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Homework Help: Changing masses on an atwood machine

  1. Nov 18, 2014 #1
    An example of this kind of a problem would be a frictionless, massless pulley holding two equal masses(say, 5kg each). But then, an additional mass is added to one side (ie, a frog jumping on to mass 1, 1 kg). What would happen to the system? What if the frog jumps off?

    Initially, I know that the system has a net force of 0. F(net) = T -Fg while T=Fg.
    It is not accelerating.
    But after the frog jumps on, the system is no longer in equilibrium and mass 1 accelerates downward because the force of gravity on the "new" mass 1 (5+1= 6 kg) is larger than the tension in the string . So, mass 1 would be accelerating downwards at approximately 1.8 m/s^2. This is because a = (m1*g-m2*g)/(m1+m2). And tension, calculated using mass 2 where T>Fg would be T=Fnet+Fg=5*1.8+5*9.8= 58N.

    But I don't think that the actual test question would be this simple. Is there anything different I need to look at? For example, does the system still have a constant acceleration just after the equilibrium is disturbed? Does the fact that a mass was added during our observations cause the situation to differ from if we were to look at it from two different events (ie, before the frog, when the masses were equal, and then a situation where one mass is greater than the other. Then comparing the two events)
  2. jcsd
  3. Nov 19, 2014 #2


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    Hello water, welcome to PF :)
    Use the template: it's mandatory in PF and it helps you to order your thinking.

    Check your calculation: g/11 is not 1.8 m/s2.
    (Otherwise, what you do seems correct to me).

    Read what you typed before posting; your last paragraph is totally unclear.

    And when the frog jumps off (sideways): what is then the net force on each mass ?
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