A person drags a sled across a frozen lake to town. The surface of the lake is quite smooth and slippery (so the friction from the ice can be neglected), but he has studded boots with nails driven through the soles (from the inside) so he can walk across the ice without slipping. Once the sled is moving at a reasonable speed (about the pace he can walk comfortably), in order to keep the sled moving at this speed he needs to Pull with a constant force. Not pull at all.

A person drags a sled across a frozen lake to town. The surface of the lake is quite smooth and slippery (so the friction from the ice can be neglected), but he has studded boots with nails driven through the soles (from the inside) so he can walk across the ice without slipping. Once the sled is moving at a reasonable speed (about the pace he can walk comfortably), in order to keep the sled moving at this speed he needs to Pull with a constant force. Not pull at all.

Pull with a constant force.
	Not pull at all.
	Pull with a decreasing force.
	Pull with an increasing force.