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1.4 Fluids and Free-Body Diagrams

3 min readdecember 31, 2022

K

Krish Gupta

Daniella Garcia-Loos

Daniella Garcia-Loos

K

Krish Gupta

Daniella Garcia-Loos

Daniella Garcia-Loos

Fluids and Free-Body Diagrams

Force Body Diagrams, commonly known as FBDs, are one of the most powerful tools in a physicist's kit. FBDs in this course aren’t any tougher than the FBDs in your first-year physics classes. However, in this class, you won't always be awarded points for drawing FBDs, because you are expected to include them. In this course, you will have to use FBDs to solve a further problem or give a nuanced explanation of a scenario.

You should be familiar with the and the force of gravity. Another force that comes up on almost all FBDs in Physics 2 is the . The will be explored in much more detail in the next section. For now, know that the is written as Fb and acts mostly on objects in fluids in the opposite direction as gravity.

The only comes into play if the object is thrown and falls all the way to the bottom of the fluid container. Remember that is a contact force, so the object has to be stable and in contact with the sea floor to be experiencing the !

Another force that may come up is the . The in fluids is much like air resistance in Physics 1 problems. However, the resistive forces in fluids can’t be always ignored like air resistance. The acts in the direction opposite to the motion of the object and depends on the velocity of the object—so if the object is at rest there is no , but the faster the object moves in the fluid, the greater the will be.

🟦Blue: FBD for an object floating in water

An object floating in water experiences the gravitational force due to it's weight and a as it's in water ( a fluid).

🟨Yellow: FBD for an object sinking in water

The object experiences a gravitational force due to the Earth, a since it's in a fluid a because it's in motion in a fluid.

🟪Pink: FBD for an object that has sunk to the bottom of the water

The object experiences the gravitational force due to the Earth, a since it's in a fluid and a because it's in contact with the surface (the bottom of the water).

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-IToE2sih3EzB.png?alt=media&token=145bee2d-e539-4a47-bc70-653f6f7e9ec9

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-FrDv6CEZnre2.png?alt=media&token=7b953a32-36fb-4233-a054-3f9d31bc29cf

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ybsIxOBq2fUS.png?alt=media&token=d9630399-2972-489e-b327-ae9441d3d89a

Here are the steps for drawing a free-body diagram (FBD) for a fluid problem:

  1. Identify the object of interest. This is the object for which you are trying to draw the FBD.

  2. Identify the forces acting on the object. These may include the weight of the object, the acting on the object, and any other forces that are applied to the object, such as friction or tension.

  3. Draw a reference line to represent the object. This should be a horizontal line, as the object is in a fluid.

  4. Draw a vector to represent each force acting on the object. The vectors should be drawn in the direction in which the force is applied, and their length should be proportional to the magnitude of the force.

  5. Label each force vector with the symbol and magnitude of the force. For example, you might label the weight of the object as "W" and the as "Fb."

  6. Add any necessary notes or annotations to the FBD. For example, you might include the density of the fluid or the volume of the object.

  7. Check the FBD to ensure that it is complete and accurate. Make sure that all forces acting on the object are represented, and that the forces are balanced (the sum of the forces is zero).

Key Terms to Review (3)

Buoyant Force

: Buoyant force is an upward force exerted on an object submerged in a fluid (liquid or gas). It is equal in magnitude but opposite in direction to the weight of the fluid displaced by the object.

Normal Force

: The normal force is the support force exerted by a surface on an object in contact with it. It acts perpendicular to the surface and prevents objects from passing through each other.

Resistive Force

: Resistive forces are forces that oppose the motion of an object through a fluid, such as air or water. They can include drag, air resistance, and viscous forces.

1.4 Fluids and Free-Body Diagrams

3 min readdecember 31, 2022

K

Krish Gupta

Daniella Garcia-Loos

Daniella Garcia-Loos

K

Krish Gupta

Daniella Garcia-Loos

Daniella Garcia-Loos

Fluids and Free-Body Diagrams

Force Body Diagrams, commonly known as FBDs, are one of the most powerful tools in a physicist's kit. FBDs in this course aren’t any tougher than the FBDs in your first-year physics classes. However, in this class, you won't always be awarded points for drawing FBDs, because you are expected to include them. In this course, you will have to use FBDs to solve a further problem or give a nuanced explanation of a scenario.

You should be familiar with the and the force of gravity. Another force that comes up on almost all FBDs in Physics 2 is the . The will be explored in much more detail in the next section. For now, know that the is written as Fb and acts mostly on objects in fluids in the opposite direction as gravity.

The only comes into play if the object is thrown and falls all the way to the bottom of the fluid container. Remember that is a contact force, so the object has to be stable and in contact with the sea floor to be experiencing the !

Another force that may come up is the . The in fluids is much like air resistance in Physics 1 problems. However, the resistive forces in fluids can’t be always ignored like air resistance. The acts in the direction opposite to the motion of the object and depends on the velocity of the object—so if the object is at rest there is no , but the faster the object moves in the fluid, the greater the will be.

🟦Blue: FBD for an object floating in water

An object floating in water experiences the gravitational force due to it's weight and a as it's in water ( a fluid).

🟨Yellow: FBD for an object sinking in water

The object experiences a gravitational force due to the Earth, a since it's in a fluid a because it's in motion in a fluid.

🟪Pink: FBD for an object that has sunk to the bottom of the water

The object experiences the gravitational force due to the Earth, a since it's in a fluid and a because it's in contact with the surface (the bottom of the water).

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-IToE2sih3EzB.png?alt=media&token=145bee2d-e539-4a47-bc70-653f6f7e9ec9

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-FrDv6CEZnre2.png?alt=media&token=7b953a32-36fb-4233-a054-3f9d31bc29cf

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ybsIxOBq2fUS.png?alt=media&token=d9630399-2972-489e-b327-ae9441d3d89a

Here are the steps for drawing a free-body diagram (FBD) for a fluid problem:

  1. Identify the object of interest. This is the object for which you are trying to draw the FBD.

  2. Identify the forces acting on the object. These may include the weight of the object, the acting on the object, and any other forces that are applied to the object, such as friction or tension.

  3. Draw a reference line to represent the object. This should be a horizontal line, as the object is in a fluid.

  4. Draw a vector to represent each force acting on the object. The vectors should be drawn in the direction in which the force is applied, and their length should be proportional to the magnitude of the force.

  5. Label each force vector with the symbol and magnitude of the force. For example, you might label the weight of the object as "W" and the as "Fb."

  6. Add any necessary notes or annotations to the FBD. For example, you might include the density of the fluid or the volume of the object.

  7. Check the FBD to ensure that it is complete and accurate. Make sure that all forces acting on the object are represented, and that the forces are balanced (the sum of the forces is zero).

Key Terms to Review (3)

Buoyant Force

: Buoyant force is an upward force exerted on an object submerged in a fluid (liquid or gas). It is equal in magnitude but opposite in direction to the weight of the fluid displaced by the object.

Normal Force

: The normal force is the support force exerted by a surface on an object in contact with it. It acts perpendicular to the surface and prevents objects from passing through each other.

Resistive Force

: Resistive forces are forces that oppose the motion of an object through a fluid, such as air or water. They can include drag, air resistance, and viscous forces.


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.