Problem

You want to move a part in a certain direction with a certain force.

Solution

Use the BodyThrust object controlled with the force property.

local b = Instance.new('BodyThrust')
b.force = Vector3.new(0, 5000, 5000)
b.location = Vector3.new(0, 100, 100)
b.Parent = Workspace.Part

Discussion

The BodyThrust object uses its force property to control the force on each respective axis. Then it uses its location property to determine the direction in which is going to apply the force.