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Mechanical Work Sub Unit 2.1 Objectives Define work done by a force or torque in a mechanical system. Explain the relationship between work, force applied, and the distance an object moves. Solve work problems, given force and distance information in English and SI units.

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mechanical work

Mechanical Work

Sub Unit 2.1

objectives
Objectives
  • Define work done by a force or torque in a mechanical system.
  • Explain the relationship between work, force applied, and the distance an object moves.
  • Solve work problems, given force and distance information in English and SI units.
  • Explain how efficiency relates to input work and output work for a mechanical system.
  • Define radian measure of angles.
  • Explain the relationship between work, torque applied, and the angle (in radians) through which and object moves.
  • Solve work problems, given torque and angle information in English and SI units.
slide3
Work
  • Linear
    • When a force moves something a distance (like a pushing a car, moving a desk, lifting weights, etc.)
  • Rotational
    • When a torque causes rotational movement (like gears, pulleys,
    • wheels, etc.)
work done by a force
Work done by a force

Work (W) = Force (F) x Distance (d)

Work UnitsEnglishfoot • pounds (ft•lb)

SInewton • meters (N•m)

Force UnitsEnglishSI pounds newtons

Distance UnitsEnglishSI feet meters

=

x

1 Newton meter (N•m) = 1 Joule (J)

slide5
Work
  • Uses magnitude of force and displacement (both are vectors)
  • Can be positive (if both force and displacement are in same direction)
  • Can be negative (if force and displacement are in opposite direction)
change in potential kinetic energy
Change in Potential / Kinetic Energy?
  • Work equals change in energy
    • Like when the barbell gains gravitational potential energy due to the work done to lift it.
    • Or when braking, the kinetic energy of vehicle decreases due to the work done by the brakes.
efficiency

output work

Efficiency

=

input work

Efficiency
  • Machines convert work input to useful work output.
  • Theoretically, “work in” equals “work out.”
  • Realistically, “work in” is always greater than “work out.” Why? – Losses

To get percentage, multiply by 100%

rotational work

Arc length

 =

r

“Rotational Work”
  • Work done by a torque

“” must be in radians

Work (W) = torque () x angle moved ()

 = F x l

1 rev = 360º = 2 rad

summary
Summary
  • Mechanical systems use force and torque to cause desired movement and do useful work.
  • Work is done when a force or torque moves an object. Work is done only while the force or torque is applied in the direction of movement.
  • Work equals force times displacement or torque times angle. Work is measured in ft•lb or N•m (J= N•m). W=Fd; W= 
  • The displacement used to calculate work is the distance the object moves while the force is applied.
  • Efficiency describes how well a machine performs work. Efficiency is the ration of output work to input work (Eff = Wout / Win)
  • Angles can be measured in either radians or degrees. The radian is a dimensionless unit and is used in most calculations involving angles.