Physics: Machines Mr. Foxs 8 th Grade Science (Ive got nothing witty for here.)

Physics: Machines

Mr. Fox’s 8th Grade Science(I’ve got nothing witty for here.)

What is a machine?

A device that is used to make work easier. It can multiply force Change direction of force Multiply distance

Input output

Input Force – the force you exert on the handle. What you put into it.

Output Force - the force you get out of the machine. What the machine puts out.

Mechanical Advantage

A machine’s mechanical advantage is the number of times a force exerted on a machine is multiplied by the machine.

Mech Adv: Multiplying Force

Always greater than 1 for machines that multiply force!

Example: You are lifting an object with a lever. Your input force is 35N and the output force is 70N. What is the Mechanical Advantage of the lever?

Mech Adv: Multiplying Distance

Always less than 1. Because you are getting out less force, but more distance.

Example: You are putting in 20N of force, but the machine is only giving back 5N of force. What is the Mechanical Advantage?

Mech Adv: Changing direction

Always equals one. You are getting back what you put in, just in a different direction.

You hoist the main sail on the black pearl using 150N of force. The pulley is putting out 150N of force, what’s the mechanical advantage?

Efficiency of Machines

You compare the output work to the input work.

Ideal Machines

If there were no friction we would have ideal mechanical advantage.

Because there is friction, efficiency gives us the actual mechanical advantage.

The closer to 100% efficiency, the better the machine is. Nothing is lost to friction.

Homework

Practice Worksheet

Also those that were absent can make up the running lab today and TODAY ONLY!

Simple Machines

There are 6 kinds of simple machines.Each has their own use.

Inclined Plane

A flat slanted surface.It’s meant to multiply your distance.

Ideal Mech Adv = length of incline ÷ height of incline.

Ex: ramps

Wedge

A device that is thick at one end and tapers to a thin edge at the other end.

Two inclined planes put together.Meant to multiply force

Ex: Splitting maul, teeth, zipper.

Screws

An inclined plane that is wrapped around a cylinder.

Depends on how far apart the threads are. Close threads multiply distance. Far apart threads multiply force.

Levers

Lever – rigid bar that is free to pivot, or rotate, about a fixed point.

Fulcrum – the fixed point that a lever pivots

There are 3 classes of lever

Classes of Levers

First Class Lever

Fulcrum is in between the input and output forces.

Fulcrum is closer to the output force, you multiply force

Fulcrum is closer to the input force, you multiply distance.

First Class Lever

Second Class Lever

Output force is between the fulcrum and then input force.

Always multiply force!

Third Class Lever

Input force is between the fulcrum and output force.

Always multiply the distance!

Mech Adv.

Mech Adv lever = distance from fulcrum to input force ÷ distance from fulcrum to output force

Wheel and Axel

A simple machine made of two different sized circular objects.

If you turn the wheel, you multiply forceIf you turn the axel, you multiply

distance.Mech adv = radius of wheel / radius of

axel.

Pulley

Is a grooved wheel with a rope (chain, steel cable) wrapped around it.

Fixed Pulley – change direction I.M.A=1

Movable Pulley – Multiply Force I.M.A=2

Pulley System

A series of pulleys put together to multiply force and change direction.

The I.M.A of a pulley system is equal to the number of sections of the rope that support the object.

Compound Machines

Machines that use 2 or more simple machines. Gears – are wheel and axels that are all

connected in some way, either directly next to each other, or by a chain.

Homework

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