Mechanical Work

Work Force or force-like quantity enabling something to move (change speed, direction or both) – Mechanical – force or torque makes objects move – Fluid – pressure causes fluids to move – Electrical – voltage makes charge move Technology – collection of methods used to apply energy to do USEFUL work (work with purpose) _soul-amp_8.jpg

Work formula (linear) W = force * distance moved in direction of force (w = F * d) Force applied w/o movement = no work Units are newton-meters (Nm) or Joules (J) in SI Units are pound-feet or foot- pounds in English system (lb*ft)

Work vs. no work When is work done? Weightlifter pulls on barbell but it doesn’t move Weightlifter lifts barbell to shoulder height Weightlifter pauses at shoulder Weightlifter extends barbell over head

Work Force and displacement (direction of distance covered) are vectors – Force in same direction as movement = positive direction. E.g., pushing a grocery cart down an aisle – Force in opposite direction = negative direction. E.g., friction applied to cart wheels

Torque and rotational work Work in rotational system is done when torque is applied to a lever arm to turn something Torque = force * length of lever arm that force is applied to Lever arm length is perpendicular to ( ┴ ) force direction Lever arm length is radius (length of wrench handle)  = F * d Measured in Nm or J in SI system, or ft. lb in English system

Radians Angles can be measured in degrees or radians Most measurements occur in radians (rad) 1 radian = measure of angle  when length of arc between 2 radii = length of each radius. Has no units, but often denoted as “rad” Value is 57.3 o  rad = 180 o ; 2  rad= 360 o 6.28 rad = 1 complete turn or rotation

Work in rotational systems Work in rotating systems = torque applied * angle moved through (in radians) W =  Measured in Nm or J in SI system, or ft. lb in English system

Efficiency Efficiency – ratio (fraction) comparing work put out by the machine (distance * weight lifted by machine) to work input (distance * force operator applies to get machine operating) Efficiency = Work output * 100% Work input increases-power-efficiency.php/comparison-chart.jpg

Efficiency (cont.) Efficiency also calculated using mech. advantage Actual MA = F out / F in Ideal MA = D in / D out AMA / IMA = Efficiency Work output is always less than work input because of friction

Work vs. momentum Force applied while ball is being thrown = work Motion of ball after being thrown = momentum ges/js28_shot_doctor_jim_clip_image002.jpg

Potential vs. kinetic energy Potential energy – stored because of position above surface (usually above earth’s surface) Kinetic energy – energy of motion (energy object has because it is moving) stories/harnessing-the-sun/sci-media/images/potential-and-kinetic- energy/ eng-NZ/Potential-and-kinetic-energy.jpg