Physics with Technology Work Push as hard as you can on the wall. Ask: Am I working? Did I do any work? Push a piece of paper across the demo table. Ask: Am I working? I am using less force but did I do any work? Physics with Technology

Force applied across a distance. Work Force applied across a distance.

Work Lab Remind them that they should not ride the cart as roller skates. They will loose points. (“You see a roller skate, I see a cart/lab equipment.”)

Work Equation W = Fd (N)(m) = Nm Newton meter 1 Nm = 1 J (Work and energy are like sisters. It takes energy to do the work. Work is what you do. Energy is what you use).

Work If you take a short steep path then: If you take a long gradual path then: The DISTANCE is small… But the FORCE is huge! The FORCE is small… But the DISTANCE is long.

The total work (if gravity is involved) is always equal to: Wh mg

Practice Bob pushes a crate with 50 N of force for 6 meters. How much work did he do?

Practice Bob did 240 J of work yesterday when he pushed a heavy box across the floor. If he pushed it 3 meters, then how much did friction resist?

Practice Climbing in Tarzan’s tree house is hard work! Literally. It takes 1300 J of work to get up there. If he weighs 650 N, then how high is the tree house?

RAMPS A simple machine that makes work easier by extending the distance in order to decrease the force.

Mechanical Advantage: Force without machine Force with machine 100 N If we do not use a machine 20 N If we do use a machine (any device) = 5

Bob is trying to get his 210 kg four wheeler up into the back of his pick up truck. The bed of his truck is 1.5 meters above the ground. How much energy will it take to get the wheeler into the truck? If Bob just lifts it straight up like superman, how much force will it require? Bob isn’t this strong. He decides to use a ramp. It is twelve meters long. How much force will it take this time? What is the mechanical advantage of the ramp? e. What if Bob used a ramp that was 20 meters long?

Betty (65 kg) and Babby (65 kg) decide to race each other to the top of a 240 meter tall building. Betty is going to climb straight up on a rope, and Babby is going to run up the stairs. The total length of the stairs is 960 meters. How much energy will it take for either girl to get to the top of the building? How much force will Betty’s arms need to provide? How much force will Babby’s legs need to provide? Even though Babby is taking a much longer route, she is likely to win the race. Why?

It turns out that friction can do work too   It turns out that friction can do work too. We call it negative work because it is NOT helping! We use the same equation. If the force of friction on a box is 35 Newtons, and you slide the box 13 meters across the floor, how much work did friction do?

Total Work ∑W = Positive Work – Negative Work If a force HELPS displace the object it does positive work. If a force RESISTS displacement it does Negative Work..

Bob pushes a box straight forward with 210 N of force for 12 meters Bob pushes a box straight forward with 210 N of force for 12 meters. Meanwhile, friction resists with 45 N of force. 1. How much work did Bob do?   2. How much work did friction do? 3. How much work was done total?

Efficiency: How do we calculate the efficiency of a machine? Formula: Efficiency = Work out x 100 % Work in Work in = Work out = Work I did = Fd Work accomplished = Wh = mgh

Practice: Bob pushes a 4 kg crate up a ramp that is 2 meters long. He pushes with 40 N of force to a height of 1.9 meters. Calculate the: Work In: Work Out: Efficiency:

Practice: Gertrude pulls a bucket of 2.3 kg water up out of a well 6 meters. She must apply 25N of force on the rope to accomplish the task. Calculate the following: Work In: Work Out:   Efficiency:

Skills Test Skills Test 6 Have them put everything away and then demo the whole thing for them. Make copies of the skills test and use the document camera to show how you would do it by writing on the copies. Skills Test 6