Let’s say you are sitting on the couch and I ask you how many more people can sit on that couch so that it does not get damaged? Now to answer this question you will need to define the couch properly then check its capacity then only you can analyse the situation. Similarly in physics to study the effect of forces on the body we first have to define the system and draw its free body diagram so that it will be isolated from its surroundings and we can focus only on the system. In this article let’s study about the system and free body diagram.
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Two or more objects that interact with each other form a system. Defining a system is very important because the forces applied on a body will cause motion on the defined system.
If the action-reaction pair exists in the considered system, then it is known as an internal force, otherwise it is known as an external force.
For example, in system 1, as shown in the given figure, both the action and the reaction forces are inside the system. Hence, the forces are known as internal forces.
In system 2, the reaction force is outside the system. Hence, force N1 is known as external force, whereas N2 force is not considered as it is not inside the system.
Note: Internal force cannot change the motion of the system.
Let’s take an example, you can't move a car sitting inside it and pushing it on the front seat. However, if you come out of the car and push it, you may succeed since the force applied will become an external force.
In the figure shown, you can't push the truck while you are a part of it. Hence, you need to come out of the truck and push in order to move it.
For example, consider a system as shown to understand it further.
Consider the two boxes as a system. Then, the force applied by the girl is an external force on the system. Due to this external force, all parts of the system move with the same acceleration.
It is the diagrammatic representation of a body that is isolated from its surroundings, showing all the external forces acting on it.
Consider a block that has four forces acting on it. The free-body diagram for this block is as shown in the figure.
Steps to Draw FBD
Note: The entire Newtonian mechanics is applied on a body that is considered as a point. Hence, always take the body as a point while drawing forces in FBD. For visualisation, the body can be drawn to a shape.
1. Two bodies A and B of mass m and M, respectively, are being pushed by a force F towards the positive x-axis direction. Draw a free-body diagram of both the blocks.
Solution: Let blocks A and B be kept in contact on a surface. When force F is applied on block A, it pushes block B rightwards with force NBA. As a reaction, block B will push block A with the force NAB in the left direction. These forces, known as normal reactions, act normally to the surface of blocks at their contact point. Weight of both the blocks acts in a downward direction. There is a normal reaction force from ground acting on two blocks in a vertically upward direction. Free-body diagrams of both the blocks A and B having mass m and M, respectively, are shown in the figure.
2. Two blocks of mass m and M are connected to each other with a massless rope and are being pulled by a rope with a force F. Draw the free-body diagram of both the blocks.
Solution: The FBD of both the masses have been drawn separately. The tension occurred on both the blocks because the rope has also been shown in the individual FBD's of the masses.
3. A point A on a sphere of weight W rests in contact with a smooth vertical wall and is supported by a string joining point B on the sphere to point C on the wall. Draw the free-body diagram of the sphere.
Solution: Here, the weight (Fg) of the body acts downward. Normal reaction (NA) from the wall will be acting in the right direction. Tension (TB) will act along the string length and away from the body.
4. A block of mass M is placed on a wedge that is fixed to the ground as shown in the figure. Draw a free-body diagram of the block.
Solution: Two or more objects that interact with each other form a system.
Solution: It is the diagrammatic representation of a body that is isolated from its surroundings, showing all the external forces acting on it.
Solution: By using these steps you can draw FBD: (a) Isolate the body from its surroundings (free body). (b) Assume the body as a point mass. (c) Draw all the external forces acting on the body, starting from the point mass.
Can internal forces change the motion of a system?
Solution: Internal force cannot change the motion of the system.
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