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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 11
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.

You have to analyze this diagram with some attention to detail before you understand the situation. Here the 5.0-kg box is on a scale. And the scale is on a cart. And the cart is being pushed leftward across a horizontal surface. There is a leftward motion of the scale, the box and the cart. 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 box. To do so, consider the forces acting upon the box. And give some attention to direction.

There are three obvious contact forces on the box along with the downward force of gravity (the Earth pulling the box downard). The two obvious horizontal forces on the box are the force of the finger pushing the box to the left and the force of the vertical wall on the cart pushing the box to the right. Less obviously, there is likely some friction between the box surface and the scale surface. This would be a horizontal force as well. The one obvious vertical contact force is the force of the scale pushing upward on the box. This is the normal force that the scale would read. This upward normal force and the downward force of gravity must balance each other. 

So from this type of analysis or reasoning, you should be able to answer the question. It's worth noting that the analysis broke the situation down into thinking about forces upon the box and then thinking about their direction. 
 

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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