Forces work in pairs. For example when you push a shopping cart, another really good example is if you are pushing a stalled car from the street, it is more difficult to get the car moving than what it is to keep it moving. This is because inertia is working against you to keep the car at rest (object at rest will stay at rest). Once the car is moving, inertia will work with you and you will only have to push with enough force to overcome gravity and friction.
