Add a third force that will cause the object to remain at rest. Label the new force F⃗ 3. Draw the vector starting at the black
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If we are talking on the force being exerted by a segment of a rope of lenght R on the right on a point M which is being also pulled from the Left by a segment of rope R as shown in the figure attached. Then we invoke Newton's Third Law:
"Any force exerted by an object (in this case a segment of the rope) also suffers a equal and opposite force".
If we pick
whis is the tension exerted by the right segment then the left segment will also exert an equal and opposite force so we have that 
"Any force exerted by an object (in this case a segment of the rope) also suffers a equal and opposite force".
If we pick
(a) 907.5 N/m
The force applied to the spring is equal to the weight of the object suspended on it, so:
The spring obeys Hook's law:
where k is the spring constant and is the stretching of the spring. Since we know
, we can re-arrange the equation to find the spring constant:
(b) 1.45 cm
In this second case, the force applied to the spring will be different, since the weight of the new object is different:
So, by applying Hook's law again, we can find the new stretching of the spring (using the value of the spring constant that we found in the previous part):
(c) 3.5 J
The amount of work that must be done to stretch the string by a distance is equal to the elastic potential energy stored by the spring, given by:
Substituting k=907.5 N/m and , we find the amount of work that must be done: