PHYSICS 50: Lecture 7-1

1. PHYSICS 50: Lecture 7-1 RICHARD CRAIG

2. Homework #6 • Read Chapter 7 • Exercises and Problems: 7.5, 7.14, 7.29,7.38, 7.46, 7.55 • Due Thursday, 3/13

3. Potential Energy • Instead of considering how the work form gravity changes the kinetic energy of a system I can consider gravity a form of energy • Gravitational Potential energy • Ug = mgh

4. Conservation of Energy • When no “outside” forces or friction then mechanical energy is conserved • Kinetic + Gravitational + Elastic = Constant • Total energy is always conserved

5. Q7.4 Concept Question The two ramps shown are both frictionless. The heights y1 and y2 are the same for each ramp. A block of mass m is released from rest at the left-hand end of each ramp. Which block arrives at the right-hand end with the greater speed? A. the block on the curved track B. the block on the straight track C. Both blocks arrive at the right-hand end with the same speed. D. The answer depends on the shape of the curved track.

6. Work and energy along a curved path • Notice the same expression for gravitational potential energy is the same along either a curved or straight path.

7. Shoot bullet at same speed • Consider the speed of a projectile as it traverses its parabola in the absence of air resistance.

8. Conservative forces

9. v2 = 0 y2 v1 = 20.0 m/s m = 0.150 kg y1 = 0 Q7.2 Concept Questions #2 You toss a 0.150-kg baseball straight upward so that it leaves your hand moving at 20.0 m/s. The ball reaches a maximum height y2. What is the speed of the ball when it is at a height of y2/2? Ignore air resistance. A. 10.0 m/s B. less than 10.0 m/s but more than zero C. more than 10.0 m/s D. not enough information given to decide

10. Conservation of Mechanical Energy

11. Around the Loop

12. Diver on a spring Board