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PHYS16 – Lecture 13

PHYS16 – Lecture 13. Ch. 7 Work and Energy. Announcements. Test – Week of Feb. 28 during lab Formal reports and Notebooks due Tues. 5pm for M/T lab In-class Review on Friday – Ch. 1-6. Circular Motion Post-Questions. Circular Motion post-question.

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PHYS16 – Lecture 13

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  1. PHYS16 – Lecture 13 Ch. 7 Work and Energy

  2. Announcements • Test – Week of Feb. 28 during lab • Formal reports and Notebooks due Tues. 5pm for M/T lab • In-class Review on Friday – Ch. 1-6

  3. Circular Motion Post-Questions

  4. Circular Motion post-question • A bicycle speedometer sensor is mounted on the bike at a fixed location, usually the fork, and pulses when the spoke-mounted magnet passes by. If the speedometer sensor is mounted at a radius of 0.50 m and counts 320 pulses, how far has the cyclist traveled? • 0.50 km • 0.80 km • 1.0 km • 1.5 km • 1.8 km sensor magnet

  5. Circular Motion post-question • At the point P indicated in the figure the string suddenly breaks. What is the trajectory of the ball? http://daed.be/paul/temp/review.php_files/fci7.jpg

  6. Circular Motion post-question • Tarzan is swinging across a lake on a vine that may not support his weight. What is the most likely point for the vine to break at? D A B C

  7. Circular Motion post-question • An object is swung in uniform circular motion vertically. Where is the tangential acceleration the greatest? • At the top of the circle • At the bottom of the circle • At the point midway between top and bottom • At both the top and bottom of the circle • None of the above

  8. Circular Motion post-question • A pickup has a crate of apples in its bed. The pickup makes a right turn and the once stationary crate accelerates to the left with respect to the truck bed and hits the side of the pickup. In which direction is a force acting on the crate? • To the right • To the left • Not enough information

  9. Description of Motion – What else do we need? • We have: • Laws of Calculus – Displacement, Velocity and Acceleration • Newton’s Laws – F=ma

  10. We need energy…

  11. Ch. 7 Work and Energy • Mechanical Energy • Potential Energy • Kinetic Energy • Work • Constant Force → Dot Product • Variable Force → Integral • Work-Mechanical Energy theorem

  12. Energy Pre-question • A force F is used to raise a 4-kg mass from the ground to a height of 5 m, as shown in the diagram below. What is the work done by the force F? Ignore friction and assume the rope and pulleys are massless. • 50 J • 100 J • 174 J • 200 J • None of the above θ = 60 degrees F mass

  13. Demo – Name that Energy http://undsci.berkeley.edu/images/us101/falling_objects.jpg

  14. Definition of Energy • Energy • A quantity whose expenditure or transformation allows for physical activity • An ability to drive motion • A capacity for action • Scalar Quantity • Unit = Joule (J) = kg·m2/s2 • Comes in many forms • Thermal • Chemical • Mechanical!!!!!

  15. Mechanical Energy • Kinetic Energy (K)– energy stored in the movement of an object • Potential Energy (U) – energy stored in the configuration of a system • Gravitational Potential Energy • Spring Potential Energy

  16. Practice Question • A 0.50 kg vase falls from 3.0 m. What is the kinetic energy of the vase just before it hits the ground? A) 0 J B) 15 J C) 1.5 J D) 2.3 J

  17. Work

  18. Definition of Work • Mechanical Work (W) – energy transferred to an object due to the action of a force (+) transfer to object (-) transfer from object

  19. Aside on Dot Product • Dot Product is one way to multiply two vectors • Basically just multiply components and add • Dot Product is a scalar A B

  20. Aside on Dot Product • Dot Product is one way to multiply two vectors • Basically just multiply components and add • Dot Product is a scalar A B

  21. Aside on Dot Product • Dot Product is one way to multiply two vectors • Basically just multiply components and add • Dot Product is a scalar • Or multiply magnitudes and cosine angle between the vectors A θ=56° B

  22. Work with a Constant Force • Force = Constant, then can take force outside integral

  23. Work with a Variable Force • Force = Variable, then need to break into components and integrate Fx x

  24. Practice Questions • I pull a 4.0 kg sled a distance of 5.0 m. I pull the sled using a rope at a 30.0 degree angle with a force of 5.0 N. The kinetic coefficient of friction is 0.1. What is the work done by me? • A force is given by Fx = 3x2+2. What is the work done by the force for moving an object from x=0.0 m to x=4.0 m? 22 J 200J

  25. Conclusions • Mechanical Energy • Potential Energy, Gravitational • Potential Energy, Spring • Kinetic Energy • Work • Constant Force • Variable Force

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