Work and Machines Chapter 5

Work and Machines Imagine life without machines…no cell phones, no T.V.s, no cars, no appliances. Life would be much harder. Tasks would take more time. Some things would become impossible. In chapter 5 you will learn how scientists measure work and how machines make life easier.

Scientific Meaning of Work Work is another word for labor. Work can be fun, challenging, boring, or easy. To scientists work is what happens when an object changes its position by moving in the direction of the force that is being applied (Remember, a force is a push or pull). Trying to move a very heavy box.

Scientific Meaning of Work Failing to move/lift a very heavy box for an hour does not meet the scientific definition of work. No work is done, the box did not move. But, if you rolled a ball down a ramp, work was done. The ball changed its direction due to the force of gravity.

Measuring Work You can start to measure work by measuring how much force is used to do the work. In the metric system force is measured in Newtons. Example: Spring Scale

Measuring Work To measure work, you must also measure the distance (in meters) through which the force acted. Use the following formula to calculate how much work was done: Work = Force x Distance Your answer will be in newton-meters or joules. A joule is the metric unit of work.

Example Suppose a women is pushing a bike. She uses a force of 2 newtons and pushes the bike a distance of 10 meters. How much work did she do? Remember: Work = Force x Distance

Measuring Work Because force, distance, and work are always related, you can calculate any one of them if you know the other two. Example #1 Example #2

Using Levers Chapter 5: Lesson 2

Simple Machines Have you ever tried to open a can of paint with your fingers? What else could you use? Examples: These options are examples of simple machines. Simple machine: a tool with few parts that makes it easier or possible to do work. Simple machines change the direction or size of force you apply or they change the distance through which the force acts.

Using Levers A lever is a simple machine Levers can have many shapes. In basic form a lever is a bar that is free to turn around a fixed point. The fixed point is called a fulcrum.

The Lever Example of a person using a lever to move a boulder. The force that the person applies to the machine is called effort force. The object to be lifted is called resistance. The force the machine uses to move the resistance is called resistance force. The force that the machine exerts is greater than the force that the person exerts, The lever makes moving the boulder easier by taking the amount of force the person exerts and increases that force.

The Lever Label the Example:

Three Classes of Levers First Class: The fulcrum is positioned between the effort and the resistance. A first class lever changes the direction of a force and can also increase the force.

The Three Classes of Levers Second Class: The resistance is positioned between the effort and the fulcrum. Second class levers always increase the force applied to them. They do not change the direction of the force (wheelbarrows, paper cutter. nutcrackers).

The Three Classes of Levers Third Class: Increase the distance through which force moves which causes the resistance to move farther and faster (broom). You use effort force on the handle between the fulcrum and the resistance force. When you move the handle of the broom a short distance, the brush end moves a greater distance.

Work and Energy Energy cannot be created or destroyed. Because energy is the ability to do work, work cannot be created either. No simple machine can do more work than the person using it supplies. What machines can do is increase the direction of the force a person exerts.

Mechanical Advantage People often use simple machines to make tasks easier. Simple machines make tasks easier because it multiplies the force a person applies. The number of times a machine multiplies your effort force is called the mechanical advantage of the machine.

Mechanical Advantage Suppose a machine lifts a resistance that weighs 30 newtons when a person applies an effort force of 10 newtons. What is the machine’s mechanical advantage? Use the following formula to calculate a machine’s mechanical advantage:

Other Kinds of Simple Machines Chapter 5: Lesson 3

Six Types of Simple Machines Lever Pulley Inclined Plane Screw Wedge Wheel and Axle

The Pulley A pulley is a wheel with a rope, chain, or belt around it. There is a price to pay for making the objects easier to lift. You must pull twice as far on the rope as the object actually moves. Pulleys can be combined in different ways. Single fixed pulley: changes the direction of the force you apply, but does not multiply that force. You lift a heavy object by pulling down instead of up.

Single Fixed Pulley

The Pulley Movable pulley: as effort is applied the entire object attached to it will rise. You can use this type of pulley to make a lifting job easier. Because the rope supports the pulley from two directions, you need to apply only half as much force to lift the object.

Movable Pulley

The Inclined Plane An inclined plane is a simple machine made of a ramp. It has no moving parts and is used to lift an object. Inclined planes decrease the force you need to move an object. You pay for this decrease in effort force by an increase in the distance the object has to be moved.

Inclined Plane

The Screw A screw is a form of the inclined plane. Think of a screw as a straight piece of metal with an inclined plane wrapped in a spiral around t. The ridges formed by this spiral are called threads. Screws make it easier to fasten objects together. The mechanical advantage of a screw depends on the distance between the threads. The smaller the distance, the more times the inclined plane is wrapped around, making the mechanical advantage greater.

The Screw

The Wedge A wedge is an inclined plane that moves when its used. It is thick at one end and thinner at the other (wood splitting). A wedge is often made up of two inclined planes joined together. Both edges are slanted. A force applied to the thick end is multiplied and acts at the thin end, piercing the wood. The thinner and more gradual the wedge, the greater the mechanical advantage.

The Wedge

The Wheel and Axle A wheel is attached to a shaft called an axle (automobile steering wheel and doorknobs). A wheel and axle increases the twisting force you apply to the wheel. The multiplied force can then turn something else attached to the axle. The mechanical advantage of a wheel and axle depends on the size of the wheel compared to the thickness of the axle. The bigger the wheel is in comparison to the thickness of the axle, the greater the mechanical advantage.

The Wheel and Axle