**Vector diagram** is commonly referred to **parallelogram rule**.

Vector diagrams are generally divded into 3 categories:

**1)** 2 forces acting on a body, there is a resultant force. (Newton’s 2nd Law)

**2)** 3 forces acting on a body, the body is *at rest** / in equilibrium* or moving at *constant speed*. (Newton’s 1st Law, Net force = 0N)

– The resultant force due to any 2 forces is **equal and opposite** to the 3rd unknown force.

**3)** 3 forces acting on a body, the body is *at rest** / in equilibrium* or moving at *constant speed* (Newton’s 1st Law, Net force = 0N)

– Only 1 known force, angles given, using **closed loop triangle** to find the 2 unknown forces.

Let’s look at some examples for each category.

**1) 2 forces acting on a body, there is a resultant force. (Newton’s 2nd Law)**

**2) 3 forces acting on a body, the body is ***at rest** / in equilibrium* or moving at *constant speed*. (Newton’s 1st Law, Net force = 0N)

** – The resultant force due to any 2 forces is equal and opposite to the 3rd unknown force.**

** – You can either use ***parallelogram rule* or *closed looped triangle*.

Example 1: Body at rest – Using Parallelogram Rule

Example 1: Body at rest – Using Closed-Looped Triangle

Example 2: Body at constant speed – Using Parallelogram Rule

Example 2: Body at constant speed – Using Closed-Looped Triangle

**3) 3 forces acting on a body, the body is ***at rest** / in equilibrium* or moving at *constant speed* (Newton’s 1st Law, Net force = 0N)

** – Only 1 known force, angles given, using closed loop triangle to find the 2 unknown forces.**

Click here to view the construction of vector diagram using Parallelogram Method.