System Under Net Force
A system has a net force when an unbalanced force is exerted on it, in other words the sum of all forces is not zero. Acceleration and net force always exist together.
For example, consider the same system as above but suppose the object is now being lowered with an increasing velocity downwards (positive acceleration) therefore there exists a net force somewhere in the system. In this case, negative acceleration would indicate that .
In another example, suppose that two bodies A and B having masses and respectively are connected with each other by an inextensible string over a frictionless pulley. There are two forces acting on the body A: its weight pulling down, and the tension in the string pulling up. If body A has greater mass than body B, . Therefore, the net force on body A is, so .
Read more about this topic: Tension (physics)
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