Is the Normal Force the Only Thing Keeping a Block on the Ground?

In summary, when you pull on a block at an angle, the only force that will act is the y component of the original force. This force will cause the block to lift from the ground.
  • #1
CyberShot
133
2
I'm feeling really stupid right now.

But let's say a block is resting on a floor with friction. If someone were to pull to the right of the box at some angle theta above the horizontal, there would be components in the y (perp. to horizontal) and x (parallel to horizontal) of the force you pull at.

Now, when you draw the free body diagram, mg points straight down (the force of the block) and the Normal force points up (also equal to mg).

Now, doesn't that mean when you add up all the vectors the normal force and the weight of the block will cancel, and the only vector left in the y direction will be the y component of the force vector which was enacted at some angle theta above the horizontal.

Thus, since there's a net force in the y direction the block will lift from the ground. In fact, following this reasoning, any old arbitrary (non-zero) force at some non-zero angle should lift the block.

Obviously something's missing here from my free body diagram description?
 
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  • #2
You are nearly there.

It is a standard question/procedure to determine whether the block will slide or tip under the action of your force.

What happens if you consider moment equilibrium as well as horizontal and vertical?
 
  • #3
CyberShot said:
I'm feeling really stupid right now.

But let's say a block is resting on a floor with friction. If someone were to pull to the right of the box at some angle theta above the horizontal, there would be components in the y (perp. to horizontal) and x (parallel to horizontal) of the force you pull at.

Now, when you draw the free body diagram, mg points straight down (the force of the block) and the Normal force points up (also equal to mg).

Now, doesn't that mean when you add up all the vectors the normal force and the weight of the block will cancel, and the only vector left in the y direction will be the y component of the force vector which was enacted at some angle theta above the horizontal.

Thus, since there's a net force in the y direction the block will lift from the ground. In fact, following this reasoning, any old arbitrary (non-zero) force at some non-zero angle should lift the block.

Obviously something's missing here from my free body diagram description?

When you unweight the box a bit by pulling up at an angle, you reduce the normal force. Since the box is not accelerating up or down, the sum of the vertical forces must be zero. That includes the weight of the box, the upward force from the rope, and the normal force pushing up from the ground...
 
  • #4
Dagnabit. Beat out by Studiot again...
 
  • #5
Hello mike

o:)
 

FAQ: Is the Normal Force the Only Thing Keeping a Block on the Ground?

What is a dumb normal force question?

A dumb normal force question is a type of question that asks about the normal force, which is the force that an object experiences when it is in contact with a surface. These questions are often considered 'dumb' because they are frequently asked and can be easily answered by understanding basic physics principles.

How is the normal force calculated?

The normal force is calculated by multiplying the mass of the object by the acceleration due to gravity (9.8 m/s²) and the cosine of the angle between the object and the surface it is in contact with. This can be expressed as FN = mgcosθ.

What is the direction of the normal force?

The normal force always acts perpendicular to the surface that the object is in contact with. This means that if the surface is flat, the normal force will act in a vertical direction, pointing away from the surface.

How does the normal force affect an object?

The normal force is responsible for supporting an object and preventing it from falling through a surface. It also plays a role in determining the weight of an object on a surface, as it can cancel out a portion of the object's weight depending on the angle of the surface.

Can the normal force vary in different situations?

Yes, the normal force can vary depending on the object's weight, the angle and type of surface it is in contact with, and any other external forces acting on the object. It is important to consider these factors when calculating the normal force in a specific situation.

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