Summary

• Work-energy theorem
• Power
• Gravitational potential energy

• Reading Quiz
  
  
• Work to stretch a spring
  • Hooke's Law (Section 6-2)
  • work for a varying force
  • energy applet
    Example #6
    
  
  
• Mechanical energy: E = K + U
  • conservation principle
  • bowling ball pendulum applet
  • energy buckets
  • mass on a spring applet
  • roller coaster
    Example #7
    
  • projectiles applet
    Example #8
    
  
  
• Friction & nonconservative work



• Interactive Example 8-8
• Interactive Example 8-10

walker4
Reading quiz A golfer badly misjudges a putt, sending the ball only one-quarter of the distance to the hole. The original putt gave the ball an initial speed of v₀. If the force of resistance due to the grass is constant, what initial speed is needed to get the ball to the hole from its original position?
A. 2v₀
B. 3v₀
C. 4v₀
D. 16v₀
Answer















 
If it takes 8.0 J of work to stretch a spring 12.0 cm from its unstressed length, how much extra work is required to stretch it another 12.0 cm?
A. 8.0 J
B. 16 J
C. 24 J
D. 32 J
Answer















 
What speed will a frictionless roller coaster have at 3.0 m altitude if it began its run from rest at an altitude of 10 m?
A. 8.37 m/s
B. 11.7 m/s
C. 70 m/s
D. 140 m/s
Answer















 
If you throw a ball upwards with a speed of 17 m/s, how high does it go? (Try to use conservation of energy to answer the question)
A. 0.85 m
B. 1.70 m
C. 14.7 m
D. 28.9 m
Answer















 



C. 2v₀

The work done by the friction from the grass during the first putt is −½mv₀². Because work is Fd, four times the distance means friction will do four times the work during the second putt. That, in turn, means the ball needs four times the kinetic energy to make it to the hole. A ball with twice the speed will have four times the kinetic energy, so we need only double the initial speed.




















 



C. 24 J





















 



B. 11.7 m/s





















 



C. 14.7 m