Practice: You are practicing jumping as far as you can. In one attempt, you run and leave the floor with 7 m/s directed at an unknown angle. What maximum height do you reach if your speed at that point is 5 m/s? Ignore air resistance.

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Intro to Conservation of Energy | 52 mins | 0 completed | Learn |

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Motion Along Curved Paths | 107 mins | 0 completed | Learn |

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Example #1: Projectile Motion with Energy

**Transcript**

Hey guys so in this video I want to talk about using the conservation of energy equation to solve projectile motion problems let's check it out. So it says your projectile motion problems asking for speeds or heights are easier to solve using conservation of energy equation, now they're not always easy to solve they're sometimes easy to solve and you can't always do this and I'll talk about this more at the end but we can use energy because speeds have to do with kinetic energy and heights have to do with potential energy so we're going to be able to in some cases use this equation which would be preferable because instead of having to pick one of the three or four equations of motion and have to worry about directions of positive or negative and all that kind of stuff and have to worry about decomposing vectors we can just instead use the energy in some cases to solve it all at once, alright?

So here it says you throw an object 5 kilograms from the top of a 30-meter-tall building so this is 30 meters right here with 20 meters per second and so if you launch with 20 meters per second means that Va right here is 20 meters per second directed at an unknown angle so we don't know this, OK? Above the horizontal it says that the object reaches a maximum height of 40 so if you are at 30 and you get to 40 it's because this additional height here is 10 meters, you could write this or you could just write that the whole thing is 40 ignoring air resistance so there's no work done by friction calculate the object speed at points B C and D, OK? So, let's start over here and I want to find the velocity of object at point B, so I know information about point A I know my initial height at point A and I know my initial velocity at point A and I want to know information about Point B so I'm going to write an energy equation from A to B and it's going to look like this, Ka+Ua+work non-conservative=Kb+Ub, alright? There is kinetic energy at point A because I have a velocity, there's potential at A because I have a height there's no work non-conservative, remember 20 meters per second is after it's already left your hand so while it's between the time that it leaves your hand with 20 and it gets to B over here or actually through the entire motion even you're not doing anything, right? So, the work done by you 0 you did work in throwing it remember but the problem starts counting once it leaves your hand with 20 so the work done by you is 0 and there is no friction there is kinetic energy at point B, alright? Some people sometimes here might make a mistake and think that while the velocity at point B is 0 because the highest point but no the velocity in the Y axis at point B is 0 so the velocity of B is simply made up of its velocity in the X axis which if you remember velocity in the X axis never changes so it's Vax It's whatever the initial velocity in the X axis was, alright? Just as a quick reminder of how some of the stuff in projectile motion works but you didn't really need to know all that for here or remember all of that you just have to not think that the velocity would be 0 at the top, OK Vb is not 0 so I have kinetic energy because Vb is not 0 and I have potential energy because I have a height so I have all four types of energies here and let's do this, so 1/2MVa squared + MGHa = 1/2MVb squared + MGHb. By the way one of the things you could have done here is you could have said to make it a little bit simpler you could have said well since I'm going from A to B I'm going to say that this is a height of 0 and then this is a height of 10 you could have done that if you wanted, alright? I didn't do that here but it doesn't matter it won't affect the final answer it would have just made things a little simpler, here the masses cancel and if I plug in these numbers I have a 1/2(20) squared, gravity I'm going to use 10, the height at point A is 30, Vb is what looking for gravity is 10 and the height at B is 40, alright? You've got all the numbers all you have to do is solve for Vb if you move all the stuff around, OK? I'm going to fast forward a little bit here but if you move all the stuff around and you solve for Vb you get the Vb is 14.1 roughly 14.1 meters per second, OK? So, nothing special here just basic conservation of energy equation, alright? For part B we're being asked what is the velocity at point C, alright? What is Vc? Well to do this we can write an equation from A to C but we actually don't have to do that we don't have to write an equation from A to C at all I should know from conservation of energy that if A and C are at the same height they have to have the same speed, right? If they are at the same height they have to have the same speed so since the height at C is the same as the height at A I can tell you that of the velocity or the speed at C is the same as the speed at A so it's going to be just 20 meters per second we can also think of this in terms of symmetry if you remember symmetry from projectile motion the speed going up is the same as the speed going down for points that are symmetric and because the speed up is the same as the speed down whatever a Vay is the same as Vcy and the velocities in the X axis never change so we Vcx is Vax combine the fact that it's symmetric up and down and that the X axis never changes you end up having a Vc that is exactly the same as Va we talked about this in projectile motion but you can also just have a simple answer to this in terms of energy if the heights are the same the speeds have to be the same, OK? So that's how we get rid of B real quick let me now let me now solve part C. Part C is the velocity at point D, alright? So, to do this we're going to have kinetic initial I'm sorry we're going to write an energy equation from let's say A to D I could have written one from B to D I know information about B to D I can also even have written one from C to D since I know the height of C and the speed of C but it doesn't really matter they all work and they're all equally.... Equal amounts of work. Kd Ud there is a kinetic energy at point A there is a potential energy at point A we're going to say that the ground is 0 over here, there is no work there is kinetic at point D because it's right before you hit the ground and there is no potential energy there so this is a half MVa squared, MGHa, 1/2MVd squared all the masses cancel and we're looking for Vd, let's plug in numbers, 1/2(20)squared + 10 the height is 30 equals 1/2Vd squared if you move everything out of the way this is 200+300, this 2 multiplies over here and then Vd then becomes 31.6, Vd is 31.6 meters per second it should make sense that Vd is greater than Va, right? 20 you go up now you have 14 you come back down to 20 and then you go back down and you're a little bit faster at 31, OK? Vc was 20 and the last question is part what would be the velocity at point D? it says right what is the velocity at point D had it been thrown in any angle below the X axis? So, this is a velocity at point so instead of launching this up you had launched with the Va equals 20 that made some negative angle here in other words you threw it down instead of throwing up, what would it be the velocity at point D? And I can tell you real quick that without having to solve this the velocity would be exactly the same because the direction at which you launch the object doesn't matter because we are doing this using the energy equation, energies are scalars not vectors so the direction does not matter, OK? So I'm going to write here direction does not matter so you have in your notes and you can do this by look at the energy equation and realizing that it just has speed not Vx not Vy not sine of the theta or cosine of theta none of that stuff so there are no thetas, Vx, Vy just V in the energy equation, energy equation, OK? So this means Vd is 31.6 irrespective of the direction that you toss it at even if you didn't straight up it would still have been 31.6 on the way down so that's of that's an important conceptual point you should know that in respect of the direction the final total speed will always be the same because it depends only on the initial speed and the initial height, OK? And that's from conservation of energy, alright I have a second example here let's check that out.

Example #2: Projectile Motion with Energy

**Transcript**

Alright so in this example we have a ball that we're throwing with 20 meters per second at 37 degrees above the horizontal so letÕs draw this real quick, throwing it this way I'm going to call this Va=20 and it makes an angle of 37 degrees above the horizontal above the X axis like that and it say use the conservation of energy to find its maximum height so let me draw this, it's going to go like this and then hit the ground and I'm going to call this point A, B and C and here I want to know the maximum height so I want to know what is the height at point B? And it's asking us to do this using conservation of energy I know information about Point A I want information about Point B so I'm going to write the energy equation from A to B since A is my known and B is my target so here's the energy equation so there is kinetic energy at point A because you tossed it, it has a speed at that point there's no potential at point A because we're assuming that you're throwing it from the ground or close enough to the ground there's no information about height so we can assume that this is 0, the work done by non-conservative forces is 0 because again you throw a 20 it starts with 20 and then from them instant it leaves your hand with 20 and throughout the entire path on the air you're not doing anything there's no friction there is a kinetic energy at point B even though at point B is the highest point it still has a velocity, it has a velocity at point B is the same as Vx, remember Vx is whatever initial velocity you had here Vx is your Vax and this velocity in the X axis is the same everywhere, Vax is the same as Vx throughout and then in this case there is no Vy here, right? So, I can write that VBy is 0 therefore the entire all of the speed or velocity at point B comes from its X component there is no Y component, OK? So there's definitely energy here and there is energy here because there is a height so let's expand this, 1/2MVa squared = 1/2MVb squared + MGHb, the masses cancel and I'm looking for Hb let's try plugging in some numbers and you're going to notice that we're going to get stuck so 1/2 the speed here is 20, 1/2 and now realize that I don't have Vb at least it wasn't given to me we can calculate Vb but we don't have it right away, Vb square we don't have it + GHb so you got stuck not having a number and this is just the basic physics hustle question as I like to call it because you're going to get stuck and you'll need a number and you're going to have to go and find it, OK? And this is where you have to realize and remember a little bit of projectile motion realize that the velocity at point B because there is no Vby is made up entirely of its X component so I'm going to say that again so since Vby=0 then Vb is made up entirely of Vbx, OK? And that's the first point. Two since the acceleration in the X axis is 0 and it always is 0, right? And unless you have air resistance which most of the time doesn't show up then I can say that Vbx is the same as Vax because it never changes in fact it doesn't change so I like to think of it as not a Vax, Vbx or Vcx but just Vx to make the point to reinforce the point that Vx is always the same so if I want to find Vbx and if I want to find Vbx I just have to find Vax and we can do that Vax is Va cosine of theta X goes with cosine as long as the angle is against the horizontal which it is, OK? And we can plug this in, this is 20 cosine of 37 and 20 cosine of 37 is going to be 16 meters per second so 16 is the number that goes right here now let's continue this over here this is going to be 400, 200 this is 256 divided by this would be 128+Hb if I move this over here I'm going to get a 72 equals 10 Hb so Hb equals 7.2 meters, OK? that's it for this one, again the energy equation got a little weird because we didn't have all the pieces of information and sometimes you're going to get these equation with projectile motion where you are going to have almost everything but then you have to decompose an angle to figure something out and the key thing here to remember was the fact that your Vb at the highest point will always just be your Vx then you can get Vx from Vx so it says here one last point not really related to this question but just to these types of problems that problems that required time notice the neither one of these problems that we've solved require time but that problems require time to be solved cannot be solved with the energy equation and that's because the energy equation has no time it has no reference to time it has no T variable anywhere and instead we're going to have to use motion equations, OK? We're not really going to run into these here because this is from a previous chapter, right? And the whole idea of this portion is that you can do some of these questions using energy but not all of them some of the other questions that you won't be able to do using energy will have time and in that case, you have to use motion equations instead, whenever you're being asked for time use motion equations otherwise you might be able to use the energy equation that's it let me know if you guys have any questions.

Practice: You are practicing jumping as far as you can. In one attempt, you run and leave the floor with 7 m/s directed at an unknown angle. What maximum height do you reach if your speed at that point is 5 m/s? Ignore air resistance.

Practice: When you launch a 3-kg object from the ground with unknown initial speed directed at 37° above the x-axis, it hits the building shown below at 15 m above the ground with 25 m/s. Calculate the object’s launch speed. (Use *g*=10 m/s^{2}.)

Practice: A 3-kg box is nudged off the top of the path shown below, slides down, and is launched form the lower end of the path. The path is frictionless and its highest point is 10 m above the ground. The lower end is 2 m above the ground and makes 53° with the horizontal. Calculate the box’s speed:

(a) at the lowest point in the path;

(b) just before it leaves the path;

(c) at its highest point;

(d) just before it hits the ground.

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