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Net Force

How to calculate the net force on an object, and draw free body diagrams.

A free body diagram shows all forces acting on an object.

We draw forces as arrows.

Arrow shows the relative magnitude (strength) and direction of a force.

If one force is 2x as big as another force, draw it 2x as long.
If one force if 3x smaller than antoher, draw it 3x smaller.

So each arrow is a vector.

Keep it simple. Don’t draw the object itself.  Just draw a small box, or a big dot  (●)

Here’s a free body diagram of a child being pulled on a sled, overlaid on an actual photo.

F g = force of gravity = weight

F N = normal force =  force from ground, supporting the child

F kf = force of kinetic friction (acting against the pull)

F T = force of tension in rope, pulling child forward.

Here’s your lesson on
Drawing Free Body Diagrams. From PhysicsClassroom.com

Physics of a falling Slinky

The bottom of the Slinky remains at the same position because upwards-pulling tension in the spring cancels out the downwards-pulling force of gravity.

When the top of the Slinky is released, gravity pulls the top of the Slinky downwards until it has fully collapsed (no more tension) and falls to the ground.

The center of gravity of the Slinky falls with the same gravitational acceleration experienced by any dropped object.

Filmed at 1000 frames per second with a Redlake N3 high speed camera, Adam Shomsky


Tension falling Slinky

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