When a ball rolls down an inclined plane , it gains speed because of

[alyafey22]

When a ball rolls down an inclined plane , it gains speed because of gravity .When rolling up an inclined plane, it loses speed because of gravity .Why doesn't gravity play a role when it rolls on a horizontal surface ?

 

[aris1]

In an horizontal surface the force of the weight is perpendicular to the displacement so no work against gravity is done if that's what you mean 'gravity doesn't play a role'.
Only work against friction is done.