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Physics I Mechanics

Physics I Mechanics. Chapter 6 Work and Kinetic Energy. Definition of Work in Physics. bow and arrow pushing a stalled car SI : Joule / jewel / 19th century English physicist , James Prescott Joule. How much work is done ?. holding a barbell ?

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Physics I Mechanics

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  1. Physics IMechanics Chapter 6 Work and Kinetic Energy

  2. Definition of Work in Physics bow and arrow pushing a stalled car SI : Joule /jewel/ 19th century English physicist, James Prescott Joule

  3. How muchworkisdone? holding a barbell? carrying a book and walk? a ball in a string moves in uniformcircular motion? lowering a barbell? lifting a book ; workdone by the lifting force? By the gravitational force?

  4. KineticEnergy and the Work-EnergyTheorem “The work done by the net force on a particle equals the change in the particle’s kinetic energy”

  5. Example 6.4 Forces on a hammerhead In a pile driver, a steelhammerheadwith mass 200 kg islifted 3.00 m above the top of a vertical I-beambeingdriveninto the ground. The hammeristhendropped, driving the I-beam 7.4 cm fartherinto the ground. The vertical rails that guide the hammerheadexert a constant 60-N friction force on the hammerhead. Use the work-energytheorem to find (a) the speed of the hammerheadjust as it hits the I-beam and (b) the average force the hammerheadexerts on the I-beam. Ignore the effects of the air.

  6. The (Physical) Meaning of KineticEnergy Conceptual Example 6.5 Two iceboats hold a racee on a frictionless horizontal lake. The two iceboats have masses m and 2m. Each iceboat has an identical sail, so the wind exerts the same constant force on each iceboat. The two iceboats start from rest and cross the finish line a distance s away. Which iceboat crosses the finish line with greater kinetic energy?

  7. Work and KineticEnergy in Composite Systems

  8. Test YourUnderstanding of Section 6.2 Rank the following bodies in order of theirkineticenergy, from least to greatest. (i) a 2.0-kg body movingat 5.0 m/s ; (ii) a 1.0-kg body thatinitiallywasatrest and thenhad 30 J of workdone on it ; (iii) a 1.0-kg body thatinitiallywasmovingat 4.0 m/s and thenhad 20 J of workdone on it ; (iv) a 2.0-kg body thatinitiallywasmovingat 10 m/s and thendid 80 J of work on another body.

  9. 6.3 Work and EnergywithVarying Forces Work Done by a Varying Force, Straight-Line Motion a spring « Hooke’s law » If it is already stretched?

  10. Example 6.6 Work done on a spring scale A woman weighing 600 N steps on a bathroom scale containing a stiff spring. In equilibrium the spring is compressed 1.0 cm under her weight. Find the force constant of the spring and the total work done on it during the compression.

  11. Work-EnergyTheorem for Straight-Line Motion, Varying Forces Example 6.7 Motion with a varying force An air-trackglider of mass 0.100 kg isattached to the end of a horizontal air track by a springwith force constant 20.0 N/m. Initially the springisunstretched and the gliderismovingat 1.50 m/s to the right. Find the maximum distance dthat the glider moves to the right (a) if the air trackisturned on sothatthereis no friction, and (b) if the air isturned off sothatthereiskinetic friction withcoeeficientμk = 0.47.

  12. Work-EnergyTheorem for Motion Along a Curve Example 6.8 Motion on a curvedpath I At a familypicnicyou are appointed to push yourobnoxious cousin Throckmorton in a swing. Hisweightis w, the length of the chainsis R, and you push Throckyuntil the chainsmake an angle θ0with the vertical. To do this, youexert a varying horizontal force F thatstartsatzero and graduallyincreasesjustenoughsothatThrocky and the swing move veryslowly and remainverynearly in equilibrium. Whatis the total workdone on Throcky by all forces? Whatis the workdone by the tension T in the chains? Whatis the workyou do by exerting the force F? (Neglect the weight of the chains and the seat.)

  13. Example 6.9 Motion on a curved path II In the example 6.8 the infinitesimal displacement has a magnitude of ds, an x-component of dscosθ, and a y-component of dssinθ. Hence . Use this expression and Eq. (6.14) to calculate the work done during the motion by the chain tension, by the force of gravity, and by the force .

  14. Test YourUnderstanding of Section 6.3 In the example 5.21 (Section 5.4) we examined a conical pendulum. The speed of the pendulum bob remains constant as it travels around the circle shown in Fig. 5.32a. (a) Over one complete circle, how much work does the tension force F do on the bob? (i) a positive amount; (ii) a negative amount; (iii) zero. (b) Over one complete circle, how much work does the weight do on the bob? (i) a positive amount; (ii) a negative amount; (iii) zero.

  15. 6.4 Power « average power » SI : Watt English inventor, James Watt 1 hp = 0.746 kW kW.h =

  16. Example 6.10 Force and Power Each of the two jet engines in a Boeing 767 airliner develops a thrust ( a forward force on the airplane) of 197,000 N. When the airplane is flying at 250 m/s, what horsepower does each engine develop?

  17. Example 6.11 A “power climb” A 50.0-kg marathon runner runs up the stairs to the top of Chicago’s 443-m-tall Sears Tower, the tallest building in the United States. To lift herself to the top in 15.0 minutes, what must be her average power out put in watts? In kilowatts? In horsepower?

  18. Test YourUnderstanding of Section 6.4 The air surrounding an airplane in flight exerts a drag force that acts opposite to the airplane’s motion. When the Boeing 767 in Example 6.10 is flying in a straight line at a constant altitude at a constant 250 m/s, what is the rate at which the drag force does work on it? (i) 132,000 hp; (ii) 66,000 hp; (iii) 0; (iv) -66,000 hp; (v) -132,000 hp.

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