Science 8: Unit D: Science 8: Unit D: Mechanical SystemsMechanical Systems

Topic 1: Levers and Inclined Topic 1: Levers and Inclined PlanesPlanes

What is Work?What is Work?

The scientific definition of work is when a force is The scientific definition of work is when a force is done on an object causing that object to move in done on an object causing that object to move in the same direction as the force. If there is no the same direction as the force. If there is no movement, then no work is being done, no movement, then no work is being done, no matter how much force is being used. matter how much force is being used.

Example1: When you jump straight up, your leg Example1: When you jump straight up, your leg muscles do work in lifting you up, as you move muscles do work in lifting you up, as you move back down to the ground, the Earth’s gravity is back down to the ground, the Earth’s gravity is doing work in pulling you back down.doing work in pulling you back down.

Work ContinuedWork Continued

Example 2: A man runs out of gas 5 km away Example 2: A man runs out of gas 5 km away from the gas station. He tries to push the car the from the gas station. He tries to push the car the rest of the way, but cannot move it. No work is rest of the way, but cannot move it. No work is being done even though he’s pushing with force. being done even though he’s pushing with force.

The amount of work is calculated by multiplying The amount of work is calculated by multiplying the force times the distance the object moves. the force times the distance the object moves. The formula looks like this: The formula looks like this: W = F x dW = F x d

Force is measured in Newtons (N) and distance Force is measured in Newtons (N) and distance is measured in meters (m). The resulting work is measured in meters (m). The resulting work unit is called a joule (J). unit is called a joule (J).

Energy and WorkEnergy and Work

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

Work and MachinesWork and Machines

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

Work and Machines Cont’dWork and Machines Cont’d

The work you put into a machine to make it The work you put into a machine to make it work is called the input work. work is called the input work.

Work input = Force input x d input Work input = Force input x d input Work output is the work done by the machine. Work output is the work done by the machine. Work output = Force output x d outputWork output = Force output x d output According the Conservation of Energy Law According the Conservation of Energy Law

energy cannot be created or destroyed. You energy cannot be created or destroyed. You cannot get free work done in the machine. In cannot get free work done in the machine. In other words, Win = Woutother words, Win = Wout

Mechanical AdvantageMechanical Advantage

Machines are NOT designed to increase your Machines are NOT designed to increase your work, but amplify your force. The amount of work, but amplify your force. The amount of force you get compared to the amount of force force you get compared to the amount of force you input into the machine is called the you input into the machine is called the mechanical advantage.mechanical advantage.

MA = FMA = Floadload/F/Fefforteffort

Note that mechanical advantage is a simple ratio Note that mechanical advantage is a simple ratio and has no units.and has no units.

Any machine with a mechanical advantage Any machine with a mechanical advantage greater than one means that the distance you greater than one means that the distance you must move is greater than the distance the must move is greater than the distance the machine will move the load.machine will move the load.

Simple MachinesSimple Machines

Device which uses a single force over a distance Device which uses a single force over a distance to achieve work. Why use a machine to do what to achieve work. Why use a machine to do what you can do yourself? Either because you want to you can do yourself? Either because you want to increase or change the direction of your force or increase or change the direction of your force or distance that you’re moving.distance that you’re moving.

Eg. An axe is a simple machine which directs Eg. An axe is a simple machine which directs your force along a narrow edge to achieve your force along a narrow edge to achieve greater effect.greater effect.

There are traditionally six simple machines. All There are traditionally six simple machines. All other machines are combinations of these six:other machines are combinations of these six:

The Six Simple MachinesThe Six Simple Machines

The The inclined planeinclined plane or ramp or ramp The The wheel and axlewheel and axle The The leverlever The The pulleypulley The The wedgewedge The The screwscrew

I. Inclined PlaneI. Inclined Plane

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

II. LeverII. Lever

Device that is a rigid bar or plank that can Device that is a rigid bar or plank that can rotate around a fulcrum. They’re used to rotate around a fulcrum. They’re used to reduce the force needed the force needed reduce the force needed the force needed to move a very large load, but again, you to move a very large load, but again, you must move a greater distance than the must move a greater distance than the load will move.load will move.

- There are three classes of levers: - There are three classes of levers:

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

MA = Effort arm/Load ArmMA = Effort arm/Load Arm