SImple Machines: Pulleys (1954)

[Music] if you've ever tried to raise someone up into a tree you know how much force it takes to raise this much weight by pulling [Music] up but these two painters raise their own weight and the heavy scaffold too and the work doesn't look as hard with a simple machine the pulley the work can be easier let's see why here's one reason this is a single fixed pulley it's fastened to a support how much force will it take to lift a 20 lb weight 20 lbs this kind of pulley does not increase Force but it's easier to pull down than up because we pull with the weight of our body so fixed pulley can help by changing the direction of a force but the pulley on the scaffold moving with the weight can help by increasing the force now with a movable pulley let's see how much force it takes to lift the 20 lb weight only about 10 lb and because the pulleys can all turn freely the force in each rope is about the same see 10 lb in this rope and 10 lb in this rope together they lift 20 lb but how far does the force move to lift that weight with this pulley machine the weight is lifted one foot while the force moves 2 feet so we trade distance for Force we move the force farther to lift the weight to gain more Force we can add more pulley now we have two fixed pulley and two movable pulleys there are four ropes supporting the weight how much force will it take to lift the weight about 5 lb in each rope about 5 lb four ropes support 20 lb and the force must move four times as far as the weight trading distance for Force but there are five ropes why isn't the force multiplied by five notice only four ropes support the weight the fifth rope just changes the direction of the force we pull down to move the weight up of course the there could be five supporting ropes if we fastened one end to the movable block and pulley five ropes lifting so the force required here to lift 20 lb is about 4 lb but it has to move five times as far as the weight now we've shown the pulleys hanging separately but sometimes you see two pulleys together or three fre the frame and the pulleys in it are called a pulley block and the whole pulley system is called a block and tackle here again with five supporting ropes you can lift a weight of 20 lb with a force of 4 lb so we can rig pulleys to help us in two ways fixed pulley change the direction of our Force lifting when we pull down sometimes fixed pulleys help us lift when we pull sideways sometimes fixed pulley balance one weight against another so we can lift loads [Music] easier movable pulley help us in another way multiplying force movable pulley trade distance for Force so we can lift heavy loads making our work easier with simple machines