Simple machines make our lives easier. They make it easier to lift, move and build things. Chances are that you use simple machines more than you think. If you have ever screwed in a light bulb, put the lid on a jam jar, put keys on a keychain, pierced food with a fork, walked up a ramp, or propped open a door, you've made good use of simple machines. A block and tackle setup is also a simple machine.
Block and tackle refers to pulleys and rope (in that order). One kid can drag ten adults across the room with this simple setup – we've done this class lots of times with kids and parents, and it really works! Be careful with this experiment - you'll want to keep your fingers away from the rope and don't pull too hard (kids really get carried away with this one!)
If you haven't already, make sure you try out the broomstick version of this activity first.
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Block and tackle refers to pulleys and rope (in that order). One kid can drag ten adults across the room with this simple setup – we've done this class lots of times with kids and parents, and it really works! Be careful with this experiment - you'll want to keep your fingers away from the rope and don't pull too hard (kids really get carried away with this one!)
If you haven't already, make sure you try out the broomstick version of this activity first.
Please login or register to read the rest of this content.
You can purchase them at the hardware store, or make your own (search for “homemade pulley” on the site).
Hi,
Where can we buy the pulleys from? Do they have to be the same size?
Compared to the broomstick version of this experiment, the pulley version should be much easier. In the broomstick version, the rope is rubbing against the wood of the handles at each point where it loops over a handle, creating a lot of friction between the rope and wood at each loop point. The friction is so strong that the rope might not want to slide easily over the wood, and when it does slide the rope probably makes a strange creaking sound against the wood. This doesn’t happen with the pulley version because the string is no longer sliding against anything. The only sliding happens between the wheel of each pulley and the bar running through the center of that pulley wheel, and that contact point has probably been oiled to make them slide easily against each other. So using the pulley version of the experiment, the rope should move much more readily through the pulleys without creating all that friction. This will make it easier for the person pulling the end of the rope to pull the other two people together.
How does looping the rope through many pulleys make it easier on the person pulling the end of the rope to pull the other two people together?
This is a physics problem involving force, tension, mechanical advantage, and work. The people pulling on the two broomsticks and the person pulling on the end of the rope exert forces on the system, and the force of the taught rope as it pulls on the pulleys or the broomsticks is called tension. It is the total force on the system, how that force is divided up between the lengths of rope between the pulleys, and the resulting mechanical advantage of the system that makes it easy for the person at the end of the rope to pull the two people with the sticks together. Let’s draw some simple example diagrams to help with this explanation.
Figure (a) shows a single pulley fixed to the ceiling with a weight W attached to one end of the rope. Remember, weight is actually the force of gravity pulling down on mass. The rope experiences a tension equal to W, and this tension is the same along the entire length of the rope. A person pulling the other end of the rope must apply a force to it that keeps the weight hanging motionless – this means the system is in equilibrium. To keep the system in equilibrium, the person must pull down with a force equal to the tension in the rope. Now, to determine the mechanical advantage of the system, you must divide the output force – the force of the weight – by the input force – the force applied on the rope by the person. Since the person pulls down with force W, which is the same force the weight exerts on the other side of the rope, this system has a mechanical advantage of 1.
Consider the system in Figure (b). Now the pulley is moveable; as the free end of the rope is pulled up, the pulley will also move upward. The weight is attached to the center of this moveable pulley. In this setup, the weight is supported both by the end of the rope attached to the ceiling AND the free end of the rope pulled up by the person. Since the tension everywhere in the rope must be the same, the two ends of the rope must support the weight equally, meaning that each end supports half of W. As mentioned before, the person pulling up on the rope has to apply a force equal to the tension in the rope. This means he or she only has to exert a force equal to half the weight! The mechanical advantage of this system is 2 – the output force of W divided by the input force of W/2.
The person pulling up on the rope pays a price for reducing the force they need to exert on the system, though. That’s because the work done by the person on the system must equal the work done on it by the weight W. Work is the force exerted on an object multiplied by the distance the object moves. The person pulling on the rope only has to apply a force equal to half of W, but in order to move the weight up a distance d, the person must pull the rope a distance of 2d. This is shown in the equations below.
Figure (c) shows a similar setup but with two pulleys and three sections of the rope supporting weight W. As mentioned before, the tension needs to be the same all along the rope, and so the force of the weight is supported equally between all three sections of the rope – the tension in each section is W/3. A person pulling on the free end of the rope only has to pull with a force of W/3, meaning the mechanical advantage of the system is now 3! Again, the person pays the price of having to pull the rope farther to get the weight to move a certain distance – to move W a distance d, the person must pull the rope a distance 3d.
The pulley and broomstick experiment works the same way as the pulleys shown above. The more pulleys that are added to the sticks to be strung together, the more mechanical advantage the system has, and the easier it is for the person pulling the rope to bring the others holding the broomsticks together.
How does the pully make the string work?