# Draw the F.B.D. (Free Body diagram) of two blocks

• Shivam
In summary, the problem involves a system where all surfaces are frictionless, the pulley is ideal, and the string is light. The question is to find the acceleration of block 2, given the force F = mg/2. By analyzing the system and horizontal external forces, it is determined that the pulley applies a horizontal force F to the right on the box through the attachment, and also pushes down on the box with force F. The FBD for the two blocks can be drawn separately, with F acting horizontally on block 2 and the tension in the string acting vertically on the smaller block. A 3-body diagram can also be used if considering the force in the pulley connection, but it is not necessary for solving
Shivam
Summary: In the situation given, all the surfaces are friction less, pulley is ideal and string is light, F=mg/2, Find the acceleration of block 2 ?

Here's the diagram--
My attempt- (i) I tried to solve the question by making FBD of the two blocks, but i am not able to draw all the forces, the forces that i drew are not able to explain the motion of the blocks.
[Moderator's note: Moved from a technical forum and thus no template.]

Last edited by a moderator:
What forces act on the pulley?

scottdave said:
What forces act on the pulley?
Net √2F will be acting on pulley due to tension making an angle of -45° with +x-axis, and this Force has to be balanced by the opposite Force but I don't know how to associate this opposite Force and to whom should I associate .

You don't need to analyse the bodies separately to answer the question. Just consider the system and the horizontal external forces. The body 2 had no vertical acceleration.

Shivam said:
Net √2F will be acting on pulley due to tension making an angle of -45° with +x-axis, and this Force has to be balanced by the opposite Force but I don't know how to associate this opposite Force and to whom should I associate .
The pulley is rigidly attached to the box (not with a rope). @nasu is also correct but there is nothing wrong with your method. You do need to include the pulley applying F to the right on the box through the attachment. It will also push down on the box with force F .

nasu said:
You don't need to analyse the bodies separately to answer the question. Just consider the system and the horizontal external forces. The body 2 had no vertical acceleration.
I saw one method where these boxes are considered under a system. I understood that method , but I wanted to know FBD of the boxes separately and actually which force is opposing the contact force .. if can draw the FBD then please do post here.

Your FBD is almost correct. Just that F is horizontal and not vertical. The force acting vertically on the the small body is the tension in the string. As the pulley is rigidly connected to body 2, F acts horizontally on body 2. This is where is supposed to be in the FBD, if you split the system into two objects. If you consider the force in the pulley connection then you should have a 3 body diagram: body 1, body 2 and pulley. But you don't need this for the problem. Even 2-body is not necessary.

## 1. What is a Free Body Diagram (F.B.D.)?

A Free Body Diagram, also known as an F.B.D., is a visual representation of the forces acting on an object. It is a simplified diagram that shows all the external forces acting on an object, without showing the object itself.

## 2. Why is it important to draw a Free Body Diagram?

Drawing a Free Body Diagram is important because it helps to identify and visualize all the forces acting on an object. This allows for a better understanding of the motion and acceleration of the object, and can aid in solving problems involving forces.

## 3. How do you draw a Free Body Diagram of two blocks?

To draw a Free Body Diagram of two blocks, first draw a box to represent each block with arrows pointing away from the box to represent all the forces acting on it. Then, label each arrow with the type of force and its direction. Make sure to include all the external forces acting on both blocks, such as gravity, normal force, friction, and applied forces.

## 4. Can a Free Body Diagram of two blocks be drawn in any orientation?

Yes, a Free Body Diagram of two blocks can be drawn in any orientation as long as all the forces acting on the blocks are accurately represented in magnitude and direction. The orientation of the diagram does not affect the accuracy of the analysis.

## 5. What is the purpose of drawing a Free Body Diagram of two blocks?

The purpose of drawing a Free Body Diagram of two blocks is to simplify the analysis of the forces acting on the blocks and to determine the net force and resulting motion. It is an essential tool for solving problems involving forces and is commonly used in physics and engineering.

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