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Physical ScienceCh 5 (Part II): Simple Machines
• A pulley is a grooved wheel with a rope or chain in the groove, which rotates around a fixed point.
• A single fixed pulley is attached to an immovable surface.
It changes the direction of the force, but not the size. So the M.A. is 1.
• A single movable pulley is not attached to a stationary surface, and is free to move.
• A single movable pulley has a mechanical advantage of 2.
• Does the mechanical advantage of 2 for a single movable pulley mean that you’ll increase the amount of work you get out of the pulley?
• No, it doesn’t.
If you get twice as much
force out then you have to
pull out twice as much rope,
but the amount of work (f x d)
would not be effected.
So how could you change the amount of work produced by the machine?
• A block and tackle is a combination of fixed and movable pulleys.
• The M.A. is equal to the number of rope segments supporting the weight.
What is the mechanical advantage of this block & tackle?
• A wheel and axle consists of 2 different sized wheels which rotate together around the same axis.
• The mechanical advantage of a wheel and axle is determined by dividing the radius of the wheel by the radius of the axle.
M.A. = Rw / RA
• A gear is a wheel with interlocking teeth which prevent slippage.
• An inclined plane (ramp) is a slanted surface used to raise or lower objects.
Inclined planes are usually stationary.
• A wedge is a movable inclined plane.
• What does sharpening a knife do to it’s mechanical advantage?
• Is a sharper object always going to perform better than a dull one?
• A screw is an inclined plane wound around a cylinder.
• The closer the threads, the greater the mechanical advantage.
• Which screw would have the greater M.A.?
• Which would be hardest to drive into a block of wood?
• Which would require more turns?
A B
The front sprocket on a bike has 2 gears which act as axles, while the circle created by the pedal acts as the wheel. The outer gear has a radius of 6 inches and the smaller inner one has a radius of 4 inches. If the pedal goes in a circle with a radius of 12 inches, calculate the mechanical advantage provided by each of the different gears.
• In the previous example, which gear would you want to be in if you were pedaling up a steep hill? Why?
The lugnut on a large dump truck has a radius of 5 cm, and requires a force of 1,800 N to loosen. Lenny, however, can only produce 150 N of force. He knows that if he gets a long enough wrench then he can produce that much force. How long would the wrench need to be?
• Carl is applying 200 N of force to push a wheelbarrow up a ramp. If the ramp is 6 m long and allows him to roll the wheelbarrow to a height of 1.5 m, what is the mechanical advantage of the ramp?
• How much force would Carl need to apply if he were not using the ramp?
• How much force would Waylon need to apply in order to lift a 375 N weight, using the pulley shown?
• How much rope would he need to pull out in order to raise the weight 4 m?
• Using the 3 pulleys shown below, how much force would Ned need to apply to each to lift a 400 N weight?
• Why wouldn’t he just always use the one which requires the least force.
• In the diagram below, the inclined plane is 5 m long and 2 m high. The wheel and axle have radii of 25 inches and 10 inches respectively. How much force would be required to move the weight up the inclined plane? (assume no friction)
• How much work is being done by the machine?