What is an example of when momentum is not conserved?

Example:

In this question, it is important to understand the trick that is hidden in it. Initially it might look that the problem is unsolvable, but it is definitely solvable. Now let us first write down all the information's that are provided.

Force = 100 Newtons

= 100 kg m/s^2

Time = 5 seconds

Now we know the unit of momentum to be kg m/s. So if we multiply the time with momentum then we can get the momentum very easily.

Then

Momentum = (100 * 5) kg m/s

= 500 kg m/s

So the momentum of the wagon is 500 kg m/s