Work, Power, and Mechanical Advantage Review

LG 1: SWBAT define work and calculate the work done on an object.

d W F Joules Newtons meters Amount of energy used by person or machine How much force is applied to the object The distance for which the force is applied to the object W F d

Example 1 Krystal lifted a box with a force of 50 N. She lifted the box 2 m. How much work did she do?

Answer Force = 50 N distance = 2 m Work = Force x distance W = 50 N x 2 m W = 100 Joules (100 J) Krystal did 100 Joules of work.

Example 2 600 N  600 N  10 m  A rock climber weighs 600 N. (The force of gravity acting on him is 600 N downward.) To make himself move up on a rock, he needs to push up with 600 N of force to overcome the force of gravity. If he pushes himself 10 m up the rock, how much work has he done?

Answer 600 N  600 N  10 m  Force = 600N distance = 10m Work = Force x distance W = 600N x 10m W = 6000 Joules (6000 J) The climber does 6000 Joules of work during this climb.

Check For Understanding Which of the following is the best example of work being done on an object? Holding a 50 kg barbell Lifting a bag of groceries Keeping a board in place Pushing a car that will not move

LG 2: SWBAT define power and calculate the power a person or machine uses.

Joules Watts seconds How long it takes for the work to be done How quickly work is done How much work is done on the object How long it takes for the work to be done

Example 1 Starlan does 500 J of work lifting a heavy box into a truck. It takes him 10 seconds to lift the box. How much power did Starlan use?

Answer Work = 500 J time = 10 s Starlan used 50 watts of power lifting the box.

Example 2 Kendrick lifts a 1000 N weight a height of 1 m in 2 seconds. Timberland lifts the same 1000 N weight a height of 1 m in 4 seconds. How much work does Kendrick do? How much power does Kendrick use? How much work does Timberland do? How much power does Timberland use?

Answer W = Fd = 1000N x 1m = 1000 J

Example 3 A lightbulb has a power of 80 watts. If the lightbulb is on for 40 seconds, how much work will the lightbulb do? (How much energy will it use?) 80 watts 40 seconds

Answer Power = 80 W time = 40 s The lightbulb would do 3200 J of work (release 3200 J of energy) in 40 seconds. 80 watts 40 seconds

Challenge Question How much power is required to lift a 30 N box a vertical distance of 20 m if this work is accomplished in 40 seconds?

Check For Understanding A crane lifts two 100 kg loads to the top of a building. The first load is lifted in 10 seconds and the second load is lifted in 15 seconds. Which statement below is true? The crane does more work and expends more power for the first lift. The crane does the same work and expends the same power for each lift. The crane does the same work for each lift but expends more power for the first lift. The crane does more work in the second lift but expends the same power for each lift.