Can momentum be negative ?

Yes, momentum can be negative. Momentum is a vector quantity, meaning it has both magnitude and direction. The momentum of an object is given by the formula:

p = mv

where:

p is the momentum,
m is the mass of the object,
v is the velocity of the object.
Momentum will have a negative value if the velocity of the object is directed opposite to the chosen reference direction (usually the positive direction). In other words, if an object is moving in the opposite direction to the positive axis (such as to the left if we define right as positive), its velocity will be negative, and so will its momentum.

For example, if an object has a mass of 2 kg and is moving to the left at 3 m/s, its momentum would be:

p = 2 kg * (-3 m/s) = -6 kg·m/s

Thus, the negative sign indicates that the momentum is directed opposite to the reference direction.