Internal Force: Definition & Examples

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: External Force: Definition & Examples

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
 Replay
Your next lesson will play in 10 seconds
  • 0:04 What Are Internal Forces?
  • 0:55 Equilibrium
  • 2:37 Sign Convention
  • 3:12 Numerical Example
  • 3:51 Lesson Summary
Save Save Save

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Log in or Sign up

Timeline
Autoplay
Autoplay
Speed Speed

Recommended Lessons and Courses for You

Lesson Transcript
Instructor: Ali Motamedi

Ali received his MSc degree in Civil Engineering and PhD degree in Risk Management. He has been teaching engineering and analytics courses for several years.

In this lesson you will learn what the internal forces are and what they do in a structure member such as pole, beam and column. We also review the commonly used sign convention and solve a numerical example to understand how to calculate an internal force in a structure member.

What are Internal Forces?

Internal forces are produced from the external forces acting on structure members such as poled, beamd or columnd. Generally, we have three types of internal forces: axial, dhear and moment. Axial force, sometimes called 'normal force,' is a compression or tension force acting aligned with the extension of a structure member. Shear force is a force acting in a direction perpendicular to the alignment of the member. Moment force, lastly, is a turning result of a force multiplied by the distance from its acting location to the turning point. The number of these components varies in one-dimensional, two-dimensional and three-dimensional cases. Now the questions are what each of these components do and how to calculate these internal forces.

Equilibrium

When a structure member such as a pole, beam or column is in equilibrium, it means that it isn't moving as it is supposed to be. Therefore, the combination or resultant of all the external forces applied to the member equals zero. In fact, the internal forces maintain the equilibrium of a structure member in different directions.

1D Case

In the simplest example, let's consider a pole in a building structure (1D case). Depending on the applied external loading, this pole can be in either tension or compression. Now, what if the pole is cut from left to right of an arbitrary size (it's called a free body diagram)? Since the pole is not moving, as we discussed, the resultant of all the applied forces, including the external and internal forces, still needs to equal zero (see Figure 1).


Internal forces figure 1


2D Case

We will have the same concept in a 2D case with a beam. Since we assume that the beam is fixed and not moving, the resultant of all applied forces from different directions equal zero (see Figure 2).


Internal forces figure 2


3D Case

Similar to the 1D case example, cutting the beam from left or right of an arbitrary size and section will result in a combination of external and internal forces with zero resultant (see Figure 3).


Internal forces figure 3


In the 3D case, we will need more force components to maintain the equilibrium: one axial and two shears, one twisting and two bending moments.

To unlock this lesson you must be a Study.com Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account
Support