Chapter 5 Machines and Mechanical Systems

Forces in Machines How do you move something that is too heavy to carry? How were the pyramids built? Simple machines are devices that make work easier A complex machine, like a bicycle, is many simple machines that work together to perform a task. This is also called a mechanical system

Machines, Cont. A machine is a device that makes doing work easier – they can do this in 3 ways Increase the force being applied – screwdriver Increase the force being applied – screwdriver Increasing the distance over which the force is applied – ramp Increasing the distance over which the force is applied – ramp Changing the direction of the applied force - pulley Changing the direction of the applied force - pulley

Machines Every machine has a input force and an output force The input force is what is put in to make the machines work (effort force) The output force is what the machine does (resistance force)

Machines and energy Remember – Law of conservation of energy A machine can never put out more work than goes in W out is never greater than W in Most of the time energy is actually lost due to friction Ideal Machine - W out = W in

Mechanical Advantage Number of times larger the output (resistance) force is compared to the input (effort) force How much the machine multiples the force MA= F r /F e Can also be calculated with distance MA= d e /d r No units

Simple Machines An un-powered mechanical device, like a lever Other examples: Pulley, Wheel and Axle, Gears, Inclined Plane, Screw, Wedge

More Pictures

Lever Made up of the effort arm, the resistance arm, and the fulcrum Examples: crowbar, hammer, scissors, see-saw, human arm, pliers, nutcracker, can opener The arms rotate around the fulcrum Three types, 1 st class, 2 nd class and 3 rd class MA= Effort arm/Resistance arm

Classes of Levers

First class- load  fulcrum  effort Second class- fulcrum  load  effort Third class- load  effort  fulcrum

Pulleys A pulley system is also known as a block and tackle Pulleys use tension (pulling) force (not pushing force– have you ever tried to push anything with a rope?) to make work easier The more ropes you have, the less input (effort) force you have to put in MA of a pulley is the number of ropes (don’t count the rope you pull unless it goes against gravity)

Pulleys Three types of pulley Fixed – attached to something that does not move (flag pole) Moveable – one end of the rope is attached to something and the wheel is free to move (zip line) Block and tackle – a system of pulleys made up of fixed and moveable pulleys

Fixed Pulleys A fixed pulley is attached to something that doesn't move, such as a ceiling or wall. Because a fixed pulley changes only the direction of force, the IMA is 1.

Moveable Pulleys A pulley in which one end of the rope is fixed and the wheel is free to move is called a movable pulley. Unlike a fixed pulley, a movable pulley does multiply force. With a movable pulley, the attached side of the rope supports half of the 4-N weight. You have to apply a 2-N force to lift the weight. The output force exerted on the weight is 4 N, and the applied input force is 2 N. Therefore the IMA of the movable pulley is 2.

Block and Tackle A system of pulleys consisting of fixed and movable pulleys is called a block and tackle. The IMA of a pulley system is equal to the number of rope segments that support the weight. The block and tackle shown has a IMA of 4.

Pulleys Fixed Pulley Block and Tackle Movable Pulley

Wheel and Axle Simple machine consisting of a shaft of axle attached to the center of a larger wheel so that the wheel and axle rotate together Doorknobs, faucet handles are examples Actually a modified lever MA= radius of the wheel in meters/radius of the axle in meters

Wheel and Axle

Inclined Plane A sloping surface that reduces the force required to do work Ex, ramp, winding mountain road MA= Length of slope/ heigth of slope

The Screw Inclined plane wrapped around a cylindrical post Treads form a tiny ramp all the way to the tip Jar cap, screw The IMA of a screw is related to the spacing of the threads. The IMA is larger if the threads are closer together. However, if the IMA is larger, more turns of the screw are needed to drive it into some material.

The Wedge Inclined plane with two sloping sides Downwards force turns into horizontal force, splitting an object Knife, axe

Compound Machines Several Simple machines that work together Can opener had a wheel and axle, lever and wedge

Work The transfer of energy when a force makes an object move W= F x D Work is in Joules because it is a transfer of energy (Joules) Work is only done if the Force and the distance are in the SAME direction

Calculating Work W= F X d W is work in Joules F is force applied in newtons d is distance in meters