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Your tests and a printout are in your folder. Your mid-term grade is on the printout.

Your tests and a printout are in your folder. Your mid-term grade is on the printout. Mid-term: 6A, 2B, 2C. In this class typically, the final grades are higher than the min-terms. Chapter C12. Power, Collisions and Impacts 12B.1, B.3, B.6 Due Wednesday R.1 part of the practice test.

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Your tests and a printout are in your folder. Your mid-term grade is on the printout.

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  1. Your tests and a printout are in your folder. • Your mid-term grade is on the printout. • Mid-term: 6A, 2B, 2C. • In this class typically, the final grades are higher than the min-terms

  2. Chapter C12 Power, Collisions and Impacts 12B.1, B.3, B.6 Due Wednesday R.1 part of the practice test

  3. One glass of beer has 80 Calories. How many times would you need to lift barbells with 20 kg (~50 lbs) 60 cm to use up this energy? (note that Calories = kilocalories) • Write the principle • How many joules in 80 Calories? • 334,880j • How many joules are used each time you lift the barbells? • 117.6j • How many times do you need to lift? • 2848 times

  4. Chapter 11 example • Beth (m = 52 kg) climbs 400 meters and loses .5 kg of water. What is the minimum food energy she utilized? (Page 203) • Write the principle on your paper, raise your hand when finished. • Food energy > Energy to climb (P.E.) + energy to evaporate water • The > symbol represents the fact that the body loses some energy to heat (not all food energy is converted to work and vaporization)

  5. Power • Power is how fast I can do work, how fast I use energy. • Power = • Which equation I use depends on the type of energy I have, kinetic, potential or internal energy. • Power is the time rate of change of energy.

  6. Units of power • 1 joule/sec = 1 watt • 3.6 x 106 w = 3.6 Mj = 1 kwh (kilowatt hour) • 746 w = 1 horsepower = 1 hp

  7. What is the power of a 90 kg person who climbs to the top of a flight of stairs 4 m high in 10 seconds? • Write the principle. • Calculate the work done. • 3528J • What is the power? • 353 Watts • How many horsepower? • .473 hp • Does it matter how steep the stairs are? • Why not? • What would be the power if the person if they kept this up for an hour? • What would be the work if the person if they kept this up for an hour?

  8. Example problem (p-213) • A 1200 kg car descends a 5% incline at a constant speed of 11 m/s. At what rate is thermal energy being generated in the brakes? • Write the principle on your paper and raise your hand when you are done. • Energy is conserved so • 0=ΔK+ ΔP+ ΔUth • 0=0+ ΔP/dt+ ΔUth/dt • 0=0+ mg dz/dt+ ΔUth/dt

  9. From our problem we know that • dz = -.05 ds • ds/dt = v = velocity • Therefore we have • 0=0+ mgv(.05)+ ΔUth/dt • ΔUth/dt= 6468 w.

  10. Collisions • Male and female rebounds • Elastic collisions – kinetic energy is conserved • Inelastic collisions – some K.E. is changed to thermal or deformation energy. • Perfectly inelastic – maximum K.E. is changed to heat, the two objects stick together after the collision.

  11. M1 v1i v2i M2 • Consider a collision between two objects. • In all cases momentum is conserved. • Write the equations on your paper and raise your hand when your are done. • m1v1i+ m2v2i= m1v1f+ m2v2f • Write the equations for energy • ½m1v21i+ ½m2v22i=½m1v21i+ ½m2v22i

  12. case 1 • The two masses are equal • m2 is initially at rest. • Collision is elastic. • All motion is along the same straight line. • Find the final velocities.

  13. case 2 • m2=2m1 • m2 is initially at rest. Collision is elastic. • Find the final velocities. • All motion is along the same straight line. • v1i = 10 m/s

  14. 1 2 3 V1f = - 3.33 m/s 4

  15. V2=0 is not a physically possible solution • This would mean m1 passed through m2 with no effect. • When we substitute the value for v2 back into equation 3, we obtain: • V1f= -3.33 m/s • When we put these numbers back in equations 1 and 2, we see they are correct.

  16. Asteroid impacts • The asteroid we think hit the Caribbean 65 million years ago and caused the extinction of the dinosaurs. • Diameter of asteroid ~10 km • Diameter of crater = 175 km • Mass = 1015 kg • Velocity = 20 km/sec = 20,000 m/s. • K.E. = ½mv2= 2 x 1023 j • An atomic bomb = 4 x 1014 j

  17. The atmosphere does not slow the asteroid at all. • Immediately after impact the ejected material enters the hole punched in the atmosphere and the light produced is so bright it immediately ignites all material within 1000 miles. • The whole Earth is showered with vaporized rock emitting a power 5 times that of the sun (catches many other things on fire.) • See page 226

  18. Problems C12 • 12B.1, B.3, B.6 Due Wednesday • R.1 part of the practice test

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