Work, Power, and Simple Machines

What is Work?  The scientific meaning of work is a force acting through a distance.  Force must be in the same direction as the movement.

Work – Mathematically  Work = Force X Distance  W=FD  F = Newtons, D= Meters  W = N*m or Joules (J)

Here’s a thought.  In Greek mythology, Atlas held the world on his shoulders. Did he do any work?

Practice Problems  A force of 800 Newtons is needed to push a car across a lot. Two students push the car 40 meters. How much work is done?

Practice Problems  How much work is done in lifting a 60Kg crate a vertical distance of 10 meters?

Practice Problems  A 1000 N mountain climber scales a 100 m cliff. How much work is done?

What is Power?  The rate at which work is done.  The rate at which energy is used.

Power - Mathematically  Power = Work/Time  Power = ForcexDistance/Time  Watt is the unit for Power  1 Watt = 1 J/s

More Power  Having a power tool does not make for less work. It just does the work faster.

Practice Problems  A machine produces 4000 Joules of work in 5 seconds. How much power does the machine produce?

Practice Problems  A box that weighs 1000 Newtons is lifted a distance of 20 meters in 10 seconds. How many kilowatts of power are produced?

 Machine – anything that makes work easier to do.  They can change The size of the force needed The direction of the force needed The distance over which the force acts Work and Machines

Machines  Work input = work output  Fxd = Fxd  Small force over a large distance = large force over a small distance.

Efficiency  Don’t forget about friction  The amount of work output is always less than work input because of friction.

Simple Machines  Machines do not change the amount of work you have to do.  They simply make the work easier by applying a smaller force over a larger distance.

Simple Machines  Levers  Pulleys  Wheel and Axle  Inclined Plane  Screw  Wedge

Levers  Parts of a lever Fulcrum – point at which the lever pivots Effort arm – where the force is applied to the lever Resistance arm – where the mass is being lifted.

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Three Classes of Levers Figure 13

Figure 7

Pulleys

Block and Tackle - a combination of fixed and movable pulleys

Wheel and Axle

Section 14.4

Screw

Wedge