Free Body Diagram Examples

Example 1: The Sled

Question

The child next door wants to take his German Shepherd (named Spot) for a ride on his sled. He chases Spot (who is running away) while dragging his sled behind him. What forces are acting on the sled?

Hints

The forces we discussed were:

1. gravity
2. normal
3. friction
4. push or pull

Let's consider each of the above forces in turn.

1. Since the question does not state otherwise, we will assume that this is all happening on Earth. So gravity is a force acting on the sled. The gravitational force, Fmg, vector points downward.
2. The sled is being dragged on the ground, which we can take to be another surface. This surface exerts a force on the sled, preventing it from falling into the ground. Thus there is a normal force N on the sled.
3. Because the sled is in motion, and the surface with which it has contact (i.e. the ground) is not frictionless then there is kinetic friction.
4. The child is pulling on the sled, and so there is a pull force, Fpull. The pull may not be parallel to the ground but at an angle as shown in the drawing.

The sled is represented by the red dot in the FBD.

Example 2: Dot and Spot

Question

Spot, the German Shepherd, is being walked by his best pal, Dot, the Siamese cat. Actually, Dot is pulling Spot across the ice rink,

and disturbing the neighbourhood hockey game. What are the forces acting on Spot?

Hints

The forces we discussed in the introduction were:

1. gravity
2. normal
3. friction
4. pull or push by Dot

Since this is taking place on Earth, we can assume a gravitational force Fmg on Spot.

Also, because Spot is on the surface of the ice, the ice is exerting a normal force N on him.

We assume that ice is frictionless, and so neither static friction nor kinetic friction acts on Spot (which he probably appreciates).

The push/pull force may be misleading. While it is true that Spot is being pulled along by Dot, this is not the force acting on Spot. This is because Dot is actually pulling on the rope. The rope, in turn through its tension T, exerts a force on Spot.

The FBD is

Example 3: King Kong's Pendulum

Question

While climbing the Empire State  Building, King Kong peaks into one of the corporate windows, and sees a poster with Tarzan swinging from a vine. Our furry friend decides he'd much rather be swinging from vines instead of hanging outside of skyscrapers When King Kong is swinging from a vine, what forces are acting on him?

Hints

The forces we discussed were

1. gravity
2. normal
3. friction
4. push or pull

1. We assume that King Kong has travelled to the jungles on Earth, so he still experiences the Earth's gravitational force, Fmg.
2. The overgrown gorilla is not in contact with any surface, so there is no normal force.
3. Again, he is not in contact with any surface, so neither static friction nor kinetic friction is exerted on him.
4. No one or thing is pushing or pulling on King Kong (and who or what would want to?), but there is a tension T in the vine holding him up.

Example 4: The Elevator

Question

Our furry friend, King Kong, has had enough of monkey-ing around, and has returned to the Big Apple. Once again he is up to his tricks with the Empire State Building. Only this time, he has taken the elevator instead. While he is riding up to the top of the building, what forces are acting on him?

Hints

The forces we discussed were

1. gravity
2. normal
3. friction
4. push or pull