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S E C T I O N 5 . 7 • Some Applications of Newton’s Law 129 If a is downward so that a y " # 2.00 m/s 2 , then (2) gives us 31.8
N " T " F g $ a y g $ 1 % " (40.0
N)
$ # 2.00
m/s 2 9.80
m/s 2 $ 1 % 48.2
N " " F g $ a y g $ 1 % " (40.0
N)
$ 2.00
m/s 2 9.80
m/s 2 $ 1 % (2)
T " ma y $ mg " mg
$ a y g $ 1 % To finalize this problem, take this advice—if you buy a fish in an elevator, make sure the fish is weighed while What If? Suppose the elevator cable breaks, so that the elevator and its contents are in free-fall. What happens to the Answer If the elevator falls freely, its acceleration is y " # g. We see from (2) that the scale reading T is zero in this case; that is, the fish appears to be weightless. Example 5.9 The Atwood Machine downward. Because the objects are connected by an inex- T ex- erted by the string and the downward gravitational force. In We must be very careful with signs in problems such as this. In Figure 5.14a, notice that if object 1 accelerates up- 1 g, and the y component of the net force exerted on object 2 is m 2 g # T. Notice that we have chosen the signs of the forces to be consistent with the choices of 2 + m 1 , then m 1 must accelerate upward, while m 2 must accelerate downward. When Newton’s second law is applied to object 1, we obtain Similarly, for object 2 we find When (2) is added to (1), T cancels and we have $ m 2 # m 1 m 1 $ m 2 % g (3)
a y " # m 1 g $ m 2 g " m 1 a y $ m 2 a y (2)
# F y " m 2 g # T " m 2 a y (1)
# F y " T # m 1 g " m 1 a y (b) m 1 T m 1 g T m 2 g (a) m 1 m 2 a a m 2 When two objects of unequal mass are hung vertically over a Solution Conceptualize the situation pictured in Figure Active Figure 5.14 (Example 5.9) The Atwood machine. (a) Two objects (m 2 + m 1 ) connected by a massless inextensible cord over a frictionless pulley. (b) Free-body diagrams for the two objects. Interactive At the Active Figures link at http://www.pse6.com, you can adjust the masses of the objects on the Atwood machine and observe the motion. |