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

Solution

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.