In the figure below, when nothing is holding object A, object A falls to the floor. However, object B over the table does not fall to the floor. What does the table do to object B? As we discussed previously, for all objects near the surface of the Earth, the weight of the object represents the attractive force acting over the objects. Thus, over objects A and B  a force, corresponding to their respective weights, pulls the objects down. Object A falls because wA is the only force acting on the object; therefore, the net force acting on object A is

However, object B is not accelerating implying that the net force acting on it must be zero, . What is holding object B is the table; therefore, the table must apply a force on object B equal and opposite to the weight of the object. This force is called the normal force.  Why does the table applies a force over the object? The object is pushing down on the table with a force equals to the weight of the object (action); then, the table reacts by applying a force of the same magnitude but opposite direction to the object (reaction).

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Characteristics of the Normal Force.

  1. The normal force is the reaction of the surface to the action or push of the object over the surface.
  2. The magnitude of the normal force is equal to the magnitude of the push against the surface exerted by the object. If the surface is horizontal, the magnitude of the normal force is equal to the weight of the object.
  3. The direction of the normal force is always perpendicular to the surface and points out of the surface.

 

On the figure at the right, the unbalance force makes the object to slide down the inclined. Inclined planes will be study in details later on this notes. Additionally, the normal force is equal in magnitude only to the push of the object against the surface. Only horizontal surfaces produce reaction forces equal to the weight of the object.

In the case of a man leaning against a wall, the following forces can be identified:

  1. Weight of the man. Attractive force of the Earth acting on the man.

  2. Man pushing against the floor. The man push against the floor with a force equals to his weight.

  3. Normal force floor on man. As a reaction to the man exerting a force against the floor, the floor reacts by applying a force of the same magnitude but opposite direction on the man. This force balance the weight of the man.

  4. Man pushing against wall. By leaning against the wall, the man exerts a force against the wall, the magnitude of this force depends on the angle of the legs. The man exerts this force because of the tendency that the body will have to rotate counterclockwise in this figure.

  5. Normal force wall on man. As a reaction to the previous force, the wall exerts a force on the man (perpendicular to the surface). This force prevents the actual rotation of the man (see section on equilibrium).

  6. Static Friction on Man. The shoes of the man will have the tendency to slide forward because of the pushing of the wall on the man. The static frictional force prevents this for happening. Static friction is study later in this notes.

 

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Examples:

 

The following questions refer to the figure on the left.

 

1)

J

 

Q 1

 

 

Over a table is an object (B) of weight 50 N, on top of object B is object A of weight 30 N. What is the normal force acting on object B.

 

 

a)

 50 N

N

b)

 80 N

 

c)

 20 N

 

d)

 30N

 

e)

None of the above.

 

2)

J

 

Q 2

 

 

Over a table is an object (B) of weight 50 N, on top of object B is object A of weight 30 N. What is the normal force acting on object A.

 

 

a)

 50 N

 

b)

 80 N

 

c)

 20 N

N

d)

 30N

 

e)

None of the above.

 

 

* Tension

Consider a string pulled from the two extremes in opposite direction by a force F,

The string is under the tension . If the string is pulled from one side only, the tension on the string is zero. In this case, the force applied to the string has for result the acceleration of the string

 

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Examples:

 

1)

J

 

Q 1

 

 

Consider the case when the string is pulled from the left by a force of 10 N and from the right by a force of 12 N. What is the tension of the string?

N

a)

 10 N

 

b)

 12 N

 

c)

 2 N

 

d)

 11N

 

e)

None of the above.

 

2)

J

 

 

 

 

In the spring scale shown in the left figure, two identical weights, w, are attached to the scale as drawn. What is the reading of the scale?

 

 

a)

 Zero

 

b)

 2 w

N

c)

 w

 

d)

  w

 

e)

None of the above.

   

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Tension Forces and Action and Reaction

 

The tension of a string transmit forces from one body to another. The magnitude of the force transmitted is equal to the magnitude of the tension. The direction of the force can be understood following the direction of the action and reaction forces. 

Suppose that two blocks are pulling on the ideal string ( it can not stretch and is massless) in opposite direction with forces of the same magnitude, see below.

The block labeled L pulls on the string  toward the left and the block labeled R pulls on the string toward the right.

Thus, the tension on the string is .

For the block labeled L, the block acts applying a force F to the string pulling toward the left, the string reacts by applying a force of the same magnitude but opposite direction (toward the right) over the block,

For the block labeled R, the block acts applying a force F to the string pulling it toward the right, the string reacts by applying a force of the same magnitude but opposite direction (toward the left) over the block,

 

 

 

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by Luis F. Sez, Ph. D.    Comments and Suggestions: LSaez@dallaswinwin.com