# What is a free body diagram?

I have this problem:

A 10 kg box is resting on a frictionless table and someone drags it by a string at a 30 degree angle to the horizontal with a force of 40 Newtons. Find the acceleration of the box. Draw a free body diagram of the system.

What is a free body diagram?

• Unbelievable. This site is not for such trivial questions which can be found in any introductory physics book, come on!!
– Revo
Feb 5, 2012 at 23:44
• @Revo Depending on what you mean by "trivial," that may or may not be true... anyway, after some thought I raised a meta question about this, and I'd welcome your input. Feb 6, 2012 at 3:39

## 2 Answers

To draw a free body diagram, start with a picture of the system. Draw arrows to represent all the forces exerted on parts of the system, with the tail of the arrow attached to the object that force acts on. Label the forces. Then erase everything but the arrows.

To solve the problem, you will need to determine what force accelerates the box. This should be the horizontal component of the tension in the string. To get this component, use the formula $F_x=F cos\theta$. Then apply Newton's Second Law.

• "with the tail of the arrow attached to the object that force acts on"... is that right? Surely it's from the object that that force comes from, the head indicates the object that that force acts on, or am I confused? en.wikipedia.org/wiki/Free_body_diagram
– AJP
Mar 3, 2012 at 23:53

A FBD (Free Body Diagram) is a diagram where all connections have been replaced by forces. Each body at which you want to apply $\sum F = m a$ you add up vectorially the force components acting on that body. In your case, you are interested only in the vertical component of the forces acting, so add them up and make sure they add up to zero (since the body is not moving the vertical direction at all).