You want to push a sled which has two dogs in it across a very slippery icy surface (you will have footwear with good traction and can provide a steady force, but otherwise the sled will not have external forces interacting with its horizontal motion).
In order to push the sled without making it rotate, where should you push?
Refer to the diagram to help you conceptualize the situation. All objects in the system you are about to push have some mass, though their quantities are unknown. The sled is motionless initially and the surface is completely flat. Assume you are going to push on the side that is closer to you, as shown in the diagram.