Simple MachinesAs well as Work Efficiency and Mechanical advantage

What is a machine?A machine may be thought of as a device that makes work easier .

How do machines make work easier?

Machines make work easier by:multiplying the size of the force you exert (such as in this pulley system).

Machines also make work easier by: changing the direction of that force (such as in the gear, roller & rope assembly in these mini-blinds)

A pulley lifting a mass or a pulley on a flagpole.

A machine can also do both: multiplying the amount of the force, as well as changing the direction of the force. (such as this pry bar)

Other things machines do:Machines transfer force from one place to another. Example: A bicycle chain transfers force to the pedals.

belt driven

chain driven

feet driven

Machines can convert energy from one form to another. Example: This electric generator converts chemical energy (gasoline) to mechanical energy to electrical energy. – Not a simple machine.

Machines can multiply speed or distance.During one revolution of the bicycle’s pedals, the rim of the wheel travels faster than the pedals.

What are simple machines?

A Simple Machine is a device that does work with one movement.

When you use a pry bar to remove a lid, you are working against friction----the friction between the nails in the lid and the crate.

• When using a simple machine, you are trying to move something that resists being moved.

Applying Force & Doing Work• Two forces are involved when a machine is

used to do work.

First: Effort force (Fe ) – the force applied to the machine

Second: Resistance force (Fr) –the force applied by the machine to overcome resistance.

• The pry bar is used to multiply the effort force and open the crate. However, to gain force the effort must push through a greater effort distance than the resistance distance.

Law of Work and the Ideal Machine

• In an “Ideal Machine” is frictionless ---therefore there is no energy loss from friction and heat.

• For an ideal machine, the work input is equal to the work output.

• This is called the “Law of Work.”

Ideal Machines Equation

So in order that work input equals work output, the effort force must travel a greater distance than the resistance force.

Law of Work Calculation -Ideal Machines• A 500 N barrel is rolled up a 4.00 m ramp to a

platform 1.20m above the ground. What force must be applied to the barrel.

How much force would be used to lift the barrel to then platform without a ramp?

Since the barrel has a weight of 500 N, the effort force required to lift the barrel would be 500N. The work done on the barrel would be W = Fd (500N x 1.20m = 600N).

500 N Effort = 112 lbs

150 N Effort= 34 lbs

THE MECHANICAL ADVANTAGE:The Number of times a machine multiplies the effort force is the Mechanical Advantage (MA) of the machine.

Mechanical Advantage Calculation –for Ideal Machines

• For the barrel-ramp problem:

The ramp, therefore multiplies the effort put into moving the barrel to the platform over three times.

What about a less than ideal machine?

• In the real world, friction acts against a machine’s work output, whether it is a pulley system or a ramp.

The less friction the more efficient the machine, and the greater the work output.

Machine Efficiency Calculation

Sample Calculation• A sofa weighing 1500 N must be placed in a truck

bed 1.0 m off the ground. A mover uses a force of 500 N to push the sofa up an inclined plane that has a slope length of 4.0m. What is the efficiency of this inclined plane?