Free Body Diagrams

Free Body Diagrams

Middle School

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Learning Objectives

• Define a free-body diagram and understand its purpose in physics.

• Identify and represent common forces like gravity, normal force, friction, and tension.

• Use free-body diagrams to find the total net force on an object.

• Distinguish between balanced and unbalanced forces and understand the meaning of equilibrium.

Key Vocabulary

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Net Force

The overall force acting on an object when all individual forces are added together.

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Normal Force

The support force that a surface exerts on an object that is pressing on it.

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Friction

A force that opposes motion between two surfaces that are in contact with each other.

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Equilibrium

The state of an object when the net force is zero, which results in no acceleration.

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Vector

A physical quantity, such as force, that has both a magnitude and a specific direction.

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What is a Free-Body Diagram?

• A diagram showing all external forces acting on a single object.

• The object is simplified and is often drawn as a dot or box.

• Forces are vectors, meaning they have a magnitude and a direction.

• They help find the net force to see if forces are balanced.

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Multiple Choice

What is the primary reason for drawing a free-body diagram?

• To determine the mass of the object

• To calculate the net force acting on the object

• To visualize the object's speed

• To measure the object's volume

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Common Forces in Mechanics

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Multiple Choice

Which of these forces always acts perpendicular to the surface an object is resting on?

• Gravity

• Normal Force

• Friction

• Tension

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Balanced Forces and Equilibrium

Balanced Forces

• When forces are balanced, the total force on an object adds up to a net force of zero.

• An object with balanced forces is in equilibrium, which means it is not accelerating.

• Objects in equilibrium are either completely at rest or moving at a constant speed and direction.

Unbalanced Forces

Multiple Choice

If an object is in a state of equilibrium, what can be said about the net force acting on it?

• The net force is zero

• The net force causes it to speed up

• The net force is negative

• The net force is equal to its weight

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Forces on an Object on a Horizontal Surface

• Consider a box at rest on a flat table; two main forces act on it.

• The force of gravity, or weight, is a force that pulls the box down.

• The normal force is an upward push on the box from the table's surface.

• Because the box is not moving, these two opposite forces are perfectly balanced.

Multiple Choice

For a book at rest on a horizontal table, which force directly balances the downward force of gravity?

• Friction

• The Normal Force

• Tension

• Applied Force

FBD on an Inclined Plane

• Normal force is perpendicular to the ramp, and gravity acts straight down.

• Gravity is split into two components: parallel and perpendicular to the ramp.

• The parallel component pulls the object down, opposed by friction.

Multiple Choice

On a ramp, the normal force acts in which way relative to the object’s weight?

• Opposite to the total weight of the object

• Opposite to the component of weight parallel to the ramp

• Opposite to the component of weight perpendicular to the ramp, balancing it

• In the same direction as gravity

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Common Misconceptions

Misconception	Correction
The normal force is always equal to an object's weight.	This is only true on a horizontal surface with no other vertical forces.
Force pairs from Newton's Third Law cancel each other out.	These forces act on two different objects, so they do not cancel.
An object at rest has no forces acting on it.	An object at rest has balanced forces, so the net force is zero.
Mass and weight are the same thing.	Mass is the amount of matter, while weight is the force of gravity.

Multiple Choice

A 5 kg box is on a table. If you push it to the right with a 30 N force and the force of friction is 10 N to the left, what is the net force on the box?

• 40 N to the right

• 20 N to the right

• 20 N to the left

• 0 N

Multiple Choice

A block is sliding down a ramp. If the ramp is made rougher, which of the following changes will occur?

• The normal force increases, but friction stays the same

• Friction increases, causing the block to accelerate more slowly

• The block will accelerate faster because rougher surfaces reduce friction

• The weight of the block decreases

Multiple Choice

A block rests on a ramp. If you slowly increase the ramp's angle, which two forces will eventually become unbalanced, causing the block to slide?

• Normal force and the perpendicular component of gravity.

• Tension and the normal force.

• The parallel component of gravity and the force of static friction.

• Weight and the applied force.

Multiple Choice

A box rests on two different surfaces: one horizontal and one inclined. Which of the following statements is true about the normal force acting on the box?

• The normal force is the same on both surfaces because the box’s weight doesn’t change.

• The normal force is greater on the inclined plane than on the horizontal surface.

• The normal force is smaller on the inclined plane than on the horizontal surface because it only balances the perpendicular component of the weight.

• The normal force is zero on the inclined plane.

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Summary

Poll

On a scale of 1-4, how confident are you with drawing and interpreting free-body diagrams?

• 1 (Not confident)

• 2 (A little confident)

• 3 (Mostly confident)

• 4 (Very confident)