Mechanical Work

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# Mechanical Work - PowerPoint PPT Presentation

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

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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.
• 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.
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 (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)

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?
• 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.

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%

Arc length

 =

r

“Rotational Work”
• Work done by a torque

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

 = F x l

1 rev = 360º = 2 rad

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.