Man has always tried to use less effort when doing work. The earliest method of using less energy to perform a task was the use of the simple machines. These 6 simple machines, the lever, inclined plane, pulley, wedge, screw and wheel & axle can also be used together and make what we call a compound machine. But lets not get ahead of ourselves and begin by learning about these simple machines. This section will satisfy Performance Standards 5.1a and 5.1 f. for grades K-4 science.
Remember, a simple machine does not “save” work. What we get is a mechanical advantage (how much a machine multiples the effort force). There is an ideal mechanical advantage (IMA), that we can predict mathematically, and the actual mechanical advantage (AMA) we receive when we perform the task. If you are asking why there is a difference between the IMA and AMA, it is that friction is not involved in the IMA. When we compare the actual mechanical advantage (AMA) to the ideal mechanical advantage (IMA), we can calculate the efficiency of the machine.
These are the formulas we will use for our work with simple machines:
d= distance e= effort r= resistance f = force
IMA =de/dr AMA = fr/fe Efficiency = AMA/IMA X 100%
The type and extent of the investigations you perform will depend on your objectives.
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The Lever
The lever consists of an effort arm, fulcrum and the resistance arm. The positioning of the three components determine whether the simple machine will allow you to use less effort, gain speed and distance or change the direction of the applied force. Before we begin some hands-on activities let’s look at the movie on brainpop. This movie can be found at http://www.brainpop.com.
The lever consists of a rigid bar, fulcrum, resistance arm, and an effort arm. The positioning of the load in relation to the fulcrum and the effort arm determines the class of lever we are using.
1st class lever - Force Arm (effort) - Fulcrum - Load Arm (resistance)
Uses- Change direction of a force - Multiply force - Gain speed and distance
2nd Class lever - Fulcrum - Load Arm(resistance) - Force Arm (effort)
Use - Multiply force
3rd Class lever - Fulcrum - Force Arm (effort) - Load Arm (resistance)
Use - Gain distance
Demonstrate and/or have students perform the IMA and AMA of the three classes of levers.
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The Inclined Plane
The inclined plane is a slanted surface over which the load is moved. Again, let’s take a look at the Brainpop movie about the inclined plane. http://www.brainpop.com. You decide how you can best use the questions for this movie.
It is now time to do a demonstration or preferably a hands-on activity.
Do the actual activity using the formula, Work = Force X Distance without using the inclined plane.
Now have them set-up an investigation moving a load up the same height (vertical distance) but using an inclined plane.
Have them determine the IMA of the inclined plane.
(IMA = Length of incline/Height of incline.)
Use the formula AMA = Resistance Force/ Effort Force to find the actual advantage gained doing this task.
You may want to repeat this investigation but use an inclined plane of a different length but keeping the same height.
Pulley
A pulley is a simple machine that uses grooved wheels and a rope to move a load.
Types/Uses - Fixed - Change direction of a force.
Moveable - Multiply Force
Block & Tackle - Multiple Force & Change Direction of a Force
To determine the mechanical advantage of a pulley, you count the number of strands supporting the load, do not count the stand that allows you to change the direction of the force.
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No Mechanical Advantage
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Mechanical Advantage =2
If you do not have access to pulleys, have them work on a worksheet so they can distinguish between strands that support the load and strands that only change the direction of the force when determining the mechanical advantage.
With the last three simple machines we may want to just give the definition and examples.
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The Wedge
A wedge is a pair of inclined planes that move through and object when a force is applied.
Example: An axe, knife
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The Screw
An inclined plane wrapped around a cylinder. The width of the threads determines the mechanical advantage. The closer the threads, the easier to turn but the number of rotations increase to get to the desired depth.
Examples: Spiral Staircase, Bolts
The Wheel & Axle
A wheel & axle is made of to circular objects that are connected. The larger object is called the wheel and the smaller the axle.
Examples; Screwdriver - Doorknob