Work, Power, & Machines

Work

Work is the transfer of energy when an applied force moves an object over a distance

Work is calculated by multiplying the force by the distance over which the force is applied

  • Work = force x distance

  • w = fxd

    • Work is measured in joules (J).

  • Work is done only when…

    • Force cause an object to move

    • The object moves in the direction of the applied force

Power

Power is the rate at which work is done or how much work is done in a certain amount of time

  • Power is measured in watts

    • power = work/time (p = w/t)

Machines

  • The purpose of a machine to to make work easier

  • This is done in 3 ways

    • 1. Decreasing Effort Force- the force that YOU must apply

      • Ex. Using a screwdriver

    • 2. Increasing the distance over

      which the force is applied

      • Ex. Ramp

    • 3. Changing the direction of the applied force

      • Ex. Pulley

  • There are 2 classifications of machines that we will be discussing:

Simple Machines

  • A simple machine can do work in 1 motion/movement.

  • Simple machines do NOT change the amount of work

    done but they reduce the effort force that is required to do

    the work.

  • There are SIX types of simple machines:

Inclined Plane

  • An inclined plane is a device that increases the distance by which the effort force is applied

  • An inclined plane makes it easier to raise or lower heavy objects

    • Ex. Slides, Stairs, Ramps

  • As the angle of the inclined

    plane decreases, the force

    required to raise the object

    decreases.

    • A greater force is needed to

      raise an object using this

      inclined plane, but the force is

      applied over a smaller

      distance

Although the inclined plane

requires a smaller force to

raise the object, it requires

that the force be applied

over a greater distance.

The increase in distance is

proportional to the

decrease in force.

  • A smaller force is needed

    because the angle is smaller,

    but the force must be applied

    over a greater distance

Wedge

  • A wedge is a modified inclined plane. Therefore,

    wedges also make work easier by increasing the

    distance over which a force is applied.

  • The sides of a wedge come to a point so that the

    wedge can be driven into an object.

  • A wedge is used to split objects or hold objects in place.

    • Examples: Ax, Nail, Shovel, Knife

  • Not only does the wedge

    increase the distance over

    which a force is applied, but it

    can also make work easier by

    changing the direction of a force

    so to separate objects.

When a person uses a wedge,

he applies a downward force.

In turn, the wedge applies a

horizontal force to an object.

The horizontal force splits the

object into two parts.

Screw

  • A screw is an inclined plane wrapped around a cylinder. This creates threads.

  • Since the screw is a form of an inclined plane, it also

    makes work easier by increasing the distance over

    which the force is applied.

  • A screw is used to connect objects together.

    • Examples: Screw, Jar lid, Bottle opener

  • A screw can also make work

    easier by changing the

    direction of an applied force.

    You apply a rotational force

    to vertically drive a screw

    into an object.

IMPORTANT: The more threads

on a screw, the less force

needed to drive the screw into

an object. However, you must

rotate the screw more times

to drive it into an object.

Wheel and Axle

  • A wheel and axle is simple

    machine that consists of a

    wheel attached to a rod (known

    as an axle).

  • The W&A makes work easier by

    decreasing the effort force

    applied AND increasing the

    distance that the force is

    applied over.

  • A wheel and axle is used to lift

    or move objects with less

    frictional force.

    • Example: Wagon wheels, Bike

      wheels, Car tires

  • A wheel and axel decreases

    the force you must apply to

    move an object across a

    distance but you must apply

    that force over a greater

    distance.

  • The wheel and axle rotate

    together. If force is applied to

    the wheel, the axle turns. If

    force is applied to the axle, the

    wheel turns.

  • IMPORTANT: The larger the radius

    of the wheel, the easier it is to

    use the wheel and axle to do

    work.

Lever

  • A lever is a board or bar that sits on a fixed point called a fulcrum. The level pivots or moves on the fulcrum.

  • A lever makes it easier to lift and lower objects by decreasing the effort force.

    • Examples: Hammer, Scissors, Rake, using a screwdriver to open a paint can.

  • A lever can ALSO change the direction of a force. If you

    apply a downward force to a lever, you can raise an

    object.

Pulley

  • A pulley is a wheel with a groove around its outside edge. The groove is for a rope or belt that can move around the pulley.

  • A pulley makes work easier by changing the direction of the applied force.

  • A pulley is used to lift or lower objects.

    • Examples: Water well, Window shade, Crane

  • A pulley can also decrease the force needed to raise an object. In

    doing so, the force must be applied over a greater distance.

Complex Machines

  • A complex machine (aka compound machine) consist of two or more simple machines that work together to perform a function

  • Generally, compound machines perform more complex functions than the simple examples

    • Example: Bicycle