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Content Teaching Academy 2014 James Madison University Intermediate Physics-Graduate Credit Lesson Plan 1 Lesson Plan Teacher Name: Ms. Kristen Roscoe Unit: 5-Net Force Lesson Title: Elevator Lab/Introduction to Net Force Instructional Goals: The amount by which the forces acting on an object are unbalanced is called the net force. When the forces acting on an object are unbalanced, the object will accelerate. Because acceleration is a change in velocity, and velocity includes both speed and direction, a net force will change the speed and/or the direction of an object's motion. Newton's 2nd Law: The acceleration of an object is directly proportional to the net force acting on it F and inversely proportional to its mass, a µ . For convenience, this is usually m expressed as Fnet = ma. Force is measured in units of Newtons. One Newton of net force on a onekilogram object produces an acceleration of 1 m/s2. Therefore, a Newton is the same as a kilogram*meter/second2. (N = kgm/s2) Solve quantitative problems involving forces, mass and acceleration using Newton's 2nd Law. a. use force diagram analysis to find the net (unbalanced) amount of force. b. list known and unknown force and motion variables Virginia Standards of Learning: PH.5 The student will investigate and understand the interrelationships among mass, distance, force, and time through mathematical and experimental processes. Key concepts include a) linear motion; b) uniform circular motion; c) projectile motion; d) Newton’s laws of motion; e) gravitation; f) planetary motion; and g) work, power, and energy. Objectives: SWBAT Draw the force diagram for a person on a scale in an elevator Label the direction of the net force and the acceleration on an object under different scenarios Calculate the net force and the acceleration on a person while the elevator moves upward Calculate the net force and the acceleration on a person while the elevator moves downward Calculate the net force and the acceleration on a person while the elevator is at rest. Evaluation: Elevator Lab and Worksheet 2 Materials: Scale Elevator Lab Handout Whiteboards Markers Erasers SMART Notebook Worksheet 2 Procedures: Hand out elevator lab sheet and complete: Students will complete an introductory lab on Newton’s second law concepts and problems. Students will analyze changes to the force of gravity on an object as an elevator travels up and down. Students will answer preliminary lab questions then in small groups complete the lab. Whiteboard elevator lab: Once a group has completed the lab, they will answer all of the questions and then whiteboard their results. Once all groups are finished, we will have a whiteboard discussion as a class. Notes Newton’s Second Law: Students will complete notes on Newton’s second law, which states that net force on an object is equal to the mass of the object multiplied by the acceleration of the object. The net force comes from the sum of all of the forces acting on the object in all directions. In the last unit, the students focused on balanced forces, where the net force on the object was zero meaning that the object was either moving at a constant speed or was at rest. Now, the students will focus on objects that are experiencing unbalanced forces. We will use the results from the elevator lab Homework-Worksheet 2: For homework, students will complete worksheet 2 on Newton’s 2nd Law problems. Assignments: Worksheet 2 Name Date Pd Net Force Particle Model: Elevator Lab In today's activity you will look at how the forces change on you as you ride in an elevator. Prep: Read the following questions before going over to the elevator, and note the observations you need to make: 1. At what times in the motion of the elevator does the bathroom scale show your weight (or the weight of the person standing on the scale)? Make a mental note of the scale reading. 2. At what times in the motion does the scale show a value greater than your weight? Make a mental note of the scale reading. 3. At what times in the motion does the scale show a value less than your weight? Make a mental note of the scale reading. Experiment: Take the bathroom scales with you and take a ride in the elevator. Have someone stand on the bathroom scale that doesn't mind revealing his or her weight to others. Observe and remember the forces for numbers 1, 2 and 3 above. Data/Observations: When you return to the classroom record your observations. Force in pounds Force in Newtons (1 pound = 4.5 Newtons) Scale reading at rest: _______________ _______________ Maximum scale reading: _______________ _______________ Minimum scale reading: _______________ _______________ Label the following as equal to, greater than, or less than the scale reading at rest. _____________ At rest at the bottom _____________ Starting to go up _____________ Going up at constant speed _____________ Slowing to stop at the top _____________ Stopped at the top _____________ Starting to go down _____________ Going down at constant speed. _____________ Slowing to stop at the bottom. Calculate the mass of the person on the scale in kilograms: _____________________ Force Analysis: Draw a quantitative force diagram for the passenger in each of the following situations during the elevator ride. Label the forces in Newtons. To the right of each diagram draw a velocity and acceleration vector that describes the motion of person in the elevator. Calculate the net force and the acceleration of the person. 1. At rest at the bottom Quantitative force diagram 2. Starting to go up Quantitative force diagram velocity vector: velocity vector: acceleration vector: acceleration vector: net force = net force = acceleration = acceleration = 3. Going up at constant speed Quantitative force diagram 4. Slowing to stop at the top Quantitative force diagram velocity vector: velocity vector: acceleration vector: acceleration vector: net force = net force = acceleration = acceleration = 5. Stopped at the top Quantitative force diagram 6. Starting to go down Quantitative force diagram velocity vector: velocity vector: acceleration vector: acceleration vector: net force = net force = acceleration = acceleration = 7. Going down at constant speed. Quantitative force diagram 8. Slowing to stop at the bottom. Quantitative force diagram velocity vector: velocity vector: acceleration vector: acceleration vector: net force = net force = acceleration = acceleration = 9. How do the upward accelerations compare to the downward accelerations? Explain why. Name Date Pd Net Force Particle Model Worksheet 2: Newton's 2nd Law 1. A 4600 kg helicopter accelerates upward at 2.0 m/s2. Determine the lift force exerted on the propellers by the air. Make a quantitative force diagram. Write a net force equation for the axis along which forces are not balanced. 2. The maximum force that a grocery bag can withstand without ripping is 250 N. Suppose that the bag is filled with 20 kg of groceries and lifted with an acceleration of 5.0 m/s2. Do the groceries stay in the bag? Make a quantitative force diagram. Write a net force equation for the axis along which forces are not balanced. 3. A student, standing on a scale in an elevator at rest, sees that his weight is 840 N. As the elevator rises, the scale reading increases to 1050 N, then returns to normal. When the elevator slows to a stop at the 10th floor, the scale reading drops to 588 N, then returns to normal. Draw a motion map for the student during his elevator ride. Determine the acceleration at the beginning and end of the trip. Make quantitative force diagrams. Write a net force equation for the axis along which forces are not balanced. 4. A sign in an elevator states that the maximum occupancy is 20 persons. Suppose that the safety engineers assume the mass of the average rider is 75 kg. The elevator itself has a mass of 500 kg. The cable supporting the elevator can tolerate a maximum force of 30, 000 N. What is the greatest acceleration that the elevator's motor can produce without snapping the cable? Make a quantitative force diagram. Write a net force equation for the axis along which forces are not balanced.