10.2 Work 1

Chapter 10 Objectives Calculate the mechanical advantage for a lever or rope and pulleys. Calculate the work done in joules for situations involving force and distance. Give examples of energy and transformation of energy from one form to another. Calculate potential and kinetic energy. Apply the law of energy conservation to systems involving potential and kinetic energy. 2

Chapter Vocabulary chemical energy closed system law of conservation of energy electrical energy fulcrum gears input input arm input force joule kinetic energy lever machine mechanical advantage mechanical energy mechanical system nuclear energy output output arm output force potential energy pressure energy radiant energy ramp rope and pulley screw simple machine thermal energy work

Inv 10.2 Work Investigation Key Question: What is the trade-off for multiplying forces in a machine? 4

10.2 Work In physics, work has a very specific meaning. In physics, work represents a measurable change in a system, caused by a force.

10.2 Work If you push a box with a force of one newton for a distance of one meter, you have done exactly one joule of work.

10.2 Work (force is parallel to distance) Force (N) Work (joules) W = F x d Distance (m)

10.2 Work (force at angle to distance) Force (N) Work (joules) W = Fd cos (q) Angle Distance (m)

10.2 Work done against gravity Mass (g) Height object raised (m) Work (joules) W = mgh Gravity (m/sec2)

Calculate work done against gravity A crane lifts a steel beam with a mass of 1,500 kg. Calculate how much work is done against gravity if the beam is lifted 50 meters in the air. How much time does it take to lift the beam if the motor of the crane can do 10,000 joules of work per second? You are asked for the work and time it takes to do work. You are given mass, height, and work done per second. Use: W = mgh. Solve: W = (1,500 kg) ( 9.8 N/kg) (50 m) = 735,000 J At a rate of 10,000 J/s, it takes 73.5 s to lift the beam.

10.2 Work done by a machine Work is usually done when a force is applied to a simple machine. All machines can be described in terms of input work and output work. In any machine, some of the input work goes to overcoming friction. The output work is always less than the input work because of the energy lost to friction.