Bell Work: Work Intro1. What do the F and d stand for?2. What are the units for F and d?3. What do the W and t stand for?4. What is the unit for t?5. Work is a form of energy. What is the unit for
work and energy?
Work and Power
Work Work is a force causing something to move a
distance. Work = force x distance W=f x d Units for work: Newton-meter (N-m) or
Joule Direction of motion and force must be the
same.
Work is NOT Work does not involve time. Work is not motionless.
Examples Example of work:
weight lifter pushing barbells from shoulder to over head
Two men moving 20 boxes to another room.
Example of NO work: weight lifter holding barbells over his head Two men taking a break from moving boxes
Power Power - how fast work is done. Power is the rate at which work is done, or
amount of work done per unit of time. Power = work divided by time P = W/t
Power Units Units for power: Newton-meter/second (N-
m/s) or Joule/second (J/sec) or Watt (W) Kilowatt (kW) – used to measure large
quantities of power like electrical power. 1000 W in a kW Horsepower (hp) – also a unit of power. 1 hp
= 750 watts
Examples Lifting barbell in 2 seconds Two men moving 20 boxes to another room in
10 minutes.
Machines
A machine is a device that makes work easier.
Examples: hammer, bicycle ramp, scissors, shovel, and doorknob
Machines Machines can do a variety of jobs: pump
water from a well, hoist a sail, plow a field, catch a fish.
Machines make work easier by changing the size or direction of the applied force.
Two forces involved in using a machine1. The force applied to a machine is the effort
force (FE) Example: when you pull down on the handle
of a crowbar, you are applying an effort force.
Two forces…2. The force applied by the machine is the
resistance force (FR) Example: Motor turns fan
Input Work input (WI) – work done on a machine WI = FE x dE
Example: work input of the crowbar (WI) is equal to the force you apply to the handle (FE) times the distance the handle moves (dE)
Output Work Output –Work done by a machine . WO = FR x dR
Example: The work output of the crowbar (WO) is equal to the force the crowbar exerts on the nail (FR ) times the distance the nail moves (dR).
What do machines do?? Machines make work easier, but do not
multiply work. So, work output can never be greater than
work input
Mechanical Advantage The number of times a machine multiplies
the effort force is called the mechanical advantage (MA)
MA = FR/FE
Example: if a crowbar allows you to exert only 20 newtons of force to raise a 200 newton object, its mechanical advantage is 10.
Efficiency The comparison of work output to work
input is called the efficiency of a machine. Efficiency = WO/WI x 100 High efficiency means that much of the
work input is changed to useful work output. Low efficiency means that much of the work
input is lost and a great deal of useful work output does not result.
Simple MachinesI. Inclined Plane – a slanted surface used to
raise an object. Example: a ramp Wedges and Screws – wedge is an inclined
plane that moves. Have an edge that is thinner at one end. Examples: knife, ax, wedge
A screw is also an inclined plane. It is an inclined plane wrapped around a cylinder to form a spiral.
Simple MachinesII. Levers – a bar that is free to pivot a fixed
point when an effort force is applied. The fixed point of the pivot is called the fulcrum.
Levers****Three classes of levers: Fulcrum is between the effort force and the
resistance force. Example: crowbar, pliers, scissors and seesaws
Resistance force is between fulcrum and effort force. Examples: wheelbarrows, doors
Effort force is greater than the resistance force and the fulcrum
Simple machinesIII. Pulleys – a chain, belt, or rope wrapped
around a wheel. A pulley can change either the direction or the amount of an effort force.
IV. Wheel and Axle – a lever that rotates in a circle. Made of 2 wheels of different sizes. Examples: bicycles, Ferris wheels, water wheels and gears.
