Question

why is it easy to pull a lawn roller than to push it?what is the scientific reason behind this phenomenon?

Answer 1

When you pull with a force F, the vertical component of force will be upward. So the net vertical force will be less than weight of lawn mower. So the normal force will be less(N = mg - F sinФ, where Ф is the angle with which you are pulling) and hence friction will  be less. When you push with a force F, the vertical component of force will be downward. So the net vertical force will be more than weight of lawn mower. So the normal force will be more(N = mg + F sinФ, where Ф is the angle with which you are pushing) and hence friction will  be more. So pulling is generally easier than pushing.
That is the main reason. Apart from that, in lawn mower, when you push, the grass gets cut. So you have to apply extra force to for cutting the grass. But while pulling, you don't have to apply any force to cut grass which also make the pulling easier.