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How does it do that?. 3.2.1 Introduction to Work & Energy. Definitions. energy: ABILITY TO DO WORK Unit: JOULE work: A CHANGE IN TOTAL ENERGY requires MOTION Unit: JOULE. Equation. Example #1 – No Motion.
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How does it do that? 3.2.1 Introduction to Work & Energy
Definitions • energy: ABILITY TO DO WORK • Unit: JOULE • work: A CHANGE IN TOTAL ENERGY • requires MOTION • Unit: JOULE Equation
Example #1 – No Motion • A student is trying to push a heavy crate across the floor, but it does not move. • Is work being done ‘on the crate’? No, work is NOT being done ‘on the object’ unless it moves.
Example #1 – No Motion • But it feels to the student like energy is being used -- what’s going on? Energy IS being expended and work IS being done, just not ‘on the object’. Work is being done inside the student’s body.
Example #2 – Flat Ground • A box is pushed with a force of 100 newtons across a frictionless surface for a distance of 10 meters. • How much work is done on the box? W = Fd = ΔET W = (100N)(10m) W = 1000 J 100 N 10 m
Example #3 – Incline • A box is pulled up a 3.0 meter long plane that is inclined at 20° using a force of 10 newtons. • How much work is done in moving the box? W = Fd = ΔET W = (10N)(3.0m) W = 30 J 10 N 3.0 m 20°
Example #4 – Force on an Angle • A sled is pulled a horizontal distance of 4.0 meters across a frictionless surface using a force of 15 newtons at an angle of 30° above the horizontal. • How much work is done on the sled? W = Fd cos θ =ΔET W = (15N)(4.0m) cos(30°) W = 52 J 15 N 30° 4.0 m
