Science 8: Unit D: Mechanical Systems Topic 1: Levers and Inclined Planes

What is Work?  The scientific definition of work is when a force is done on an object causing that object to move in the same direction as the force. If there is no movement, then no work is being done, no matter how much force is being used.  Example1: When you jump straight up, your leg muscles do work in lifting you up, as you move back down to the ground, the Earth’s gravity is doing work in pulling you back down.

Work Continued  Example 2: A man runs out of gas 5 km away from the gas station. He tries to push the car the rest of the way, but cannot move it. No work is being done even though he’s pushing with force.  The amount of work is calculated by multiplying the force times the distance the object moves. The formula looks like this: W = F x d  Force is measured in Newtons (N) and distance is measured in meters (m). The resulting work unit is called a joule (J).

Energy and Work Work is done when there is a transfer of energy and movement occurs. Energy provides the force needed to make an object move. A machine transfers energy from its source to the object, causing the object to move. A bicycle transfers the energy from your moving legs to the energy of moving bicycle tires.

Work and Machines  The job of MOST machines is to decrease the amount of force you have to use to perform a specific task. A car jack allows you to lift up a car without you exerting a large amount of force to do it. The amount of work to do a job never changes whether you use a machine or not. So if force decreases and work stays the same that would mean that distance increases which it does. When you use a car-jack you have to move your arm a greater distance than if you lifted up the car straight up with your arms.

Work and Machines Cont’d  The work you put into a machine to make it work is called the input work.  Work input = Force input x d input  Work output is the work done by the machine.  Work output = Force output x d output  According the Conservation of Energy Law energy cannot be created or destroyed. You cannot get free work done in the machine. In other words, Win = Wout

Mechanical Advantage  Machines are NOT designed to increase your work, but amplify your force. The amount of force you get compared to the amount of force you input into the machine is called the mechanical advantage.  MA = F load /F effort  Note that mechanical advantage is a simple ratio and has no units.  Any machine with a mechanical advantage greater than one means that the distance you must move is greater than the distance the machine will move the load.

Simple Machines  Device which uses a single force over a distance to achieve work. Why use a machine to do what you can do yourself? Either because you want to increase or change the direction of your force or distance that you’re moving.  Eg. An axe is a simple machine which directs your force along a narrow edge to achieve greater effect.  There are traditionally six simple machines. All other machines are combinations of these six:

The Six Simple Machines  The inclined plane or ramp inclined planeinclined plane  The wheel and axle wheel and axlewheel and axle  The lever lever  The pulley pulley  The wedge wedge  The screw screw

I. Inclined Plane  Device which makes it easier to elevate a load. The downside is that you must move the load a greater distance.

II. Lever  Device that is a rigid bar or plank that can rotate around a fulcrum. They’re used to reduce the force needed the force needed to move a very large load, but again, you must move a greater distance than the load will move.  - There are three classes of levers:

 Measuring forces is hard to do with normal instruments. It’s much easier to calculate mechanical advantage of a lever by measuring the lever and effort arms. To calculate the mechanical advantage of a lever, we can also use the following formula:  MA = Effort arm/Load Arm