DAY 41

#Goals: SWBAT...

1. Understand the relationship between work and power

2. Solve basic work & power problems

Warm-Up (4min): Power

1. Is work a scalar or vector? What about time? What about Power?
2. Suppose that Ben Pumpiniron elevates his 80-kg body up the 2.0-meter stairwell in 1.8 seconds. If this were the case, then we could calculate Ben's power rating.
3. It can be assumed that Ben must apply an 784-Newton downward force upon the stairs to elevate his body. By so doing, the stairs would push upward on Ben's body with just enough force to lift his body up the stairs. It can also be assumed that the angle between the force of the stairs on Ben and Ben's displacement is 0 degrees.
4. Find Ben's Power Rating

CLASSWORK

1. #041A: Power Notes

Power

• Definition: Power = the rate at which work is done
• Equation: P=W/t or Work divided by the change in time
• Unit: Watt
• 1 Horsepower = 746 Watts
• Try this: Suppose that a 40-horsepower engine could accelerate the car from 0 mi/hr to 60 mi/hr in 16 seconds. If this were the case, then a car with four times the horsepower (a 160-horsepower engine) could accelerate the same car from 0 mi/hr to 60 mi/hr in ___ seconds.
  • Another Formula for Power
    • The expression for power is work/time. And since the expression for work is force*displacement, the expression for power can be rewritten as (force*displacement)/time.

2. #041B: Practice Problems

• Let's try some problems

• Check Your Understanding
• Use your understanding of work and power to answer the following questions. When finished, click the button to view the answers.

• 6. An escalator is used to move 20 passengers every minute from the first floor of a department store to the second. The second floor is located 5.20 meters above the first floor. The average passenger's mass is 54.9 kg. Determine the power requirement of the escalator in order to move this number of passengers in this amount of time.

At Home Learning (HW)

1. Complete all 6 questions/problems from #041B

If you got stuck, ask a classmate, or contact me via the Remind App. Super stuck? Solutions are here: LINK

2. Thursday we will have a problem solving quiz. You'll have three problems to solve. Review the following:

• Friction Force problems (see day 34 & 35 & 38)
• Work: The third floor of a house is 8.00 m above street level. How much work is needed to move a 150.0-kg refrigerator to the third floor? (answer: 11,760J, or to three sigfigs, 11,800J)
• Work with angles: Shani is pushing a lawn mower with a force of 88.0 N along a handle that makes an angle of 41.0° with the horizontal. How much work is done by Shani in moving the lawn mower 1.2 km to mow the yard?

3. #041C: Thursday we will revisit TME, with a focus on internal/external forces: Watch/take notes/complete questions on the following:

• Internal and External Forces - Answer the questions for the video linked below:

1. When the bowling ball falls, is the TME lost, or converted?
2. List the internal forces:
3. An external force cannot change the TME of the system. True or False?
4. Friction is one example of an external force. Does friction add energy to the system, or take it away?
5. List as many external forces as you can. You should have at least 5.
6. Video Link --> CLICK

4. EXTRA CREDIT: For credit, you'll need to show all work

• A. A wagon is pulled by a force of 38.0 N exerted on the handle at an angle of 42.0° with the horizontal. If the wagon is pulled in a circle of radius 20.0 m, how much work is done?
• B. A 120-kg lawn tractor, shown in Figure 10-17 (pg 279 --> LINK), goes up a 21° incline that is 12.0 m long in 2.5 s. Calculate the power that is developed by the tractor.