A free body diagram is a force diagram which shows all the forces acting on an object. It is called a free body diagram because an object in question is separated from the environment counting all the forces acting on it, and NOT the forces it exerts on other objects. In a free body diagram, an object is usually represented by a rectangular block and the forces acting on it are represented by lines with arrows, the arrows showing the direction of force.
Example: A book lying on the top of a table has two forces acting on it. They are - the force that the earth acts on it in vertically downward direction (Gravity) and the force the table acts on the book in vertically upward direction (Normal Reaction). Therefore, the free body diagram of the book should contain a rectangular block with two equal arrows passing through its center, one pointing vertically upward and the other vertically downward.
Drawing a correct free body diagram of an object is extremely important step in the study of motion of the object because this helps us to apply Newton's Laws of motion accurately.
Test yourself
(1) You throw a ball from the top of a tower at an angle of thirty degrees above the horizontal. How many forces are acting on it right after the ball leaves your hand? Draw its free body diagram and apply Newton's Second Law of motion to describe its motion. You can neglect air friction in this case.
(2) How many forces are acting on an athlete (a) when he is about to jump? (b) when he is in air after he makes a jump? Draw his free body diagram in each case.
(3) How many forces are acting on an object on a platform scale lying on the floor of an elevator under the following conditions?
(a) The elevator is at rest.
(b) The elevator is moving upward with a constant velocity.
(c) The elevator is moving downward with a constant velocity.
(d) The elevator is accelerating upward.
(e) The elevator is accelerating downward.
(f) The elevator is in free fall !
Draw a free body diagram in each case. Apply Newton's Second Law of motion to describe the motion of the object mathematically.
Drawing a correct free body diagram of an object is extremely important step in the study of motion of the object because this helps us to apply Newton's Laws of motion accurately.
Test yourself
(1) You throw a ball from the top of a tower at an angle of thirty degrees above the horizontal. How many forces are acting on it right after the ball leaves your hand? Draw its free body diagram and apply Newton's Second Law of motion to describe its motion. You can neglect air friction in this case.
(2) How many forces are acting on an athlete (a) when he is about to jump? (b) when he is in air after he makes a jump? Draw his free body diagram in each case.
(3) How many forces are acting on an object on a platform scale lying on the floor of an elevator under the following conditions?
(a) The elevator is at rest.
(b) The elevator is moving upward with a constant velocity.
(c) The elevator is moving downward with a constant velocity.
(d) The elevator is accelerating upward.
(e) The elevator is accelerating downward.
(f) The elevator is in free fall !
Draw a free body diagram in each case. Apply Newton's Second Law of motion to describe the motion of the object mathematically.
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