|
|
SECTION 8.2 • The Isolated System–Conservation of Mechanical Energy 225 You are designing an apparatus to support an actor of mass smoothly over two frictionless pulleys, as in Figure 8.8a. You Note that this result is consistent with the expression from kinematics, where y i " h. Fur- thermore, this result is valid even if the initial velocity is at v yf
2 " v 2 yi # 2g(y f # y i ) √ v 2 i % 2g
(h # y) v f " v 2 f " v 2 i % 2g (h # y) gravitational potential energy depends only on the change What If? What if the initial velocity v i in part (B) were down- ward? How would this affect the speed of the ball at position y ? Answer We might be tempted to claim that throwing it Example 8.3 The Pendulum A pendulum consists of a sphere of mass m attached to a A with the vertical, and the pivot at P is frictionless. (A) Find the speed of the sphere when it is at the lowest point ". Solution The only force that does work on the sphere is has regained its initial potential energy, and the kinetic en- If we measure the y coordinates of the sphere from the center of rotation, then y A " # L cos & A and y B " # L. Therefore, U A " # mg L cos & A and U B " # mgL . Applying the principle of conservation of mechanical en- ergy to the system gives (1) (B) What is the tension T B in the cord at "? Solution Because the tension force does no work, it does B , we can ap- ply Newton’s second law to the radial direction. First, recall 2 /r directed toward the center of rotation. Because r " L in this example, Newton’s second law gives Substituting Equation (1) into Equation (2) gives the ten- A : " From Equation (2) we see that the tension at " is greater B " mg when the initial angle & A " 0. Note also that part (A) of this example is catego- rized as an energy problem while part (B) is categorized as a mg(3 # 2 cos & A
) (3) T B " mg % 2mg
(1 # cos & A ) (2)
$ F r " mg # T B " ma r " # m
v B
2 L v B " √ 2gL(1 # cos & A ) 1 2 mv B 2 # mgL " 0 # mgL
cos & A K B % U B " K A % U A # " ! θ A L cos θ A L T P m
g θ θ Figure 8.7 (Example 8.3) If the sphere is released from rest at the angle & A , it will never swing above this position during its motion. At the start of the motion, when the sphere is at position !, the energy of the sphere–Earth system is entirely potential. This initial potential energy is transformed into kinetic energy when the sphere is at the lowest elevation ". As the sphere continues to move along the arc, the energy again becomes entirely potential energy when the sphere is at #. Example 8.4 A Grand Entrance Compare the effect of upward, downward, and zero initial velocities at the Interactive Worked Example link at Interactive |