Mechanical weathering is the breaking down of substances, often rocks, into smaller pieces. There is no molecular change. In chemical weathering, molecular change occurs. Molecular change means that a few of the atoms that create the molecule could become unattached and separate; and some new atoms might be introduced into the mixture during this process. If mechanical weathering is the physical breaking down of a substance, and it breaks down into single molecules(remember, this doesn't change the molecule itself), the single molecules could join with other molecules and make new substances.
A slump is exactly what it sounds like: a large vertical slab of land that "slumps" and sags under the force of gravity. A slump does not move at quick speeds like a landslide. A landslide is composed of chunks of rock and dirt that move downhill, often at high speeds. The landslide often starts from a pile of loose dirt. It also can start when a mountain with partially solid rock formations undergoes an earthquake and rock pieces are shaken free of the loose formations. The image I've included shows the difference.