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Hi guys. Hi Dr. D. What have you been doing? Just went for a bike ride to get a

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little exercise. How's your project coming? We're excited about using pulleys

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to lift Jacob into the treehouse, but we ended up with a lot of excess rope. Are

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we doing something wrong? I figured you'd have some questions about pulleys, so I

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set up a model over here. Let's try it out.

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I have four kilograms or about nine pounds attached to this pulley system. Go

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ahead and pull on the string. That's really easy. Why don't you lift the weight,

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which we call the load one meter. We'll call this the load distance.

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Wait a minute, Anthony. Where did all this string come from? I don't know. What's

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wrong? Nothing's wrong. Let me explain. Simple machines allow you to multiply

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your force. In this case, you can lift an object eight times as heavy as your

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force. Right. We learned from the Cyphos Kids Club that you count the number of

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strings supporting the load. I count eight strings, and that's why we can

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multiply our force eight times. Very good, but there is a trade-off. What do you

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mean? Remember that work is equal to force times distance. Okay. With simple

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machines like pulleys, you can multiply your force, but not the work. So in this

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case, you've multiplied your force eight times, but how much string did you have

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to pull in? We'll call this your distance.

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Eight meters. That means our distance is eight times the load distance. Does it

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always work that way? Yes, it does. If you multiply your force, the trade-off is that

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your distance will always be greater than the load distance. Take a lever, for

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example.

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Why don't I stand over here, and have you try to lift me up by pushing over there.

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Try it with just one hand. Okay.

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I can't believe I just did that. That was easy. Well, if your force is five

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times further from the fulcrum, or rotational point, than the load, you will

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multiply your force by five times. But your distance, how far you push down, will

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also be five times greater than the load distance, how far I move up.

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This is really cool. So if simple machines don't allow us to do less work,

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or need less energy, why do we use them? I guess it's because they make the work

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easier. Excellent. We still need to make our work easier. Right now, we've

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multiplied our force three times by using three pulleys, but that isn't

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enough to easily lift the load of 120 pounds. But if we add more pulleys to

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multiply the force even more, that means that we have to pull in a lot more rope.

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Wow. Maybe we need to use a different kind of simple machine. Don't scrap your

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pulley idea. You might want to consider combining other simple machines with

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your pulley system. You mean like use a lever with our pulley system? That's a

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possibility. Another one is a wheel and axle. Take my bike, for example. This could

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also be called a crank and axle. This wheel with the chain attached is the

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axle. It's attached to this crank with the pedal. To find out how this machine

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multiplies the force, all we have to do is divide the radius of the crank by the

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radius of the axle. R.D., would you make the measurements, please?

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The crank is 16 centimeters and the axle is 8 centimeters, so that means the

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machine multiplies the force by two. That's great. Another possibility you may

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want to explore is gears. I'm going to visit my grandparents in San Diego. We're

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planning on going to Legoland. I bet there are a lot of gears in Legoland.

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I'll send them an email and see if I can speak with someone about gears. Great.

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Bye, Dr. D. Thanks.