Free Body DiagramsThe first step in setting up

force problems

Drawing

Start with a “free body”

Here is an automobile being towed… as in your book.

free body

*Draw ONLY forces acting ON the object

Let’s start with the force of the tow truck.Then move it to the geo-center of the objectThis is referred to as the center of gravity.

*Next: the gravitational force

Gravity acts… downI’ve drawn it longer since the weight of the car is much greater than the force needed to tow it.

*Remember: Draw ONLY forces acting ON the car

The car is pushing down on the ground… don’t draw that force vector.The ground is pushing up on the car… how do we know this?I’ve drawn it almost the same size as the weight of the car vector… why?

*Any other forces?

That’s right… don’t forget the frictional force of the rotating wheels.But let’s move that vector to the center of gravity.What do we know if the vector sum of all of the forces adds up to zero?We refer to this condition as a “net force = 0” condition.

Equilibrium

*Let’s look at the vector-sum

Vectors can be moved.Do not change either the magnitude (represented by the length) or the direction they point.Let’s look at just the vectors.Now, let’s move them around, any order, so that they are “head-to-tail” to get a “vector-sum.”

*Let’s do it in a different order

Vectors can be moved.Let’s add them up in a different order “head-to-tail.”