Oh miss. Hello, Jackson. How are you today? Great. Good. So I found some past papers past paper questions that we can have a look at the questions that they ask on work, power efficiency, kinetic energy, gravitational potential energy. So are you ready to do some past paper type questions? This is the way they ask this topic. And are you doing exl or etxl international? Do you know which example you're doing the he says, is it the international a level or the a level? Perhaps you could find out. I don't know exactly, but I will ask my physics school teacher to know more about this, the international exam. So. This is paper 22. 2022, question 14. Okay. So basically you have to work out what they're asking and how to solve this. So a regenerative braking system allows an electric car to use its kinetic energy to charge a battery as the car decelerates. So as the car slows down, the the motion is used to charge up the battery to help its range get longer. A car is traveling at 13m per second, decelerated to rest the energy transferred to the car battery during the deceleration was 73.9 kilojoules. Calculate the efficiency of the regenerative braking system. If we know the MaaS of the car is that. Firstly, the energy is the kinetic energy. So that is a half m square. So not point five times not point five times what, 15, 60 times 13 square. 1.32 times ten to the three. Continue you the file, so let me write it. Did you square 13? Yeah. Yeah, 12345. So calculate the efficiency of the regenerative braking system. So how much of the kinetic energy is converted to electrical chemical energy stored in the battery? So if Yeah it is given 33.9. So if that's 100%. 13 one H2 nois hot. Over 100, I want to I think wait miss let me try, 13122 zero, divided by 739, Oh Oh. Yeah, it's over 100. I've we've got a power of tenure. Oh Yeah, so that's my mistake. I beyour power today. That's our 100%, isn't it? That's our so this is the kinetic energy and this is the energy given to the batteries. So what percentage is that? That makes sense, doesn't it? Yeah. So thatbe 739 zero zero times. 100 divided by. What percentage do we get? Well, the percentage is that. 56%. Okay. So that's where three Marks drag forces act on the car as it moves through the ash state. How the drag forces vary with the velocity of the car. What do we mean by drag forces? Drag forces move through the air forces draforces. The force upwards. Not quite. If you think if the car is moving in this direction. The resistive forces move in that direction. So if the car is moving in this direction, the resistive forces act to oppose the motion. Yeah. How does that how to draforces. Change with the velocity of the car. Do you think the drag forces increase with the speed of the car? Speed of the car. Speed well the force strike force is in the opposite direction of the car moving Yeah so of the carmoving being so if the kinetic energy is the photo with with how the force which miss is just just one Marso, maybe that is opposite to the thrust. Does the drag force change if your velocity changes? Does the force due to air resistance increase or decrease as speed decreases or increase decreases or increases? That's really what it's asking. Let's say as speed increases. The drag force, do you think itgo up or go down? Resies the track force in. So the velocity of the car and a speed increases the drug g force. So if the drug force increases, the velocity will increases as well. So the thrust increases well, the faster you go, the more air you're hitting. So the bigger the drag force. When it says state, you're right, you nothing to calculate, you just have to state, okay. The graph shows how the efficiency of the regenerative braking system depends on the initial velocity of the car for initial velocities in the range of 20 to 40m per second. So remember, when we're breaking, after traveling between 20 and 40m per second, we get a decrease inefficiencies. The efficiency of regenerating some of that energy in the battery decreases. Why do you think this is? Explain why the efficiency of the regenerative braking system varies as shown in the graph. So if this is 40 and if this is 20, the of regenerative breaking system varies as shown in the graph. So Yeah this question asme, why this is why why it is not constant because it is variable. If your initial velocity is 40 as opposed to 20, there's a lower efficiency, there's less kinetic energy regenerated as electrical energy for the car. Why do you think that might be bearing in mind our previous question was about drag? Jack force. So Jackson, what have we said in the previous question? Read this in the previous questions. Yeah, I see the dragon Force Act on the car. Yeah, I'm thinking about that. Why? So the drug force and speed increases, the drug force will increases. So so if the drug force increases, so is efficient, the draforce will increase as well. In this graph, if there is higher initial balothe, efficiency is lower. And the efficiency is the energy output divided by energy input. And we can know that when the higher, when the speed is higher, there is some energy waste as thermal energy. So not all kinetic energy will transferred to the thst and the drug. So that is the reason why when the initial velocity increases, the efficiency decreases. Yeah some energy is converser to thermal energy. In the air. Good. So as. I think that would get us the four Marks. That's what they're looking for. Okay. 23. Okay, this is a different paper. Again, these papers have ten multiple choice questions at the beginning, and then about four or five longer questions at the end. So these are a bit of calculating, a bit of explaining. So here we go. A high speed train accelerates from rest along a straight horizontal track. The final speed of the train as it speeds up is 76m per second. Show that the useful work done to accelerate the train is about two gigajoules. If we know the MaaS of the train. So. So what do we know? We know the MaaS, we know the speed verthe accelories from rest. So the rest final speed and the walk down is 2G jtwo gigajles. So giga, so let me think a kilo is ten to the three. Megan is ten to the six. Nine giga, maybe nine. Correct. So gigar, so we know v, we know m, so work has been done to give the train what type of energy. Show that wait, show that the the useful work down to x just two Marks, so maybe W equal to a half mv squared. So that is 7.2 times ten to the five kilograms times a half, times 76m. The second square. 2.08 times ten to the nine right? So that's. We've done what we've been asked. The train accelerates for 180s. The train has an input power of 16 megawatts. While accelerating, determine the work done against air resistance as the train accelerates. Not against the air resistance as the train accelerates, accelerates for 180s input power or done against irresistors. Two Marks, okay, power equal to work down, divided by time, so same, so work down equal to power times the time. So about that. W, so the train is 16, 16 million watts and make a times not okay, that is mea, okay, mea, okay. Mea mili is lower ercase, mea is uppercase. Okay. Okay. Yeah. Mea mea, so megawatts 1.6 times ten to the six, ten to the seven, maybe 16 times ten to the six times 180. 2.88 times ten to the nine. So as we worked out this, the work done was 2.08. The useful work done was 2.08 by ten to the nine. So but the power input leads to this amount of input work. So what do we do next? So maybe we subtract. 2.08 by ten times nine. 21 zero eight times ten. Oh Yeah. The last answer. So so if that is so there's input power and that is the output, the work done. So out input minus output and that is every resistance energy. So that is not point Yeah not point eight times ten to the nine, but maybe that is eight times ten to the seven. Yeah. So 2.88-2.0a's. Then 2024. Here's a multiple choice question, again, the way they can ask questions. An engine is used to remove water from a mine. The engine transfers energy to the water at a rate of 180 kilowatts and waste energy at the rate of 60 kilowatts. Which of the following expression gives the efficiency of our engine? Efficiency. Energy was. Now you two vision transfer energy to water at a rate and wasted energy at the rate above some key diagram. So the sum of energy, that is energy to the water is usually remove water for a for a mine. So that is 200, 240 kio watts. So maybe it's between a and b. So it gives the efficiency. Efficiency is 180. So I choose a. 嗯。Oh. Then there is a long question. The train moves up an inclined track at a constant velocity. The train moves through a vertical height of 25m. Show that the gravitational potential energy gained by the train is about 400 megajoules if we know the MaaS of the train. So again, you're beginning to see similar types of questions. But instead of kinetic energy, we have gravitational potential energy. We know the vertical height, we know the MaaS. So gravitational potential energy, what is the equation for that, Jackson? Rectational M G. M we know g we know 1.7 times ten to the six times 25m, times 9.81 tons per newtons per kilograms. So Yeah of that we know. So 25 times one 1.7 times ten to the six. Times 25. 1.06 times ten to the nine but not the 40 megajles. 25 times nothing. Now it is Yeah for home 417 times ten to the six th. Yeah. The trains engine has an output power of 20 megawatts. The train takes a time of 35s to move through the vertical height of 25m. Calculate the work done by the engine against the resistive forces. Okay, walk down by engine against recsiforce, so transenergy output power is 20 million. Work down equal to the power, equal to power times time. So that is 20 mea two times ten to the seven. Times 35. Seven times, 700 times ten to the six and the last one we get. 417 times ten to the six so 400 and and 17 divided by 400 and no, 700 times 100. Maybe we don't need to do that. So it is ff Oh no, no, No Uh 700-417, 283 that is worked down by the engine against the rectivity ity force. Yes, good. Right 21. Okay. So here is a multiple choice question, Jackson. Okay, a hydroelectric power station has an efficiency of 32% in one hour. The useful energy output of the power station is 1.2 by ten to the 13 joules, which is the following expression, gives the total input power to the power station in watts. Okay, inwards so output useful energy ine output is 1.2 times ten to the 13 juws. The total money, total power input in ts watts. Wait. An hour, the time, so one point. So that is 32 over 100, equal to the 1.2 times ten to the 13 divided by money input, so 1.2 times. 20 is 13 times 100 divided by 32, 3.7 form times ten to the which? Other than 13 times 0.32. So I think this one. The Marks came gives b. 手割言了。I suppose this is energy. So we didn't we could work out joules per second. Remember, if you think of a definition of power, is work done per unit time. So the work done. Divided by the number of seconds in an hour. Divided by the percentage will give you the so it's mixing work and power, unfortunately. Okay. So that was the answer for that one. Question 14. A spear moving at 25m per second of a ramp. The ramp is at an angle upward at ten degree to the horizontal, as shown. There is a flat surface at the start that starts 30m from the ramp. So there's their flat surface, and that distance is 30m. The flat surface is 2.9m below the ramp. To duce, whether the skier reaches the flat surface before landing, do they land here or do they land here? We assume there's no air resistance. Yeah, and. What's this one? Guy reaches the flight surface before landing. This is worth five Marks. Yeah, Oh, my God, five Marks. So this mix es so bad. Well, it's really about I chose this question because the second bit of the question, it. Involves work and energy. But this question is mixing sothat. And the next bit of this question mixes work and energy. So we can get the. Vertical component and the horizontal component. V and H maybe. So that's 25 times costs ten. And this is 25 times sine ten. Ten degrees. Then we can use those to work out how the time that the Kier is in the air and work out where they land. So that's. 25 times sine ten. His. 25 sine 1095 cosine Oh 4.3. And this is 25 times cos ten. 24.6m per second. So if we use the vertical component, sorry, the horizontal component, and see how long it would take at this speed to cover thirshe meahso. S is equal to the horizontal times t. Yeah. So 30m is the distance we're trying to cover is 24.6. Times the time it takes to travel. So t will be equal to. 1.22s. Yeah us v plus t so 24.6 is a horizontal velocity. So so as Yeah as I see as it's a horizontal distance, so t equto 1.22Yeah. So the distance is s is equal to U T. Yes, half a two squared. So that's equal to. In the vertical sense going up. It's four point. What is it? It's one, three, four. Times 1.2. Minus five. Times 9.81. Times 1.2 squared. 1.22 squared. When we work all that out. That means itfall 1.99m. That is a minus nine point. Oh Yeah because Yeah because it is decreasing drodown. Yeah. So but so miss is so I see. So if that is horizontal, that is constant velocity. But if that is vertical, that is a variable. Raise a velocity. What do you mean? I mean, in the horizontal, we just use one equation as equal to vt, but if that is vertical, we should use Sumet. Yeah. So two. So this distance, if it's falling 1.99m. I'm covering 30m. But it has. So 2.99. -1.99. So 2.9-1.99. Means that it. No point 91. Itonly Ely have fallen this amount by the time it's covered 30m. So itland somewhere beyond this point in the flat surface. So deduce by working out how far, how the time it would take to travel horizontally using the horizontal velocity, working out the time, substituting that for the time it would take to land. In the same amount of time. Travel, it would only have fallen 1.99m, so it would keep falling, keep traveling in this direction. So it land after the flat surface. So lands. After. 30m. Okay, another skier travels along the horizontal surface with an initial speed of 23m per second. She comes to rest after traveling a distance of 43m. Calculate the average force required to bring the skier to a halt to a stop if the MaaS of the skier is 63 kilograms. Average bridge, okay, average force required to bring the skto the rest, horizontal surface, initial speed 23m per second and a distance of 43m to rest. And average force, average force when we know the MaaS. Firstly, f equal to M A MaaS times acceleration. So now we know the final velocity, zero, initial speed is 23 and the distance is 43. And we want to know about the acceleration, v square equal v square. It could use square plus two as so that is 23 square. 23. Times 23. Divided by two, divided by 43, 6.15. Right? And this number we can time 3063. So 20 23 squared. So a is. F equals ma? 387. Yeah, 387. Good. So those are real past paper questions and and. You've done very well with them. You can see sometimes they mix sothat work done, etcetera. What would you like to do after this topic? Because I'm running out of questions to give you maybe. I want you. Yeah. I don't think it is maybe it is a very necessary problem for me to do some calculations, but I want to know about some brief answers, like, for example, to explain why and if this changes, for example, a. Blbasketball post is there post maybe? Let me, sir. Welcome to my research. Basketball op hoop for example, basketball hoop maybe usually people, usually annormally people use water to increase its weight. So if people use sand sand inside of water, so what what it will change the what the MaaS H will change like that. I think I maybe I should practice this because they takes part in takes takes up take takes up a big part of the whole exam. So I think maybe it's very necessary for me to in to practice this kind of question. Yeah, I think I've seen this question that you're talking about. Instead of water. Yeah so Yeah, I know the answer. So because Yeah I did an exam yesterday. Double physics. So one question is about this. What did you put? Do you remember what answer you put? Yeah. So Yeah, maybe I'm not sure about that answer because that is a basketball hoop and said that said about a force of wind of this. So Yeah, the force of wind to flow it to to lish over top lish over it over Yeah so. It give it, it give us, give me a an angle and let me calculate the minimum force for the force of the wind. So I think maybe this question maybe if that is a minimum force, so maybe that the weight should be minimum as well. So I didn't calculate the weight of the water. I just calculate the empty base. Yeah. And the second question is about the sand and water. So if they put sand into the base, what will happen? So I firstly, I write write an equation and the MaaS equal to the density times the volume. So and I said that if the sof, the, if the sand has the more in the larger density than water, the the total MaaS of the space will increase. So the, and third point is the force of wind will increase. Yeah. It's to do with moments, isn't that? Yeah, I think it's about a moment. Okay, so. I have to just delete a couple of. On finding the question, I have seen this question already. Okay. Is this it? So a portable basketball case has Yeah, Yeah, that is a, that is a question I met yesterday of the question. The base can be filled with water to increase stability. The base has a capacity shows that the maximum weight of the base is due to the large area of the backboard. The baseball may topple over when the wind blows. So Yeah, this is Yeah, that is so question. So if you know that angle, you know that angle too. So that's 15 degrees as well and you this is your pivot. Yeah so. The weight of the post. Calculate the minimum force of the wind that will cause the basketball to be blown over when it is at the angle shown. Ignore the effect of the wind on the base. Thank you. So that is 27 newtons. Yeah. So we have with water in it. So 13.5. So that's 4.5. The base has a capacity of 85 liters, so 85, 88 and a half kilograms. Milligrams times g. Remember, we have to deal with forces so MaaS times g. So we're told that. 85 plus the weight of the empty one, so the MaaS so 80, 817 nusions okay, 870 nusions. So this we can do that distance times, that force. Clockwise is equal to. This distance times this force 27 musions antit clockwise that we have to take into account the angle. Yeah. Okay. So why that is 870 newtons, because the 870 newtons is the maximum weight. So is that the minimum weight, minimum force of wind? As you're told, it can take 85 liters each leisure. One leisure is one kilogram. And then we have 85 leiers Yeah I know that Yeah I know how to calculate this Yeah and times, Yeah we know that the maximum weight that is fourth water, but the minimum minimum weight that is. To mean so so we should take 870 newtons into the weight of the base. 870Yeah. So. Okay, I know that again, it's using the data. Any data that you're given, you'll be expected to be used. So 88.5 kilograms times g is about 868. Their rounding is up to 870 nutance. So then you do. Are the clockwise moments. Nopoint 45 times 870 times costs 15 because it's acting at an angle is equal to. Two. Two times. 27 plus. Force of the weight. So that's plus 15 plus the force of the wind. Times. 2.4 measures. Times costs 15. So if you work that out, that should give you the answer. Yeah. Sand instead of water. The density of sand is greater than the density of water. So the density means the MaaS per unit volume will be bigger. So this will mean that you will get a greater force than 870, which will mean that sand will be able to withstand a bigger toppling force than water because the density of sand is higher. Density equmass per unit volume. Okay? So that's what they're looking for there. State and justify what would happen with force of the wind. So your force of the wind would be able to be bigger because sand has a greater MaaS per unit volume. So you will need a bigger force of wind to tliover. Okay. Yeah, that question is worth ten Marks. Okay. Did you get any of these questions? Jackson, nearly no, no, no, I saw it before by noin test. Okay, Jackson, so more descriptive type questions. I'll let you go now. Okay. Okay Jackson, bye bye bye.