Simple Machines: Levers and Pulleys

• A mechanism involves multiple machines working together
• Mechanisms are also classified as mechanical systems
• A simple machine requires the application of a single force to do work

Levers

• A lever is a rigid bar supported at one point
• First class levers have the fulcrum between the input and output forces
• The output and input forces in a first class lever are in opposite directions
• Second class levers have the output force between the fulcrum and the input force and the fulcrum is at the very end
• Third class levers have the input force between the output and the fulcrum

Calculating IMA of a lever

• IMA is length of input arm/length of output arm
• If the input is 6.0 m from the fulcrum and output force is 1.5 m, then the IMA is 4
• If a meter stick is used as a lever with force at 0cm and output at 100 cm, with a fulcrum at 75 cm, then the input arm is measured at 75 cm and the output arm is 25 cm and the IMA is 3

Pulleys

• A pulley consists of a grooved wheel with a rope or cable looped around it and it is free to spin
• It can change the direction or increase the output force
• Fixed pulleys change only the direction of the force with an IMA of 1
• Moveable pulleys connect one end of the rope and is supported by the rope and the IMA is 2
• The IMA of a moveable pulley is equal to the number of support ropes

Wheel and Axle

• This simple machine consists of a shaft or axle attached to a larger disk called the wheel
• The wheel is often input, while the axle is often output
• If the input is applied to the axle, IMA = radius of the axle/radius of the wheel
• If the input is applied to the wheel, IMA = radius of the wheel/radius of the axle
• The handle of a garden tap of radius 3.0 cm connected to a shaft of radius 0.50 cm has an IMA of 6

Inclined plane

• An inclined plane is a sloping surface for moving objects, commonly known as a ramp
• It reduces the force needed but increases the distance
• IMA = length of ramp/height of ramp
• An object raised 4m by a loading ramp of 12 m has an IMA of 3

Screw

• This is an inclined plane wrapped around a rod
• There is a winding inclined plane that starts at the top and wraps around the rod called the thread
• The thread is what allows it to move smoothly

Wedge

• A wedge is an inclined plane that moves through an object or material
• It allows for an object or material to be broken apart into two or more pieces

Efficiency

• The efficiency of a machine measures the useful work compared to the work input
• Useful work is the work the machine is designed to perform
• Efficiency = useful output work (joules) x 100%/input work (joules)
• If a machine does 35 J of work with 50 J of work put in, its efficiency is 70%
• A car transforms chemical energy into kinetic, sound, light, and thermal energy, and the output work is its ability to move

Calculating Efficiency

• For an inclined plane with a 500 N crate is moved up a 5.0 m long ramp where the person pushes with a force of 400 N to raise the crate vertically by 2.0 m: Work = Force x Distance
• Work = (500)(2.0) = 1000J (Work done by the crate)
• Work= (400)(5.0) = 2000 J (Work done by person)
• Efficiency = (1000/2000)(100) = 50 percent, and the rest is friction

Wedge Efficiency

• Efficiency = (W out/W in) x 100%
• If a force of 900 N pushes a wedge 0.10 m into a log, the work done is 90J, so W = (900N)(0.10m)= 90 J
• If the work done on the log is 50, then the efficiency is (50/90)(100)or 55.5%