# Physical Science B – Lesson #7 (2-5-2020)

### Topic:

Introduction to Physical Science B!

### Unit:

Physical Science B – Unit 7 (Mr. Tyler Version)

### Objectives:

1. The students will be able to calculate work, force, and distance using the formula Work=Force X Distance;
2. The students will be able to calculate power, work, and time using the formula Power=Work/Time;
3. The students will be able to explain the relationship between Work & Power.

### Prior Knowledge:

The students should have prior knowledge of:

• formula: Weight (newtons) = Mass x Acceleration due to gravity
• Mass (kg) = Weight(lbs)/2.2
• Definitions: Force, Weight, Distance, Time, Gravity
• Units: Acceleration due to gravity on Earth (9.8 N/kg), Weight (newtons), Mass (kg), Distance (meters), Time (seconds)

### Guiding Questions:

1. What is work?
2. What happens to work when distance equals zero?
3. What happened to the amount of work done after you climbed the stairs twice, each time at a different speed?
4. How do you determine your weight in newtons?
5. What is power?
6. Why does power have a time unit?
7. Can you use more than one formula to find “work”?
8. How are work and power related?

### Agenda:

• Unit 6 Recap. How are we on everything? Anything that needs to be done in the unit?

• Unit 6 Test. – 40 Questions

### Task #3 (Unit 7 Intro)

What is Work?

In classical physics terms, you do work on an object when you exert a force on the object causing it to move some distance. The amount of work you actually do may have little relationship to the amount of effort you apply. For example, if you push on a car stuck in a snow drift, you may exert a lot of force (and effort) but if the car does not budge, you have not done any work! In order for work to be done on an object, the object must move some distance as a result of the force you apply. There are also constraints on the force you apply. Only force exerted in the same direction as the movement of the object result in work. You may think that you do a lot of work if you carry an arm full of books from home to school. In reality you do no work at all! In carrying the stack of books, you exert an upward force to hold the books so they don’t fall to the ground. There is no movement associated with this force. As you walk, the motion of the books is horizontal not vertical. Since the force applied to the books is vertical, and the motion is horizontal, you do not do any work on the books.

Work is a transfer of energy so work is done on an object when you transfer energy to that object. The amount of work done on an object depends on the amount of force exerted on the object and the amount of distance the object moves.

Work = Force x Distance

According to Newton’s Second Law of Motion, the net force on an object is dependent on the mass of the object, and its acceleration during the movement.

Force = Mass x Acceleration

The common unit of force is the Newton (N). One Newton is the force required to accelerate one kilogram of mass at 1 meter per second per second.

1 N = 1kg m/s2

The amount of work done to push a 10,000 N car a distance of 10 meters would be

10,000 N x 10 m = 100,000 N m or 100,000 J

The Newton-meters are termed joules (J). The joule is named after James Prescott Joule (1818-1889) who first calculated the amount of electrical work needed to produce a unit of heat. In his experiments, Joule discovered that the same amount of heat was produced by the same amount of either electrical or mechanical work (“the mechanical equivalent of heat”).

• I will be putting these definitions and formulas of work and power on the smart board/board:
• Work – The transfer of energy that results from applying a force over a distance
• Work = Force X Distance
• Power – The rate of doing work.
• Power = Work/Time

We will then begin practicing using these problems. I will also be handing out Unit 7 today.

### SAMPLE QUESTIONS:

Sample problem 1:

A person pushes a shopping cart with a force of 9.0 newtons in a straight line for 12 meters. How much work was done?

Sample problem 2 :

If the person in the previous question pushed the cart for 6.0 seconds, how much power was generated?

Sample problem 3: How far can you push a sled if you exert a force of 7.0 newtons in the same direction, and the amount of work on it is 56 joules?

Sample problem 4: How much time did it take a person that did 50 joules of work pushing a box to generate 10 watts of power?

Sample Problem 5:

How much power is generated by a 60.kg person climbing a flight of stairs 15. meters high for 5.0 seconds?

### Guided Practice:

You will answer 5 work and power word problems using work and power equations.

1. How much work does an elephant do while moving a wagon 40 meters with a pulling force of 400N?
2. If it takes 15 seconds for you to do 3000 J of work, what is your power output?
3. A 900 N mountain climber scales a 200 m cliff. How much work is done by the mountain climber?
4. A small motor does 8000 J of work in 20 seconds. What is the power of the motor in watts?
5. A woman runs a kilometer using a force of 250 N and a power output of 500 W. How long in minutes does it take this woman to complete 1 kilometer?

### Power Climb Lab

• Each student will be handed a worksheet with instructions to complete the activity.
• The students will be: calculating weight(force) of person climbing the stairs, measuring the vertical distance of the flight of stairs, calculating work, calculating power and sharing the results on the board.
• We will have an oral discussion (guided questions)
##### Special Materials Needed for Lab:
1. 1 flight of stairs/bleachers (5 yards distance between groups)
2. metric ruler
3. stopwatches
4. bathroom scale (lbs)
5. calculator

### Summative Assessment

Each student will complete a 10 question written assignment related to work and power:

1. What is the work formula equation that you used in class today?
2. What is the power formula equation that you used in class today?
3. John uses 50 N of force to push a shopping cart 10 meters. How much work does he do in joules?
4. Liz does 4800. J of work in climbing a set of stairs. If she does the work in 6.00 seconds, what is her power output?
5. How much work is required to pull a sled in order to generate 30 W of power for 5 seconds?
6. 1 newton-meter is equal to 1 _______ of work.
7. Power is usually measured in _______.
8. How much work does a 50 kg person on Earth do if they lift up a barbell 2 meters above the ground?
9. Thinking about the experiment you did in class today, EXPLAIN why the amount of work did not change after climbing the stairs twice at different speeds?
10. Thinking about the experiment you did in class today, EXPLAIN why the amount of power did change after climbing the stairs twice at different speeds?

### Next Generation Science Standards:

HS-PS 3-1 Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other components and energy flows in and out of the system are known.

HS-PS 3-3 Design, build and refine a device that works within given constraints to convert one form of energy to another form of energy. 