Question

Looking at a FBD(Free Body Diagram) how can you tell which way it is accelerating?

Answer 1

We can look at the free body diagram and tell which way a body is accelerating by seeing which direction has an unbalanced force.

Fix a direction as positive(say positive x-axis as positive) every force acting in the direction of negative x-axis should be taken as negative. Now add the forces along the positive x-axis with that along negative x-axis. If the result is negative the object is accelerating towards the negative x-axis, whereas if the result is positive the object is accelerating towards positive x-axis.

For example: If someone pushes a block horizontally with a force 10 N and pulls its with 20 N, 10-20=-10 , therefore, the block will accelerate in the direction of the person pulling the block.