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Investigation
Lesson – Work - Physics
LESSON PLAN – Work
Name(s):
Evelyn Baldwin
Title of Lesson:
Work
Source of Lesson:
Alex Economou
Chicago Vocational H. S.
Length of Lesson:
50 minutes
Description of the Class:
Physics – Mars Rover Curriculum
TEKS Address - Physics:
(2) Scientific processes. The student uses
critical thinking and scientific problem solving to make informed decisions.
The student is expected to:
(A) Analyze, review, and critique scientific
explanations, including hypotheses and theories, as to their strengths and
weaknesses using scientific evidence and information;
(B) Express laws symbolically and employ
mathematical procedures including vector addition and right-triangle geometry
to solve physical problems;
(5) Science
concepts. The student knows that changes occur within a physical system and
recognizes that energy and momentum are conserved. The student is expected
to:
(A) Interpret evidence for the work-energy
theorem;
Objectives: To show work (w) equals force (f) times distance (d) and the connection between work and potential energy, kinetic energy, and heat. Equipment: Single and double pulleys to produce mechanical advantages of 2 to 5, Stands and supports for such String masses from 0.1 to 2 kg Newton force scales to 5N Wooden inclined plane about 2 m long 1 wood disk or block that can support masses up to 10 kg 1 liquid crystal strip about 20 cm long 5 cm wide (the strip should have a temperature change at 25oC to 30oC) 2 beakers Ice Boiling water 1 thermoelectric converter (PASCO SCIENTIFIC, 1876 SABRE STREET, HAYWARD CALIFORNIA, 94545. 1-800-772-8700) Procedure: Set up a number of pulley assemblies depending on the number of class experimental groups each with a different mechanical advantage and mass. Determine the force needed to lift at constant speed the provided weight, the distance the weight is lifted, and the distance the force at the other end of the string moves as the weight is lifted. A data table can be constructed thus: mass of weight of distance force used distance column column object object object is to lift the force 2*3 4*5 lifted lifted is lifted object travels kg N m N m Column 6 should be equal to column 7. But column 6 is mgh and column 7 is f*d. Point out that lifting the weight is tiring, because you have done work. How does that work appear on the data table? What is constantly the same in each row? (f*d = mgh) We therefore define work done as f*d. (If f is in Newtons and d is in meters, work has units of Joules.) Mgh is also the work done, but that raised weight has the work somehow in it. Demonstrate this by dropping a weight on piece of chalk. Point out that the weight did work on the chalk, and therefore has the ability to do work when raised. We call that ability because of its position potential energy. Point out that the weight, just before it strikes the chalk has no potential energy. Since we have shown that the work we did was converted to potential energy, then it follows the potential energy was converted to something. Since the weight was moving, the new energy is energy of motion, called kinetic energy. We know that V2f-Vo2 = 2*A*S = 2*g*h. If Vo =0, then V2f = 2*g*h. Multiply both sides by m. mV2 = 2*m*g*h or mV2/2 = m*g*h. This states that mV2/2 at end is the same as the m*g*h at the start. This mathematical quantity, m*V2/2 is called kinetic energy, KE. Next set up an inclined plane so that weights piled on the wooden disc or block will not accelerate as they slide. Tape a liquid crystal strip to the plane's lower third. Demonstrate that the strip changes color with temperature by touching it. Slide the weights and support down the plane. The strip changes color showing heat was produced. Point out that the KE of the weights as it slid down the plane did not change (no change in speed) but the potential energy did. Why? If we believe that the energy at first equals the energy finally then we must add heat to make a balanced statement; m*g*h (at first) = m V2/2 (at end) + heat. Heat must be a form of energy. To show that heat can be used to make KE and work use the thermoelectric converter, one plate in hot water the other in ice water, in different beakers.
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