Work and Simple Machines

Work is the use of force to move an object some

distance.Work is done only when an

object that is being pushed or pulled actually moves.

Work is done.

Work is being done because the force and motion are in the same direction

Applied Force

Direction of motion

Work is not done.

Work is not being done because there is not movement of the box.

Applied Force

No m

oti

on

Calculating Work

Work = force (N) x distance (m)

Joules= Newton x metersremember that Joules is also the unit for energy, so work must be a form of ENERGY.

Work = Energy

How much work is done if a person lifts a barbell weighing 450 N to a height of 2 m?– F= 450 N d=2 m– W=F*d– W= (450 N) *(2 m)– W= 900 N-m– W= 900 J

Power is…

• Power= Work ( or Energy) Time

• P= E t

• The unit for power is the watt (W). One Watt equals one joule per second.

An Antarctic explorer uses 6000 Joules of work to pull his sled for 60 sec. What power does he need?– Work: 6000 J time: 60 sec– P= W/t– P= 6000 J/ 60 sec– P= 100 J/sec– P= 100 W

When you do workwork on an object, you transfertransfer the energy of that object. Energy is transferredtransferred from yourself to the objectthe object. Power is equal to the amount of energyenergy transferred over a certain timetime.

A machine makes work easier by

1. Increasing the force2. Increasing the Distance3.Changing the direction of

the force

Simple Machines

What is a Simple Machine?

A simple machine is a machine that does work only

in one movement.

What is a compound machine?

A compound machine is a machine made up of two or

more simple machines.

Inclined plane—a flat, sloped surface

1. Inclined plane gets longer, the force needed to move the object is smaller.

2. The mechanical advantage= length of the inclined plane height of the inclined plane

Inclined Planes

Longer Incline Plane:________ force is needed

to move an object

Shorter Inclined Plane:________ force is needed

to move an object.

Lifting Without an Inclined Plane:

__________________force is need to move the object

Lever—any rigid rod or plank that pivots about a point.

1. the point about which the lever pivots—fulcrum

2. Mechanical advantage=distance from fulcrum to input force distance from fulcrum to output force

Levers can be divided into classes depending on the position of the fulcrum.

Wheel and axle—two circular objects of different sizes that rotate together

1.The mechanical advantage= the radius of the wheel

radius of the axle1.If the input force is applied to the axle, the

mechanical advantage is less than one, examples:: fan or ferris wheel.

2.If the input force is applied to the wheel, the mechanical advantage is almost always more than one examples:: a doorknob or steering wheel.

Pulley—grooved wheel with a rope or chain wrapped around it

1. Fixed pulleys, such as on window blinds or flagpoles, are attached to an overhead structure and change the direction of the force you exert; mechanical advantage of one.

2. Moveable pulleys are attached to the object being lifted and allow you to exert a smaller force; mechanical advantage of two.

3. Pulley system—combination of fixed and movable pulleys

Pulleys

Type of Pulley

Effect on force

Mechanical Advantage

Fixed pulley

Movable pulley

Pulley System

Inclined plane that moves—wedge; changes the

direction of the applied force; example: your front teeth

Screw—Inclined plane wrapped around a cylinder or

post; • mechanical advantage =

the length of inclined plane around screw the length of the screw.

Assignment: