DAILY QUESTION January 26, What is the SI unit for work?

Agenda1/26/09 Daily Question Chapter 12 Section 1 Notes –BrainPop – “Work” and “Power” Assignments: Review Worksheet due 1/27

Work Is when force causes a change in the position or the motion of an object in the direction of the applied force Work = force (N) x distance (m) SI unit for work: Joules (J)

Work Practice Problem: A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder? W = f x dw = 5200 N x 25 m w = J

Power The rate at which work is done Power = work ÷ time SI unit for Power is: watts (W)

Power Practice Problem: While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts? P = W ÷ tP = 3960 J ÷ 60 s P = 66.0 Watts

Machines Help us do work by redistributing the work that we put into them They: –Multiply forces –Redirect forces

Machines They allow the same amount of work to be done by either: –Decreasing the distance while increasing the force –Decreasing the force while increasing the distance Ramp

Mechanical Advantage The ratio between the output force and the input force OR the ratio between the input distance and the output distance When greater than 1 – multiplies the input force –Car Jack When less than 1 – increases the distance and speed –Baseball Bat