Physics 151 Week 10 Day 2

1. Physics 151 Week 10 Day 2 Topics: Apparent Weight & Conservation of Energy Apparent Weight Conservation of Energy

2. Scales and Elevators (Apparent Weight) • Consider a person with a mass of 60 kg is in an elevator standing on a scale. The elevator is accelerating upward. • Draw a system schema and 2 force diagrams: One for the person and one for the scale • What does the scale read? • Use Newton’s 2nd law to determine what the scale reads (This is apparent weight) Slide 4-19

3. Scales and Elevators (Apparent Weight) • Suppose a person with a mass of 60 kg is in an elevator standing on a scale. • Use the system schema and force diagrams of the scale and the person to determine what the scale would read for the following situations: • A. If the elevator is descending at 4.9 m/s. • B. If the elevator has a downward acceleration of 4.9 m/s/s. • C. If the elevator has an upward acceleration of 4.9 m/s/s. Slide 4-19

4. Apparent Weight Slide 5-24

5. Example Problem A 50 kg student gets in a 1000 kg elevator at rest. As the elevator begins to move, she has an apparent weight of 600 N for the first 3 s. How far has the elevator moved, and in which direction, at the end of 3 s? Slide 5-25

6. Clicker Question The apparent weight of an object is • the pull of gravity on the object. • the object’s mass times the acceleration of gravity. • the magnitude of the contact force that supports the object. • the pull of gravity on an object that is accelerating upward. Slide 5-7

7. Answer • The apparent weight of an object is • the pull of gravity on the object. • the object’s mass times the acceleration of gravity. • the magnitude of the contact force that supports the object. • the pull of gravity on an object that is accelerating upward. Slide 5-8

8. Energy Model • Types of Energy • Kinetic Energy KE = 1/2 mv2 • Potential Energy: • Gravitational Potential Energy PEg = mgy • Spring Potential Energy Pes = 1/2 kL2 • Conservation of Energy (Closed System) • Visualizations: • Energy Bar Charts

9. The Basic Equation • KfUfEthKiUi • A few things to note: • Work can be positive (work in) or negative (work out) • We are, for now, ignoring heat. • Thermal energy is…special. When energy changes to thermal energy, this change is irreversible. Slide 10-24

10. Conceptual Example Problem A car sits at rest at the top of a hill. A small push sends it rolling down a hill. After its height has dropped by 5.0 m, it is moving at a good clip. Write down the equation for conservation of energy, noting the choice of system, the initial and final states, and what energy transformation has taken place. Slide 10-25

11. Checking Understanding • Three balls are thrown off a cliff with the same speed, but in different directions. Which ball has the greatest speed just before it hits the ground? • Ball A • Ball B • Ball C • All balls have the same speed Slide 10-26

12. Answer • Three balls are thrown off a cliff with the same speed, but in different directions. Which ball has the greatest speed just before it hits the ground? • Ball A • Ball B • Ball C • All balls have the same speed Slide 10-27

13. Additional Questions Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance? Slide 10-54

14. Answer Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance? E. Slide 10-55