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Current time:0:00Total duration:9:40

Identifying centripetal force for cars and satellites

AP.PHYS:
INT‑3.B (EU)
,
INT‑3.B.2 (EK)
,
INT‑3.B.2.1 (LO)

Video transcript

so here we have something that you probably have done in the last maybe in the last day and if we're in a car and we're just making a turn let's say at a constant speed on a road that is flat so it's not a banked racetrack or anything like that what is keeping the car from just veering off in a straight line and this one's a less a little bit less intuitive because we don't have any string here that's tethering the car to the center of the curve of our of our road so what's keeping it from going in a straight line here pause the video and think about that well in this situation and we could think about other forces that are at play and once again I'll assume we're in a vacuum although you could think about air resistance as well and think about what what is counteracting the air resistance it turns out that that's friction but the other forces at play you of course have the force of gravity pulling downward on the car force of gravity and that's being counteracted by the normal force that's being counteracted by the normal force force the normal force of the road on the car but what's keeping the car going in a circle and actually let's just do air resistance for fun so the air resistance the force of the air on the car that's going to be pushing in the direction opposite from the velocity of the car so we could call that let's just call that force of air you can't read that let me do this so force of the air that would be its magnitude and then that's being counteracted by and this is a little bit counterintuitive and this will actually give us a clue on the centripetal force that is this component that's going to be counteracted by this component of the friction so force of friction in the direction that the car is going think about it if you didn't have if you didn't if this was an ice on ice if the wheels didn't have traction no matter how hard the engine went and no matter how fast the wheel sped it wouldn't be able to overcome the air resistance and then the car would decelerate but the are all the forces that are that aren't acting in a radial direction that aren't keeping the car on the road so to speak are keeping it going in that circular motion around the curve the one there is once again the force of friction so this is another I guess you could say another component of the force of friction and that's all happening where the tires were the literally the rubber meets the road but this right over here you have the force of friction that is keeping and maybe I'll call it force of friction radially radially lieutenant parentheses force of friction radially that is keeping us going in a circular direction and so in this situation that is our centripetal force let's do another example and let's keep going with the theme of cars now so let's say a scenario where we are on a loop-de-loop which is always fun and kind of scary I have dreams where I'm have to drive on a loop-de-loop for some reason and I find it intimidating but let's think about the car at different points of the loopty-loop and think about what is / what is what is the centripetal force at different points so let's first think about this point right over here and once again we assume that we are dealing on a we're on a planet and so you have your force of gravity right over here force of gravity and then you also have your normal force and I'm gonna draw it a little bit larger because in order to be moved I guess you could say upwards to stay on the loop-de-loop the normal force has to be larger you have to have a net force inward so this is F this is our normal force and so in this situation the magnitude the magnitude of our centripetal force let me just in a different color the magnitude of our centripetal force is going to be the net radial inward radial force or the magnitude of their net radial inward force so this would be equal to the the magnitude of our normal force - - the magnitude of the force of gravity if this wasn't net inward right over here then you would not this car would not be able to move in a circle it would just if this netted out to zero it would go in a straight line that way and if this netted out so that it was negative it would accelerate downwards so let's go at this point right over here and we could also think about things like air resistance and friction where air resistance is pushing back on the car then the friction is overcoming it but we're gonna focus just on on the things that are driving us centripetal e inward or outward right now now what about this point for the car well we still have the force of gravity you still have the force of gravity and actually I'll make this a little bit bigger we could let me let me put the air resistance there just to be complete so this would be the air resistance force of the air and then that's being counteracted by force of friction the traction that the cars of the road over here this orange vector this would now be the combination of the force of gravity and actually you could even consider the force of gravity plus the plus the force the air resistance plus the force of the air pushing back on the car the pressure of the air and then that is being counteracted by the force of friction so the force of friction of the tires pushing pushing or against the force of friction of the tire between the tire and the road but neither of these are acting centripetal E acting radially inward so what's that going to be well here you have the normal force of the of the track the track is what's keeping this car going in this circular direction and so you have and so you have here the inward force is the normal force F normal so in this situation our centripetal force the magnitude of our centripetal force is equal to the magnitude of our normal force and these actually are even going to be the same the exact vectors now last let's now must consider one last scenario when we are at the top of the loop-de-loop pause the video and see if you can figure that out well once again we can do things like we could say hey look there's probably some air resistance that is keeping us that is trying to decelerate us so that and then that's being that's being netted out by the force of friction but let's think what's going on in the vertical direction so here pushing down this way you're going to have potentially several forces and I want this to be actually at the top of the loopty-loop although it doesn't look quite like that but actually let's just assume it is we're at the top of the loop-de-loop pushing down you're gonna have the force of gravity but what else are you going to have assuming you're going fast enough the track is also pushing down the force of gravity plus the normal force the magnitude of this vector would be the magnitude it would be the sum of the magnitudes of the gravitational force and the normal force and that is what's providing your centripetal force there and so in this scenario we would say the magnitude of our centripetal force is equal to the magnitude of our gravitational force plus the magnitude of our normal force or we could even think about it as vectors we could say hey look if we just add up these vectors these two vectors you're going to get your centripetal force vector that's what keeps the car going in that circular motion now let's just do one last scenario just for fun let's imagine that we have an object in orbit so this is our planet or any planet really you have an object in orbit some type of a satellite I'll draw what we normally associate with as a satellite but this could be even a natural satellite a moon for the planet and what I'm about to say applies to the moon as well so here we don't have air we have very minimal air resistance there might be a few Atmos you know molecules every here and there but for the most part this is in a vacuum and it's in orbit so what keeps so it's in a uniform circular motion its moves in a circular orbit around the planet what keeps it going off in a straight line pause the video and think about it well here you have the force of gravity you have the force of gravity of the planet so right there you have the force of gravity and at first people say wait wait gravity well you know I see these pictures of astronauts floating when they're in orbit well that's just cuz they're in free fall but the gravity at that point if you're a few hundred miles above the surface of the earth is not that different than the gravity on the surface of the earth you just don't have air there and if you are in orbit you're in constant freefall so it feels to you like there is no gravity but it's gravity that is keeping you on the orbital path on that circular path and keeps you from just going in a straight line out into space