Question

The scenario is as follows: A 502 g block (.502 kg) is sitting at rest on a scale. The scale reads 502 g.

1. Identify the forces acting on the block:
2. Is the block in equilibrium? How do you know?
3. Identify the forces acting on the scale.
4. Is the scale in equilibrium? How do you know?  
5. First draw a free body diagram for the block. Here you consider the block as your system of interest. Everything else is external.  Then draw a free body diagram for the scale. Here you consider the scale as your system of interest. Everything else is external.  Remember that the length of force vectors is proportional to magnitude.  Label each force vector appropriately!

                                                                                 

6. Identify all the action-reaction pairs in the two free-body diagrams. To do this connect the forces with a dotted line:

Ex:              F _{A on B} <---------------  --------------> F _{B on A}                                  

7. Remember that the two forces that comprise an action-reaction pair act on different objects.  Do you need to revise your answer to question 6?
8. Are there forces acting on the block and scale, which are not part of one of the action-reaction pairs you identified, that is, are there any forces without a dotted line? If so, list them below.

            Forces on Block                                                                   Forces on Scale

9. For the forces you identified in Question 15, do each of these actions have a reaction force?  If so, why are they not on our diagrams?
10. Now consider the forces acting on the scale.  The scale reading shows the value of one of those forces. Which force is it?
11. Does the force on the scale that produces the reading have a reaction force acting on the block as well? If so, which one?
12. Based on your answers to Questions 17 and 18, what is your conclusion about “what is measured by the scale”?

Answer 1