Final answer:
Unbalanced forces on a vehicle can be caused by sudden changes in direction or speed, wind gusts, or any changes in either the applied engine force or the resisting forces such as drag and rolling resistance. Maintaining constant speed requires forces to be balanced, and the feeling of being pushed outward while turning is due to inertia, not a real force.
Step-by-step explanation:
When driving down a highway at 70 miles per hour under perfect road and weather conditions, several scenarios could lead to unbalanced forces acting on the vehicle. These include sudden changes in direction or speed, such as swerving to avoid an obstacle or abrupt braking, which would produce a net force and change the car's state of motion according to Newton's laws. Another situation could be wind gusts, which would create an additional side force on the car. Engine force must equal the force of drag and rolling resistance to maintain constant velocity, but any change in these resisting forces or a change in the applied engine force will result in acceleration.
For example, if a car is traveling around a curve, the direction of the forces changes. Instead of all forces being in the same direction, the centripetal force is required to keep the car on the curved path. This creates a sensation of being pushed outward, which isn't a real force, but rather a lack of inertia in the passenger that wants to continue moving in a straight line.
Overall, any change that causes the force applied by the engine to not be balanced with the resisting forces will result in a net force and cause acceleration, changing the car's state of motion. Force equilibrium is depicted in a free-body diagram, which includes all the forces acting on an object, highlighting when they are balanced or unbalanced.