Work and Simple Machines

What is a machine?

• A machine is a device that uses energy to perform some activity. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work. A simple machine is a device that transforms the direction or magnitude of a force.

How do we know when work has been done on an object?• When a force acts upon an object to

cause a displacement of the object, it is said that work was done upon the object.

Three key ingredients to work - force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.

Mrs. Jern !Get your foot off

the brake !!!

Work

• The equation for work is: W = F x d• The units of work are Joules,J. (1 N●m = 1J)• The units of force are Newtons, N.• The units of distance is Meters, m.

A force is applied to an object. It is moved a distance of 1 m. If we apply twice the force to move the object 1 m, how much work did we do on the object?

Answer the following…

• A force of 120 N is used to lift a 10 kg box to a height of 5.0 m. How much work is done on the box?

• A child pulls a sled up a snow-covered hill. The child does 450 J of work on the sled. If the child walks 15 m up the hill, how large of a force must the child exert?

• You must exert a force of 6.0 N on a box to slide it across a table. If you do 3.0 J of work in the process, how far have you moved the box?

• How much work is done on a small car if a 3200 N force is exerted to move it 80.0 m to the side of the road?

Work done on a textbookNo work is done on a textbook when it is held at rest.

Positive work is done on a textbook when it is raised vertically at a constant velocity.

Positive work is also done on a textbook when it is raised diagonally at a constant velocity.

No work is done on a textbook when it is carried horizontally at a constant velocity.

Negative work is done on a textbook when it is lowered diagonally at a constant velocity.

Negative work is also done on a textbook when it is lowered vertically at a constant velocity.

Work• W = Fd - remember the d must be in the same direction

as the force. The force must cause motion in the direction of the force for work to be done.

• Ex: If a student pushes a wall with all of his strength - he has done no work on the wall (if the wall does not move)

• Also a student carrying a book does NO work on the book because the force and motion are NOT in the same direction.

Work done on a textbookNo work is done on a textbook when it is held at rest.

Positive work is done on a textbook when it is raised vertically at a constant velocity.

Positive work is also done on a textbook when it is raised diagonally at a constant velocity.

No work is done on a textbook when it is carried horizontally at a constant velocity.

Negative work is done on a textbook when it is lowered diagonally at a constant velocity.

Negative work is also done on a textbook when it is lowered vertically at a constant velocity.

What is a Simple

Machine?

• A simple machine has few or no moving parts.

• Simple machines make work easier

Wheels and Axles• The wheel and

axle are a simple machine

• The axle is a rod that goes through the wheel which allows the wheel to turn

• Gears are a form of wheels and axles

Pulleys

• Pulley are wheels and axles with a groove around the outside

• A pulley needs a rope, chain or belt around the groove to make it do work

Inclined Planes

• An inclined plane is a flat surface that is higher on one end

• Inclined planes make the work of moving things easier

Wedges• Two inclined

planes joined back to back.

• Wedges are used to split things.

Screws• A screw is an inclined

plane wrapped around a shaft or cylinder.

• The inclined plane allows the screw to move itself when rotated.

Levers-First Class• In a first class

lever the fulcrum is in the middle and the load and effort is on either side

• Think of a see-saw

Examples of 1st Class Levers

• Seesaw (also known as a teeter-totter)

• Crowbar • Pliers (double lever) • Scissors (double lever)

Levers-Second Class• In a second

class lever the fulcrum is at the end, with the load in the middle

• Think of a wheelbarrow

Examples of 2ndClass Levers

• Wheelbarrow • Nutcracker (double lever) • The handle of a pair of nail clippers • An oar

Levers-Third Class

• In a third class lever the fulcrum is again at the end, but the effort is in the middle

• Think of a pair of tweezers

Examples of 3rd Class Levers

• Human arm • Tongs (double lever) (where hinged at one end, the style with a central pivot is first- class) • Catapult • Any number of tools, such as a hoe or scythe • The main body of a pair of nail clippers, in which the handle exerts the incoming force.

Simple Machines

• Simple Machines can be put together in different ways to make complex machinery

Mechanical Advantage

• Basic Formula: IMA = E / R• IMA = Ideal Mechanical Advantage E = effort R = Resistance

IMA is a multiplier.  Meaning it represents how much the machine will multiply the force which is put into it.  It, therefore, has no unit and would be written like:  3.  This means that whatever amount of work is put into the machine, you will get 3 times the work out.

Effort will usually be a distance: cm, m

Resistance will also usually be a distance although it needs to be the same unit as the effort so we compare apples to apples. :)

  

IMA of a Lever

Length of effort arm Length of resistance armIMA =

IMA of a Wheel and Axle

Radius of Wheel Radius of AxleIMA =

IMA of an Incline Plane

Length of slope Height of slopeIMA =

IMA of a Pulley

Notice the number of ropes sharing the load !