Back To CourseHigh School Physics: Help and Review
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Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.
Last weekend, I received a really large package in the mail. It was quite heavy, and I could have tried to pick it up myself, but I didn't really like the idea of potentially hurting my back doing this. So, I called a few friends to help, but no one was available, so I was still out of luck. My third option was to use a simple machine, a non-powered device that either multiplies or changes the direction of a force.
You might be wondering what the heck multiplying or changing the direction of a force has to do with moving that package into my house. Well, according to Sir Isaac Newton, in order to move a stationary object, we need to apply a force to it. And when that force is applied over a given distance, we do work on that object. More simply, we can define work as force x distance. So, in order to get that box into my house, I had to apply a force and move it from outside to inside - I had to do some work!
And this is where simple machines come in. Multiplying the force or changing its direction helps us do work. It's important to remember that the law of conservation of energy still applies here - less work isn't actually being done, even though it may feel like it. No machine can create energy - it just transfers or transforms it. The work input will always equal the work output.
Let's see how this works with one of the simplest machines around - the lever. A lever is a beam that rotates on a support point. This lever support point is called a fulcrum. Ever been on a seesaw at the park? That's a lever! Scissors are also levers, as is the flush handle mechanism in your toilet tank.
If you want to lift something on a lever, you put the object on one end of the beam and push down on the other. You do work on your end because you apply a force, which is exerted through the distance of the beam to the fulcrum.
But work is also done on the other end of the beam as the object is lifted up from the other side of the fulcrum. And what's key here is that the work done on both ends is the same. The product of force and distance on both ends is equal. But this doesn't mean that the forces or the distances are equal on both sides. In fact, making them different is the key to effectively using a simple machine!
Say, for example, that your fulcrum, or support point, is under the middle of your lever beam. The distance on both sides of the fulcrum is the same, which means that the force will be the same on both sides. You will need to apply the same amount of force on your end as it will take to lift the object on the other end.
But if you move your fulcrum so that it's closer to the object you want to lift, you have increased the distance your force is applied over. This means that while the products of the two sides are still equal, the individual components are not. Since the distance you apply your force over is greater, the force you need to apply must decrease to compensate. On the other end of the beam, the distance has decreased, which means the output force must increase to compensate.
Here, the same amount of work is done as before when our fulcrum was in the middle, and the same amount of work is done on both ends of the lever. But simply moving the fulcrum toward the object you want to lift changes the amount of force involved in that work - on both ends.
For the same reason it would not be wise to move the fulcrum toward you and away from the object you want to lift, when the fulcrum is closer to you, the distance your force is applied over is lessened, meaning you need to apply more force. But on the other end, the distance from the fulcrum has increased, meaning there's a decrease in output force to compensate. You've just made it harder to lift the object instead of easier!
It's all about that relationship between force and distance. In all three situations, the work you did was the same. But the difficulty of that work changed depending on how you utilized the relationship of the work components - the force and the distance.
Levers are wonderful, but they certainly aren't the only type of simple machine. Nor are they always the best simple machine to use. It all depends on the work you want to do.
A wheel and axle is a simple machine where two components rotate together to transfer force from one to the other. Think of your car or bike wheels or even a round doorknob, and you'll get the idea of this machine.
When you put a rope or cable around a wheel and axle, you get a pulley. Depending on how many pulleys you have, you can either change the direction of the force (pulling down on the rope to lift an object up into the air), or you can multiply the force to move a very heavy object. When you increase the length of the rope by adding more pulleys, the same idea applies here as with the lever. Except this time, you are increasing the distance of the force over the rope instead of a beam.
Another handy simple machine is the inclined plane. This is a flat surface that is elevated on one end. Think of this like a lever that is stuck in place, almost like the fulcrum is stuck at the far end of the beam. Inclined planes are very useful for moving objects to different heights. If you've ever moved furniture, you know having a ramp (an inclined plane) available is much preferred to climbing individual stairs!
A special version of the inclined plane is a screw. This is an inclined plane that wraps around a pole or cylinder. Screws create linear motion and force from rotational ones. You turn a screwdriver, but the screw itself goes in straight. Same with a corkscrew into a wine cork - your hand goes around in circles, but that helix shape of the screw's plane creates a straightward motion of the screw itself.
If an inclined plane is slanted at one end, it can also be used as a wedge, our final simple machine. This device literally gets wedged between two surfaces: separating them, lifting one of them, or just holding something in place. Much like other simple machines, size matters with a wedge! If you have a long, narrow wedge, you will need less force than with a short, wide wedge. This is because the long wedge applies the force over a greater distance. But remember, regardless of what size wedge you use, the same amount of work is done. It's just how 'easy' you want that work to be!
If you need to move, lift, or pull something very heavy, you might consider using a simple machine. This is a non-powered device that either multiplies or changes the direction of a force. Work is done on an object when a force is applied over a given distance, so if you want to make the work seem easier, you need to change one of the components of the work itself.
The easiest way to do this is by increasing the distance the force is applied over. The greater the distance, the less input force is needed. All simple machines can do this - be it a lever and a fulcrum, a pulley with a long rope, or a screw with its long, helical inclined plane.
And remember, just because the work feels easier doesn't mean there's any less energy involved. True to the law of conservation of energy, even with a simple machine involved, the amount of work going in is the same as the amount of work coming out. It's all in how you change the force and distance involved in that work.
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Back To CourseHigh School Physics: Help and Review
22 chapters | 267 lessons