Your friend is confused about ideas discussed in chapter 4 that seem to contradict ideas discussed in this chapter. For example, in Chapter 4 we learned that the net force is zero for a car traveling along a level road at constant velocity, and in this chapter we learned that work is done in such a case. Your friend asks, "How can work be done when the net force equals zero?" What is your explanation?

The work can be increasing the potential energy of the system, for instance, if an aircraft is flying vertically at constant speed, the net force is zero, but the PE of the aircraft is increasing.

e car goes once around the track in 361 s. What is the magnitude of the centripetal acceleration of the car?

The apparent contradiction your friend is noticing can be resolved by understanding the different perspectives and concepts being discussed in the two chapters.

In Chapter 4, the idea of a car traveling along a level road at a constant velocity is described. In this scenario, it is stated that the net force acting on the car is zero. This means that all the forces acting on the car (such as friction, air resistance, and the force exerted by the engine) are balanced in such a way that they cancel each other out, resulting in no acceleration or change in velocity.

However, in this chapter, the concept of work is being discussed. Work is defined as the transfer of energy that occurs when a force is applied to an object, causing it to move in the direction of the force. Work is directly related to the displacement of the object and the force applied to it.

The important point to understand is that even though the net force on an object may be zero, work can still be done on that object. In the case of a car traveling at a constant velocity on a level road, while the net force is indeed zero, there can still be individual forces acting on the car that cause displacement. For example, if the engine of the car applies a force to push it forward, it is doing work by transferring energy to the car and causing it to move. Although the force of friction may be opposing this motion, it does not negate the work done by the engine.

So, in summary, the net force being zero does not imply that no work is being done. It simply means that the forces are balanced, resulting in no acceleration. Work can still be done on an object even when the net force is zero if there are individual forces resulting in displacement.

To help your friend understand the apparent contradiction between the idea of the net force being zero and work being done, let's break it down step by step.

First, in Chapter 4, when a car is traveling along a level road at a constant velocity, the key point is that the net force acting on the car is indeed zero. This means that the forces acting on the car, like the force of gravity or the friction force, are balanced out by an equal and opposite force.

However, in the current chapter, we learned about work being done. Work is defined as the transfer of energy that occurs when a force is applied over a displacement. In other words, work is done when an object moves in response to a force acting on it.

In the case of the car moving along a level road, while the net force is indeed zero, work can still be done. How is that possible? It's because the net force is not the only factor that determines whether work is done; displacement is also crucial.

When a car is moving at a constant velocity, the net force is zero, but the car is still covering a distance or undergoing a displacement. Even though there is no change in speed (constant velocity), the car is still moving from one point to another. This displacement is critical in the context of work because work is directly related to the displacement of an object.

So, to answer your friend's question directly, work can be done even when the net force is zero because work is not solely dependent on the net force but also on the displacement of the object. In the case of the car moving along a level road at a constant velocity, the displacement is non-zero, and therefore work is being done.

To further illustrate this, you can explain that if the car were to stop suddenly or change its velocity, then the net force would no longer be zero. Consequently, the work done would also change because the displacement and/or the net force would no longer be zero.

Overall, it's essential to understand that work and net force are related but not the same concept. The net force determines the motion of an object, while work characterizes the energy transfer that occurs when a force acts on an object over a displacement.