In the figure,block A has a mass of 3.00 kg.It rests on a smooth horizontal table and is connected by a very light horizontal string over an ideal pulley to block B,which has a mass of 2.00 kg.When block B is gently released from rest,how long does it take block B to travel 80.0 cm?
A)0.404 s
B)0.494 s
C)0.639 s
D)0.785 s
E)0.935 s

Question

In the figure,block A has a mass of 3.00 kg.It rests on a smooth horizontal table and is connected by a very light horizontal string over an ideal pulley to block B,which has a mass of 2.00 kg.When block B is gently released from rest,how long does it take block B to travel 80.0 cm? 
A)0.404 s 
B)0.494 s 
C)0.639 s 
D)0.785 s 
E)0.935 s

Quizplus · Accepted Answer

We can use the kinematic equations to solve for the time it takes block B to travel 80.0 cm. Since there is no friction, we can assume that the tension in the string is constant and equal to the weight of block A.

Using the equation: 
d = 1/2at^2 + vt

where d=80.0 cm, v=0, and a=g=9.81 m/s^2, and t is the time it takes for block B to travel the distance.

We can solve for t:

t = sqrt(2d/a)

t = sqrt((2*0.80 m)/(9.81 m/s^2))

t = 0.6389 s

Rounding to the nearest hundredth gives t = 0.64 s, which is closest to answer choice C: 0.639 s.

In the Figure,block a Has a Mass of 3