Engineering Mechanics

External Forces in Mechanics Explained for Students

Stand still on the floor. Two things are happening to you. Gravity pulls you down, and the floor pushes you up. Both of these act on you from the outside. They are external forces.

Every object you study in mechanics has forces acting on it from outside. These outside pushes and pulls decide whether the object moves, stays still, or tips over. So learning external forces is a must.

In this lesson you will learn what external forces are, the main kinds you will meet, and how they show up in a free body diagram. By the end you will be able to spot every external force acting on any object.

This follows on from the last lesson on types of forces. Quick recall: forces can be contact or non-contact, and they act together as force systems.

What Are External Forces?

An external force is a force applied to a body from outside itself.

If the source of the force is another object, and it acts on the body you are studying, then it is an external force. Your hand pushing a box, gravity pulling a ball, and the wind pressing on a wall are all external forces.

The key word is outside. External forces come from the surroundings of the body, not from within the body itself.

What Is the Difference Between External and Internal Forces?

Difference Between External and Internal Forces

This is a common point of confusion, so let us keep it short and clear.

External forces act on a body from outside, such as an applied push or gravity. Internal forces are the forces that develop inside the material to resist those external loads and hold the body together.

Think of a rope holding a weight. The pull of the weight is an external force on the rope. The tension felt inside the rope, fibre by fibre, is the internal force. We will study internal forces in full in the next lesson. For now, just remember that external means from outside.

What Are the Types of External Forces?

External forces usually fall into three groups. Knowing these makes it easy to list every force on an object.

  • Applied forces, also called action forces. These are the direct pushes and pulls placed on the body.

  • Reaction forces. These are the supporting forces from surfaces and supports that hold the body in place.

  • Weight. The pull of gravity on the body, which always acts downward.

Let us look at each one.

What Are Applied Forces?

An applied force is a force placed on the body on purpose, usually the load you are told about in a problem.

Picture pushing a shopping trolley. The push from your hands is the applied force. If a crane lifts a beam, the pull of the cable is the applied force. These are the forces that make things happen.

Applied forces are sometimes called action forces because they are the active loads on the object. They are the starting point of most problems, since the question usually tells you their size and direction.

What Are Reaction Forces?

A reaction force is the supporting force that a surface or support gives back to the body.

When you place a book on a table, the table pushes up on the book. That upward push is the reaction force, often called the normal reaction. Without it, the book would fall through the table.

Every support creates a reaction. A wall, a floor, a hinge, and a roller all push or hold the body in some way. Finding these reaction forces is one of the most common tasks in mechanics, because they are usually the unknowns you must solve for.

What Is Weight as an External Force?

Weight is the force of gravity pulling a body toward the Earth. It is always an external force because gravity comes from outside the body.

Weight is found using:

W = m × g

Where:

  • W is the weight, in newtons (N)

  • m is the mass of the body, in kilograms (kg)

  • g is the acceleration due to gravity, about 9.81 metres per second squared (m/s²)

For example, a 10 kg box has a weight of W = 10 × 9.81 = 98.1 newtons, acting straight down. Weight always acts through the centre of gravity of the body.

How Do External Forces Appear in a Free Body Diagram?

A free body diagram, or FBD, is a simple sketch that shows only the body and every external force acting on it.

To draw one, you imagine the body floating alone, then add an arrow for each external force. You include the applied forces, the weight acting downward, and the reaction forces at every support. You do not draw internal forces, because they cancel inside the body.

free body

A clear free body diagram is the single most useful tool in mechanics. Once every external force is on the sketch, the problem is halfway solved.

External Forces Example

Let us list the external forces on a lamp hanging from the ceiling by a cable.

  • Weight of the lamp, acting downward through its centre of gravity.

  • Tension in the cable, acting upward along the cable.

That is it. Only two external forces act on the lamp. If the lamp hangs still, these two must balance, so the cable tension equals the weight of the lamp. If the lamp has a mass of 2 kg, then:

Tension = weight = 2 × 9.81 = 19.62 newtons.

Simple, once you know which external forces to include.

Where Do Engineers Use External Forces?

Identifying external forces is the first real step in any design calculation.

Structural engineers list every external load on a bridge, such as traffic weight, wind, and support reactions, before they check if it is safe. Mechanical engineers find the external forces on a machine part to choose its size and material. Getting this list right is critical, because a missing force means a wrong and unsafe answer.

When these external loads become complex, engineers feed them into simulation tools to see how the part responds. That skill is covered in the FEA with ANSYS course.

Frequently Asked Questions

Q: What are external forces? A: External forces are forces applied to a body from outside, such as an applied push, the weight from gravity, and reaction forces from supports.

Q: What is the difference between external and internal forces? A: External forces act on a body from outside. Internal forces develop inside the material to resist those external loads and hold the body together.

Q: Is weight an external force? A: Yes. Weight is the pull of gravity, which comes from outside the body, so it is always an external force acting downward.

Q: What is a reaction force? A: A reaction force is the supporting force a surface or support gives back to a body, such as the upward push of a table on a book.

Q: Which forces are shown in a free body diagram? A: Only external forces are shown, including applied forces, weight, and reaction forces at the supports. Internal forces are not drawn.

Key Takeaways

  • An external force acts on a body from outside itself.

  • The three main kinds are applied forces, reaction forces, and weight.

  • Applied forces are the direct pushes and pulls on the body.

  • Reaction forces are the supporting forces from surfaces and supports.

  • Weight is the pull of gravity, found using W = m × g.

  • A free body diagram shows only the external forces on a body.

Quick Revision Box

  • External force: a force applied to a body from outside

  • Types: applied force, reaction force, weight

  • Weight formula: W = m × g, where g is about 9.81 m/s²

  • Free body diagram: shows all external forces, not internal ones

  • Reaction forces: usually the unknowns you solve for

Practice Corner

  1. In your own words, what makes a force an external force?

  2. Name the three main types of external forces.

  3. Find the weight of a 25 kg object. Take g as 9.81 m/s².

  4. A book rests on a table. Name the two external forces acting on it.

  5. Why are internal forces not shown in a free body diagram?

Answers

  1. It is a force that acts on the body from outside, coming from its surroundings rather than from within.

  2. Applied forces, reaction forces, and weight.

  3. W = 25 × 9.81 = 245.25 newtons, acting downward.

  4. The weight acting downward and the normal reaction from the table acting upward.

  5. Because internal forces act in equal and opposite pairs inside the body and cancel out, so they do not affect the overall balance.

What's Next

Next we study Internal Forces, the forces that build up inside a material to resist external loads and keep the body from breaking.

Conclusion

External forces are the pushes and pulls that act on a body from the outside. Learn to spot the three kinds, applied forces, reactions, and weight, and you can draw a correct free body diagram for any object. Since almost every mechanics problem starts with listing external forces, this skill will support everything you learn next.

Want to build on these fundamentals? Explore the free tools on the GaugeHow Learn Hub and test yourself with the Practice and MCQ tests.

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