C2.1 Free-Body Diagram (FBD) and Supports

Free-Body Diagram (FBD)

In your much dreaded class test, the first question requires you to identify all forces acting on a person who is standing. Whew, pretty easy – it must be just the person’s weight (gravitational force).

But hang on, if it’s only the weight, shouldn’t this person be accelerating towards the ground constantly? This doesn’t make sense, and after thinking harder you realise that the weight is balanced by the normal force acting on the person’s feet from the ground.

How do we analyse any static problems and identify all forces acting on a body without fail? This is where the free-body diagram (FBD) comes in. FBD is the most important step in any mechanics problem and is often overlooked by students.

The principle is to isolate the object of interest, replacing all actions/supports into its equivalent forces acting on the body. We do this because we must identify all forces acting on a body before proper mechanics analysis can be conducted.

C2.1 Free-Body Diagram (FBD) and Supports

Free-Body Diagram (FBD)

In your much dreaded class test, the first question requires you to identify all forces acting on a person who is standing. Whew, pretty easy – it must be just the person’s weight (gravitational force).

But hang on, if it’s only the weight, shouldn’t this person be accelerating towards the ground constantly? This doesn’t make sense, and after thinking harder you realise that the weight is balanced by the normal force acting on the person’s feet from the ground.

How do we analyse any static problems and identify all forces acting on a body without fail? This is where the free-body diagram (FBD) comes in. FBD is the most important step in any mechanics problem and is often overlooked by students.

The principle is to isolate the object of interest, replacing all actions/supports into its equivalent forces acting on the body. We do this because we must identify all forces acting on a body before proper mechanics analysis can be conducted.

Let’s now look at some typical action and reaction forces that are encountered in statics.

Action Forces

The typical action forces that we encounter in Statics are shown below:

Typical action forces and their examples

Reaction Forces/Supports

Since statics is a study of objects in stationary condition, there must be supports that provide reaction forces to balance the action forces. Otherwise, your weight will accelerate you indefinitely towards the centre of the earth! =)

In 2D statics analysis, we have 3 typical supports that provide:

1 reaction force (R_x or R_y)
2 reaction forces (R_x and R_y)
3 reaction forces (R_x, R_y, M)

Typical supports and their representation in FBD

Let’s look at some problems to help us master this very important concept of FBD.

Let’s now look at some typical action and reaction forces that are encountered in statics.

Action Forces

The typical action forces that we encounter in Statics are shown below:

Typical action forces and their examples

Reaction Forces/Supports

Since statics is a study of objects in stationary condition, there must be supports that provide reaction forces to balance the action forces. Otherwise, your weight will accelerate you indefinitely towards the centre of the earth! =)

In 2D statics analysis, we have 3 typical supports that provide:

1 reaction force (R_x or R_y)
2 reaction forces (R_x and R_y)
3 reaction forces (R_x, R_y, M)

Typical supports and their representation in FBD

Let’s look at some problems to help us master this very important concept of FBD.