Only forces ON the object count

A swimmer's hands and feet drive water backward, and the water drives her forward.

Now total up the forces that set her acceleration. Which of those two forces counts?

Newton's second law reads a = F_net/m. F_net admits some forces and refuses others, and this lesson draws the line.

Two different objects star in the swimmer's story: the swimmer, and the water.

The water's forward force on the swimmer acts on the swimmer. It joins her net force.

The swimmer's own net force is built from forces like these, forces acting ON the swimmer. The vertical forces balance and are left off.

The swimmer's push on the water acts on the water. It belongs to the water's net force, not the swimmer's.

This force acts on the water, so it joins the water's net force. It never appears on the swimmer's own diagram.

Here is the rule: only the forces acting on the object itself combine into the net force that sets its acceleration.

Forces the object exerts on other things never enter its own net force.

A truck tows a trailer. Which force belongs in the TRAILER's net force?

A free-body diagram enforces the rule by design: it shows only the forces acting on the chosen object.

So before combining forces, ask of each one: does it act on my object, or is it a force my object exerts on something else?

Each force joins the net force of the object it acts on, and no other.

Saskia says: when a boot kicks a soccer ball, the ball's net force includes the boot's force on the ball, but not the ball's force on the boot. Is she correct?

A rowboat moves forward while its oars drive water backward. Which force helps set the BOAT's acceleration?

For a = F_net/m, only the forces acting on the object itself combine into the net force that sets its acceleration; forces the object exerts on other things never enter.

A rope tows a wagon. Sort these three forces: the rope's tension on the wagon, friction from the ground on the wagon, and the wagon's backward force on the rope. Which belong in the wagon's net force?

The rope's tension on the wagon acts on the wagon: it belongs.

Friction from the ground on the wagon acts on the wagon: it belongs.

The wagon's backward force on the rope acts on the rope: it does not belong.

Two of the three belong. The force the wagon exerts on the rope never enters the wagon's own net force.

Your turn

Which statement about an object's net force is correct?

A physics book rests on a table. Which of these is a force ON the book?

Finding a crate's acceleration, a student lists the crate's downward force on the floor among the forces to combine. What is wrong?