7.9 Example 9

A block with a mass of 20 kg, is supported to a string, as shown the image. If the spring is stretched 4 centimeters, which is the constant spring? which are the reactions in A?

Solution with FisicaLab

Erase the chalkboard and add the necessary elements to make three free body diagrams, as show the image below:

We don’t need the reactions on the pulley, for that we don’t add this. To the element Block, write:

m

20

To the force applied to the block:

f

t

To the spring write:

k

k

d

4 @ cm

And to the forces applied to the spring, write in both:

f

t

ang

45

Here the angle is irrelevant. The magnitude of the oblique force applied to the point, is an unknown data, but is the same unknown data applied to the spring. Then write:

f

t

ang

45

We calling the reactions forces applied to the point, Ry and Rx, respectively. Then write:

f

Ry

f

Rx

Now click in the icon Solve to get the answer.

t = 196.200 N ;  k = 4905.000 N/m ;  Rx = 138.734 N ;
Ry = -138.734 N ;
Status = success.

The negative sign in the Ry data is because the sense of this force, is the opposite of the force that we add to the pulley.