2 is the denominator?
Can you let me know the time on the video which relates to your enquiry please.
No I am only squaring 2 pi r so I can put it inside the square root.
where can i find videos about magnetic fields?
Ur video rocks
Very helpful
at 18.25 if you are squaring (2 x pi x r) wouldn't that mean you are cancelling out the square root of GM/r ??
Thankyou So much SIr you explained really in great manner :)
Sir, your videos are awesome !
but there is something i awalys wanted to know... and i didnt found it in your channel...
can you explain how the formula E= -GMm/r² come? was it determined mathematically or experimentally?
Thanks !
+Otávio Nunes Guimarães it was by empirical methods (experimentally)
Great videos, thank you!
Thank you, It was very helpful.
Hi, erm, I'm a bit confused. Why does F= -GMm/r2 again? I mean, why is there a minus? Because the distance is measured in the opposite direction to the direction the force acts? Could you clarify that idea in the context of the forces being attractive? lol