Anderson Video - Rocket Launch

Professor Anderson
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>> Hello class Professor Anderson here. Let's take a look at a very simple rocket launch example and let's see if we can identify two things, the motion diagram for this rocket and what the force diagram looks like. Okay, hopefully this sounds familiar to you. If not, it's on your homework so when you see it you'll say, oh that's what he was talking about. All right, let's see what our motion diagram looks like. Motion diagram is of course velocity. So if my rocket is starting on the ground level it's right there and now it's just starting to move. And as it goes up it starts to increase its speed okay. And so maybe this is what my motion diagram looks like. It starts out moving at a small velocity and it ends up moving at a much higher velocity. Okay, these are measurements of the velocity. So let's think about the forces that are acting on it and to do that we need to draw a free body diagram, so there's my rocket. What are the forces that are acting on the rocket? What's one of the forces that's acting on the rocket? Maybe somebody that hasn't chimed in yet. Hey [inaudible], what do you think? What's the one force that's acting on the rocket? >> There is gravity going down. >> Okay, gravity going down. Okay, what else [inaudible]? >> There's also thrust from the rocket going up. >> Okay. Have thrust from the rocket going up. Anything else? >> There's also a normal force going up with the rocket. >> A normal force from what? [ Inaudible Comment ] You said it's going up like that? >> No, there's air resistance going down. >> Okay. >> Yeah. >> So is there a normal force? >> No. >> No. There's no normal force because it's not in contact with the ground right. If we're ignoring when it's sitting there at launch right, as soon as it takes off from the ground it's not in contact with the ground anymore, so there can't be any normal force. So the last thing you said there was what? >> Air resistance going down. >> Air resistance going down that is our drag force due to air resistance. So this is what my free body diagram looks like, have two forces going down, I have one force going up. Is everybody happy with my diagram? Anybody not happy with my diagram? I'll give you a hint you should not be happy with my diagram. So what are we doing wrong? Yeah Anda [phonetic], what do you think? What am I doing wrong? >> I think thrust should be longer than the force of gravity. >> Why? >> Because the rocket's accelerating upwards. >> Okay if the rock is accelerating upwards, then we have to have a positive number for this, right. This is going to be bigger than zero, which means that all my forces F thrust going up minus M G minus F drag has to be bigger than zero. And if that has to be bigger than zero then F thrust has to be bigger than the combination of M G and F drag right. And I can move these over to the other side of the equation and I get that. So you're exactly right that if this thing is going to accelerate upwards F thrust has to be bigger and in fact, it has to be bigger than the sum of both of those. So maybe something more like that right. You can measure them out and see if we're right, maybe we have to go a little bit taller, but that seems about right. Okay, if F thrust is bigger than both of those things, then it will accelerate upwards. What if it is not bigger than both of those things? If F thrust is in fact equal to M G plus F drag Anda, what do you think happens? >> It would be at constant speed. >> Okay it would be at constant speed, acceleration would be zero. It would be at constant speed that would be maybe what we would call terminal speed for the launch. Good, any questions about that one? All right, if there are definitely come see me in office hours. Cheers.
>> Hello class Professor Anderson here. Let's take a look at a very simple rocket launch example and let's see if we can identify two things, the motion diagram for this rocket and what the force diagram looks like. Okay, hopefully this sounds familiar to you. If not, it's on your homework so when you see it you'll say, oh that's what he was talking about. All right, let's see what our motion diagram looks like. Motion diagram is of course velocity. So if my rocket is starting on the ground level it's right there and now it's just starting to move. And as it goes up it starts to increase its speed okay. And so maybe this is what my motion diagram looks like. It starts out moving at a small velocity and it ends up moving at a much higher velocity. Okay, these are measurements of the velocity. So let's think about the forces that are acting on it and to do that we need to draw a free body diagram, so there's my rocket. What are the forces that are acting on the rocket? What's one of the forces that's acting on the rocket? Maybe somebody that hasn't chimed in yet. Hey [inaudible], what do you think? What's the one force that's acting on the rocket? >> There is gravity going down. >> Okay, gravity going down. Okay, what else [inaudible]? >> There's also thrust from the rocket going up. >> Okay. Have thrust from the rocket going up. Anything else? >> There's also a normal force going up with the rocket. >> A normal force from what? [ Inaudible Comment ] You said it's going up like that? >> No, there's air resistance going down. >> Okay. >> Yeah. >> So is there a normal force? >> No. >> No. There's no normal force because it's not in contact with the ground right. If we're ignoring when it's sitting there at launch right, as soon as it takes off from the ground it's not in contact with the ground anymore, so there can't be any normal force. So the last thing you said there was what? >> Air resistance going down. >> Air resistance going down that is our drag force due to air resistance. So this is what my free body diagram looks like, have two forces going down, I have one force going up. Is everybody happy with my diagram? Anybody not happy with my diagram? I'll give you a hint you should not be happy with my diagram. So what are we doing wrong? Yeah Anda [phonetic], what do you think? What am I doing wrong? >> I think thrust should be longer than the force of gravity. >> Why? >> Because the rocket's accelerating upwards. >> Okay if the rock is accelerating upwards, then we have to have a positive number for this, right. This is going to be bigger than zero, which means that all my forces F thrust going up minus M G minus F drag has to be bigger than zero. And if that has to be bigger than zero then F thrust has to be bigger than the combination of M G and F drag right. And I can move these over to the other side of the equation and I get that. So you're exactly right that if this thing is going to accelerate upwards F thrust has to be bigger and in fact, it has to be bigger than the sum of both of those. So maybe something more like that right. You can measure them out and see if we're right, maybe we have to go a little bit taller, but that seems about right. Okay, if F thrust is bigger than both of those things, then it will accelerate upwards. What if it is not bigger than both of those things? If F thrust is in fact equal to M G plus F drag Anda, what do you think happens? >> It would be at constant speed. >> Okay it would be at constant speed, acceleration would be zero. It would be at constant speed that would be maybe what we would call terminal speed for the launch. Good, any questions about that one? All right, if there are definitely come see me in office hours. Cheers.