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Problem solving tips
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Laws Of Motion

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Free Body Diagrams
A free body diagram is a visual representation of all the forces acting on an object. It is a simple way analyze the dynamics of an object by deconstructing all the forces acting on an object and studying the object as if it were in isolation. Let us understand how to draw a free body diagram for any system:
  1. Observe the object under consideration. Draw the object with its basic geometry, without any details, such that it is isolated from all elements around it.
Note: While making a free body diagram of connected objects, we can draw separate diagrams for each object
2. Now list all the external forces directly acting on the body. A free body diagram does not include internal forces of an object. External forces like, weight, reaction force, friction etc. are to be included in the free body diagram.
3. Represent all these forces acting on the objects as vectors with their correct orientation along with the object.
Note: Any acceleration possessed by the body is not to be included in the free body diagram as the acceleration arises from all the forces seen in the free body diagram being unbalanced.
Sample Problems:
Tip 2: How to write the Second Law equation using a free-body diagram?
Now that you know how to draw the free body diagram, how can you apply Newton's Second Law to it.
Let's checkout the previous example.
Let us find out how to write the Second Law equation for both blocks separately.
Tip 3: Solving problems in mechanics
Let us learn how to deconstruct a simple problem in mechanics to understand the dynamics of a mechanical system. Let us assume, we want to determine the acceleration of a block sliding down on a frictionless plane. How shall we approach this problem?
Sample Problems:
Tip 4: Elevator Problem
The force we exert on the a weighing machine determines our weight on the scale. On a stationary weighing machine the reaction force offered is the same as the actual weight mg. However if the weighing machine is accelerating upwards or downwards, it causes the reaction force to change , which shows an apparent change in weight on the scale. Lets simplify how to approach questions related to the apparent weight inside an elevator by drawing the free body diagram in different cases.
Sample Problems: