which of the following is there no net work?

A car accelerates down a hill.
A car travels at constant speed on a flat road.
A car decelerates on a flat road.
A car decelerates as it travels up a hill.

I say its the first one

and, you are wrong. Gravity is doing work on the car.

the second one

To determine which situation has no net work, we need to understand the concept of net work. Net work is the work done on an object by all the forces acting on it. Work is defined as the product of the force applied to an object and the displacement of the object in the direction of the force.

In the first scenario, a car accelerates down a hill. When the car accelerates, there is a net force acting on it in the direction of motion. This force does work on the car, resulting in net work being done.

In the second scenario, a car travels at a constant speed on a flat road. Since the car is not accelerating, the net force acting on it is zero. Therefore, no net work is done on the car in this situation.

In the third scenario, a car decelerates on a flat road. When the car decelerates, there is a net force acting on it opposite to the direction of motion. This force does negative work on the car, resulting in net work being done.

In the fourth scenario, a car decelerates as it travels up a hill. Similar to the third scenario, there is a net force acting on the car in the opposite direction of motion, resulting in net work being done.

Based on this analysis, the correct answer is the second scenario: a car traveling at constant speed on a flat road. In this situation, the net force is zero, meaning no net work is done on the car.