A scale will read a force value that is equal to the normal force exerted by the scale upon the object that is in contact with its top surface. By analyzing all the other forces that act upon that object, it is possible to predict a value for the scale reading.
 

Here is ...

Question 9
Consider the following situation and categorize it according to the relative strength of the normal force (or scale reading). Category options include:

  1. The normal force is equal to the weight of the object.
  2. The normal force is greater than (>) the weight of the object.
  3. The normal force is less than (<) the weight of the object.
  4. There is not enough information to decide.


 

 

The more difficult questions in this Concept Builder include a variety of distracting elements. The trick is to use the same type of analytical and principled-thinking that you used on the easier questions.

Here the 5.0-kg box is at rest, but wedged between two objects - a finger on the left side and a scale on the right side. You want to know how the scale reading - that is, the normal force acting between the box and the scale - compares to the weight of the 5.0-kg box. To do so, consider the forces acting upon the box. Your analysis will have to include some attention to direction of forces.

There are two obvious contact forces - the normal force (leftwards) from the scale and the normal force pushing (rightwards) from the finger. And then there is force of gravity (the Earth pulling the box downard). When analyzing the horizontal forces, one must conclude that they are balanced since the box is at rest. So the normal force from the balance (directed leftwards) is balanced by the finger pushing the box to the right. But there is no way to know the value of this rightward force. So from this type of analysis or reasoning, you should be able to answer the question.
 

Wait! One More Thing

Now if you're thinking then you might be a bit disturbed at this point (but never let that discourage you from thinking ... let it encourage you into deeper thinking). What happens with the vertical forces. If there is a force of gravity acting downwards then there must be some force acting upwards on the box.  Where does this force come from? Answer: friction. When two surfaces are pressed together (i.e. experience a normal force), then any motion or would-be motion of the two surfaces across each other results in a friction force. In this case, there is an upward frictional force between the box's surface and the surface of the scale and of the finger-tip.
 

You can learn more about force types and force diagrams by using the following links to The Physics Classroom's Tutorial section.

Types of Forces

Drawing Free Body Diagrams
 


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