Work and Simple Machines

Chapter 4

Work

•      Work occurs when a force causes an object to move in the same direction that the force is applied

–   Ex:  you push your friend forward and they move forward

–   Non example: you push your friend forward and they fall down (gravity did the work)

•      Work involves motion, not just effort

Work

•      Work is done only when the force you exert on an object is in the same direction as the object’s motion

–  Ex:  Lift a clothes basket

–  Non-example: Carrying a clothes basket

 

Work

•      When a force is exerted at an angle, only the part of the force that is in the same direction as the motion does the work

–   Ex: You push a lawnmower at a downward angle, only the force forward counts as work

Calculating Work

•      Work = force × distance

•      Force is measured in Newtons (N)

–   You push a wagon with a force of 10 N

•      Distance is measured in meters (m)

–   The distance an object moves while the force is being applied

–   You push the wagon 5 m before you let go

•      Work is measured in the Joule (J)

–   1 J = 1 N·m

–   Work = 10 N × 5 m = 50 J

 

Power

•      Power is how quickly work is done

•      Power can be calculated by using the formula:

–  Power (watt, w) = work done (J) / time (s)

•      Doing work on an object increases the object’s kinetic and potential energy

Power

•      The amount of work done is the amount of energy transferred and can be expressed in the power formula in place of work done:

–  Power = energy transferred / time

–  So if you transfer 100 J in 10 seconds, how much power did you have?

•      Power is always the rate at which energy is transferred

Using Machines

•      A machine is a device that makes doing work easier

•      Machines change the way a person does work, NOT the amount of work that needs to be done.

 

Using Machines

•      Input force – the effort, or work, force you exert on a machine

•      Output force – the resistance force, or the work a machine does to move an object over some distance

•      When using a machine, the output work can never be greater than the input work.

Mechanical Advantage

•      Mechanical advantage is the number of times the input force is multiplied by a machine

•      Calculated by the following formula:

–  Mechanical advantage = output force (N) / input force (N)

 

Mechanical Advantage

•      Some machines make work easier by allowing you to exert a smaller force over a longer distance, resulting in a mechanical advantage of more than one.

•      Other machines allow you to exert your force over a shorter distance resulting in a mechanical advantage of less than one.

•      Still other machines allow you to change the direction of input force resulting in a mechanical advantage equal to one.

Efficiency

•      Efficiency is the ability of a machine to convert input work to out work

•      Calculated with the formula:

–  Efficiency = output work (J) / input work (J) x 100%

Efficiency

•      Friction reduces efficiency by converting some work into heat

•      The efficiency of a real machine is always less than 100% because of friction

•      Oil, or another lubricant, can increase efficiency by reducing the number of contact points between surfaces

Simple Machines – Chapter 4

•      We’ll talk more about these later

•      Simple Machine

–    A machine that does work with only one movement (makes work easier)

–    There are six:

•    Inclined plane, lever, wheel and axle, screw, wedge and pulley

•      The goal is to increase the mechanical advantage

–    The amount of times easier a machine makes work

•      Compound Machine

–    A combination of simple machines – makes work even easier

Ch 4 – Simple Machines

•      Inclined Plane

–  A flat, sloped surface

–  You increase the distance over which a force is applied

•   You do the same amount of work and it takes longer, but the force required is easier

–  Which requires less force, following a winding trail up a mountain or climbing straight up?

Ch 4 – Simple Machines

•     Wedge

–  An inclined plane that moves

•     A wedge changes the direction of the applied force

–  Ex:  with a knife, you push down, and the knife pushes an apple apart

Ch. 4 – Simple Machines

•      The Screw

–  An inclined plane wrapped around a cylinder or post

•      It changes the direction of the force applied

–  Ex: When you twist a screw to the right, the screw goes into wood

Ch 4 – Simple Machines

•       Lever

–    Any rigid rod or plank that pivots, or rotates, around a point called a fulcrum

•       Three types of levers

•       First class – Applied force down on one side of fulcrum, output force up on other side

–    Ex: pliers, scissors, teeter totter

•       Second class – Applied force up on end of rod, and output force up between applied force and fulcrum

–    Ex: wheelbarrow

•       Third class – Applied force up in middle of rod, near the fulcrum, output force on end or rod opposite fulcrum

–    Ex: baseball bat, tennis racket, golf club

Ch 4 – Simple Machines

•      Wheel and Axle

–   Consists of two circular objects of different sizes that are attached in such a way that they rotate together

•      The advantage is when one wheel is bigger than the other, it requires less force to turn it.

–   Ex: in a steering wheel, it is easy to turn the big wheel, making it easier to turn the column that connects to the wheels

Ch 4 – Simple Machines

•      Pulley

–    Consists of a grooved wheel with a rope or a chain wrapped around it

•      Two types

•      Fixed Pulley

–    Just changes the direction of the force – not the amount needed

•      Movable pulley

–    Doesn’t change the direction of the force, but does change the amount needed

•      Best to use combinations of fixed and movable pulleys (block and tackle)

–    Get change in direction and change in amount of force needed

•             What is mechanical advantage?