Hi,
you need to get the local axis of the gameObject. that will give you the directions of the object relative to itself.
in the scene, select your object, and study the tripos ( red green blue arrows), and make sure you have selected "local" in the top bar of Unity interface ( Pivot | Local ) as opposed to ( CENTER | GLOBAL )
so the apply force from the left, means applying force in the positive x axis. AND you must check "Self" for the Space if you use an action such as "Add Force",
bye,
Jean