Objectives Understand the amount of force changes when different methods are used to do work. Recognize that work is done when a force moves an object a certain distance. Calculate the amount of work done. Recognize how simple machines make tasks easier.
Main Idea Simple machines allow you to do the same work more easily.
What is work?
What happens when you apply force, but nothing moves?
Newton’s 2 nd law If the object won’t move, then another equal force must be opposing your force. Look at the picture on page F16 at the bottom.
The greater the distance, the more work is done. It requires more work to lift the weight over your head than to your waist.
Measured Units Newton-meter (N.m) standard unit of force times units of distance Apply a force of 10 N to lift a book a distance of 1 m, you have done 10 N.m of work. Joule (J) Another name for newton- meter.
Simple Machines They make doing the same work easier Little to no moving parts Changes the amount of force applied, the direction of force, or both to make it easier Effort force force used to do work using a machine Load The force it overcomes
SIX Types Lever Wheel & Axle Pulley Inclined Plane Wedge Screw
LEVERS Rigid bars that pivot around a point Two forces: effort and load Effort force applied to the lever Load force that works against the lever Pivot Point (Fulcrum) move fulcrum to help with the load THREE types of levers
First-Class Levers Fulcrum between effort & load Examples: pliers, seesaws, crowbars, fingernail clippers See page F18
Second-Class Levers Load is between the fulcrum & effort force It increases the force, but doesn’t change the direction Examples: nutcracker, wheelbarrows, bottle openers
Third-Class Levers Effort is applied between the fulcrum and the load Only type of lever that ALWAYS reduces the effort force instead of increasing it Examples: chopsticks, fishing rod, shovel, stapler
Wheel, Axles, Pulleys Wheel & Axle changes the amount of force applied to an object. Examples: steer a truck, open a water valve, seal a submarine hatch Pulley wheel with a groove along its edge Types: fixed pulley; movable pulley; pulley system
Fixed Pulley Changes direction but not the size of the force needed to move an object Allows you to pull down instead of pushing up, which is much easier
Movable Pulley Reduces the amount of force needed to lift a load Pulley attaches to the load, which doubles the pulling force Drawback – need to pull twice as much rope
Pulley System Combining two or more pulleys together The more pulleys – the less force needed to lift a load Use when lifting very heavy loads.
Inclined Planes Changes the ratio of force to distance (also called a ramp) It decreases the amount of force needed to move an object, but it increases the distance the object must travel See page F22 for an example
Wedges and Screws Consists of two inclined planes back to back Changes the direction of the force Screws are inclined planes wrapped around a cylinder or cone Changes both the direction of force and the ratio of force to distance See page F23
Compound Machines Tools made of two or more simple machines Scissors is an example: it consists of two levers with the fulcrum in the middle, the cutting blades are wedges Bike – pedals are levers; wheels and axles Piano – many levers