When a truck collides with van moves backwards and forwards why?

When the truck collides with the van there will be exchange of energy between the truck and the van. The direction in which the will move will depend on various factors such as the masses and speeds of the two vehicles, as well as the direction of impact.

Initially the truck will transfer some of its kinetic energy to the van, causing the van to move forward. However the collision also causes the truck to slow down, which in turn creates an unbalanced force on the van in the opposite direction (Newton's Third Law of motion) causing the van to move backwards.

The extent of the forward and backwards movement will depend on how elastic the collision is. In an elastic collision, energy is conserved and the total initial kinetic energy of the system will equal the total final kinetic energy. In an inelastic collision some energy is lost due to deformation or damage to the vehicles, and the final kinetic energy of the system will be less than the initial energy.

In other words, the van moves backwards and forward due to the conservation of momentum and energy. The initial impact causes the van to move forward while the subsequent unbalanced force (due to the collision) causes it to move backwards. The extent of the forward and backwards movement depends on the specific details of the collision.