Machines, Work, and Energy

Reminder: Definition of “Work”

● Work is force exerted over a distance (W = Fd)● Unit is Joules (same as energy)● Zero if motion is not in the direction of the force● Negative if it’s in the opposite direction

Work and Energy

● Doing work transfers energy● But it might not be to the object you’re doing work on● So the object might not gain the energy● For a force at an angle only some of the force does

work

Work and Gravity

● Work done to lift an object is the same as the gain in potential energy

● Work done to overcome gravity is the same regardless of path

● Using a ramp means less force over a longer distance

Definition of power

● ...the rate at which work is done.

● P = W/t● Unit: Watt, after James

Watt, 1736 - 1819

Power Example

● If Michael and Jim both lift 100 N weights to 2 m, how much work do they each do?

● If Michael lifts in 1 second and Jim lifts in 10 seconds, what is the Power of each?

Classwork

P. 106 - 107: Do the Vocabulary, Concepts, and Problems for 4.1

This is the last section that will be on the test.

These questions also might appear on the quiz.

Simple machines

Machine: device with moving parts that work to accomplish a task.

Input: what you do to run the machine, such as pushing a lever, pulling a rope, or turning a screw.

Output: what the machine does. Such as the level lifting the load, the pulley lifting a weight, or the screw entering the material.

Simple machines

Common examples of simple machines:

Wheel and axle

Pulley

Ramp

Gears

Lever

Mechanical Advantage

MA = output force / input force

Try the practice on page 93.

Work

Machines require input work to create output work

Friction and wear can make the output work smaller than input

(It can never be greater)

We can often assume input and output work is equal

Machine Example

A force of 50 N pushes down a lever by 2 m. The other end rises by ½ meter. What is the output force?

Output Fd = Input Fd

(F)(½) = (50)(2)

(½)F = 100

F = 200 N. Try the examples on page 95.

Levers

Mechanical Advantage: input arm / output arm

diFi = doFo

di / do = Fo / Fi = MA

Try the examples on page 97.

Pulleys

Tension - the force in a rope (or chain, or similar) under load

MA = # of lines supporting the load (for simple setups like pictured

Since each line has the same tension as the input force it can lift twice as much weight

Pulleys

Since each line has the same tension as the input force it can lift twice as much weight.

To lift the load one meter both lines supporting it must shorten by _____ meter, meaning the input line must be pulled by _______ meters.

GearsGear Ratio = output teeth / input teeth

Since these are circular, distance becomes

A high GR means the input gear turns many times to make the output gear turn once.

The input requires more distance but less force.

This is a low gear in your transmission.

Higher gears use a lower GR.

Ramps

Input distance = length of the slope

Output distance = the height

diFi = mgh

Fi = mgh / di , and remember Fo = weight = mg

Fi / Fo = h / di

Flip the fractions over to get MA = di / h

Screws

Can be seen as a ramp curled around in a circle.

https://www.boltdepot.com/fastener-information/measuring/tpiandpitch.aspx

MA = circumference along the thread / pitch

Work from Book

4.1, 4.2: vocabulary, concepts, and problems starting on page 106